Geological biology: an introduction to the geological history of organisms

THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

GEOLOGY LIBRARY
IN MEMORY OF

PROFESSOR

GEORGE D. LOUDERBACK
1874-1957

GEOLOGICAL BIOLOGY

AN INTRODUCTION TO THE

GEOLOGICAL HISTORY OF
ORGANISMS

HENRY SHALER WILLIAMS

SILLIMAN PROFESSOR OF GEOLOGY
IN YALE COLLEGE

NEW YORK

HENRY HOLT AND COMPANY
1895

Copyright, 1895,

BY

HENRY HOLT & CO.

ROBERT DRUMMOND, PRINTER AND ELECTROTYPBR, NEW YORK.

can

IV C
W7

TARTH
SCIENCES
UBfiAfiY

PREFACE.

THE following chapters were originally written in the form
of lectures, delivered first at Cornell University, where they
were supplemented by special laboratory work and illustrated
by actual specimens of the organisms or fossils described. The
attempt was made to replace the ordinary treatment of the
dry statistics of historical geology and paleontology by some-
thing which would bring the chief problems of the history of
organisms within the comprehension of the ordinary college
student, and kindle in the special student enthusiasm for
deeper research. In preparing them for publication the lec-
ture form was dropped, such revision of the language and
treatment was made as to provide for readers who might not
have at hand full museums from which to draw illustrative
material, and a few of the more characteristic examples, used
in elucidating the principles discussed, were selected and more
fully and precisely elaborated, so as to make a text-book for
use in earnest and exact study as well as an exposition of gen-
eral principles.

Two classes of readers were considered in giving the book
its present form, viz., students in colleges and universities
who have begun to appreciate the importance of understanding
the principles of the nature and history of organisms, either
as a preparation for further special studies or as a part of a
liberal education ; and second, the general reader, who is sup-
posed to know something of the present popular theories re-
garding organic life, and has, perhaps, already become aware
of the increasing sense of disappointment which those are
meeting who have attempted seriously to apply them to the
solution of the problems of human life. It is not assumed
that the reader has any special knowledge of biology or geol-
ogy to start with. On this account some details have been
given which would be unnecessary for the specialist, while, on
the other hand, many elaborations which would interest him

108

IV PREFACE.

have been omitted in order to bring under discussion as many
as possible of the essential problems.

The book is not intended to be a complete treatise upon
paleontology, nor a detailed report of the relation of fossils to
geological formations or to time. It is rather a reconnaissance
of a fascinating region, from which the few explorers who have
already penetrated it have brought back accounts of the most
remarkable and unexpected discoveries. A reconnaissance
aims to discover the characteristic features and the relative
importance of the various elements making up the territory
traversed, and is merely introductory to a more minute and
careful survey; its purpose is to aid the judgment, to direct
the 'jcourse of further research, and when difficulties of
travel and distances are great, it is particularly useful in pre-
venting distraction from the most expeditious way to the facts
of chief importance.

The tendency of modern science is now, and for more
than a quarter of a century has been, so much to specializa-
tion, and our minds have become so fascinated by the minute
and the particular, that our common judgments of the true
proportion of things have become more or less distorted.
Theories and ideas which have been drummed into our ears
have come to appear the most important truths in the world,
and all our;, thoughts have become colored by them. We
cannot reaii £he newspapers or listen to the talk on the street
without being convinced that the thought of the people, how-
ever little they may know of the sciences involved, is thus
biased by current theories about life and organisms. The
bearing of biological theories upon our judgments of the right-
ness or wrongness of conduct, both of ourselves and of so-
ciety, is too direct to admit of any uncertainty regarding the
validity of their foundations or their precise import. While
the facts and phenomena upon which some of the theories rest
are purely biological, others of them, which concern man
most intimately, have their chief evidence in the historical
records of geology.

Among the latter none is more important than those gath-
ered about the phenomena of evolution ; but it is evident
upon reflection that the biologist proper, who deals alone

PREFACE. V

with the organisms now living upon the earth, must rest with
a theoretical interpretation of the laws of evolution. To the
geologist the records of evolution are open for direct exami-
nation, and geological biology is a scientific treatment of the
observed facts of evolution.

While there are no end of books on evolution, and modern
biologists seem content to assume that some theory of evolu-
tion is true, without being able to decide which it shall be ;
and although the students of sociology, the moralist, and the
theologian are basing their theories about man on the " work-
ing hypotheses" of the naturalist as if " law and gospel," — it
seems to have escaped serious attention that we have open
for study a genuine record of the actual evolution of organ-
isms, extending from near the beginning of life up to the
present time. Men have been speculating in all conceivable
directions to form some theory as to how evolution ought to
work, and as to what the history of organisms ought to be : it
is the province of geological biology to tell us what the his-
tory of organisms has actually been. The geologist does not
ask what is the theory of evolution, but what are the facts of
evolution. " The primary and direct evidence in favor of
evolution can be furnished only by paleontology. The geo-
logical record, so soon as it approaches completeness, must,
when properly questioned, yield either an affirmative or a
negative answer: if evolution has taken place, there will its
mark be left ; if it has not taken place, there will lie its refu-
tation." The late Professor Huxley, who framed this most
true and pertinent sentence, knew very well the evidence
which those records furnish, although he often treated evolu-
tion as if it were a doctrine requiring argumentative defense,
rather than a science which only needs elucidation.

The treatment which evolution receives in these pages is
designed for those who wish to know what the chief facts and
factors of evolution are, not those who are looking for further
debate of the arguments either for or against a theory of evo-
lution. To the student who approaches the subject from the
historical side evolution becomes the very key to the mystery
of organic life. The phenomena of growth are fairly well
understood, the development of the individual has been sys-

VI PREFA CE.

tematized into a science of embryology ; and as we also dis-
cover the grand features of the evolution of species and races
and kinds of organisms, life begins to assume the proportions
of one of the fundamental forces in the world. When con-
sidered from this point of view the question what causes the
evolution of organisms seems as impertinent as what causes
the motion of the celestial spheres. The answer to both is
the same.

That the form and functions of successive organisms should
be accurately adjusted to their organic and physical environ-
ments is no more surprising than that the size and weight of
the revolving planets should be accurately adjusted to the
orbits in which they swing; but once grant that the systems
are in motion, and it is not reasonable to suppose in either
case that at any point in the succession of phenomena misad-
justment should occur which would require any hypothetical
selective force to put them right again. Evolution thus becomes
one of the fundamental expressions of life force, requiring no
theory to support it, but calling only for investigation to re-
veal its laws ; and it is in geological biology that we find the
direct evidences of the course of its operation. But evolu-
tion is not all of biology, and therefore sufficient illustration
of their respective phenomena has been borrowed from physi-
ology and embryology to present a comprehensive view of all
the three great factors of organic life, viz., growth, develop-
ment, and evolution.

A few of the chapters are somewhat technical in their
language, and deal with particulars of slight interest to those
unfamiliar with the nomenclature of natural history. These
chapters may be omitted by readers willing to take the
author's statements without verification. Such persons may
omit the purely geological part of the book by passing di-
rectly from Chapter I to Chapter V, where the discussion of
the biological problem begins. The more technical passages
are Chapter II; Chapter IV, except the summary at the
close ; pages 98 to 1 10 of Chapter V ; all but the summary of
Chapter VII ; the latter parts of Chapters XII and XIII ; and
the fine print of Chapters XVIII and XIX. The remainder of
the book, although occasionally expressed in scientific terms,

PREFACE. Vll

will be found, it is hoped, fully intelligible to the ordinary
reader.

Special students of paleontology and geology will miss the
expected descriptions of fossils and the means for identifying
them and for recognizing the horizons they indicate. To
such readers the author has to say that this book is offered
only as an introduction to the grand field of study open before
them, with the hope that it may be useful in guiding and
suggesting methods of investigation, and in encouraging that
deep research which will be found necessary to interpret the
full story of the history of organisms, of which only a glimpse
is here attempted.

H. S. W.

NEW HAVEN, October 5, 1895.

CONTENTS.

(The numbers refer to the pages of the text.)

CHAPTER I.

THE HISTORY OF ORGANISMS, ITS SCOPE AND IMPORTANCE.

Man an organism among organisms, i. — History of organisms and man's
relationship to living things — The discussion not from the zoologi-
cal and botanical side, 2. — The geological aspect of the history of
organisms — Geological history not a repetition of like events, but a
progressive change of phenomena, 3. — Investigation of the laws of
evolution — Old notion of an organism contrasted with the new — Work
of the paleontologist, 4. — Botanists and zoologists observe individual
characters — Paleontologists interested in the history of species, of
races, and of groups of organisms, 5. — Organisms and environment, 6.
— Geological formations — The organism— Races and their history —
The chronological scale, 7. — Theories regarding the length of geologi-
cal time, 8.

CHAPTER II.

THE MAKING OF THE GEOLOGICAL TIME-SCALE.

The heterogeneous names now in use — Importance of a systematic classi-
fication, 10. — Ancient notions of Geology — Beginnings of a scientific
system of classification, u. — Lehmann's classification according to order
of formation — Cuvier's, Brongniart's, and Reboul's contributions, 12.
— Werner's perfection of the Lehmann classification, 13. — Richard
Kirwan, and Geology at the close of the last century, 14. — Geological
mountains (Gebirge} and formations, 15. — The formation of sedimentary
rocks according to Werner and his school, 16. — Werner's classification
of rocks by their mineral characters, 17. — Conybeare and Phillips's per-
fection of the Wernerian system — De la Beche — Maclure's application
of the system to American rocks, 18. — Amos Eaton's classification of
the New York rocks — Principles involved in the Wernerian system of
classification, 19. — Fossils substituted for minerals in classifying strati-
fied rocks — Cuvier and Brongniart, 20. — William Smith and Lyell —
Lyell's classification of the Tertiary into Eocene, Miocene, and Plio-
cene, 21. — Extension of the Lyellian system by Forbes, Sedgwick, and
Murchison, 22. — Phillips' scheme, 23. — Chronological succession in-
cluded in Lyell's system — Dana's elaboration of a geological time-scale,
24. — Biological classification of Oppel — Geological terranes and time-
periods contrasted, 28. — United States Geological Survey definitions of
formation and period — English usage, 30. — Geological systems the
standard units of the time-scale — Cambrian system, 31. — Ordovician
system, 32. — Silurian system — Devonian system, 33. — Carboniferous

X CONTENTS.

system — The Post-paleozoic or Appalachian revolution, 34. — Triassic
system, 35. — Jurassic system — Cretaceous system, 36. — Tertiary sys-
tem— Quaternary system — Fossils the means by which the age of a
system is determined, 37.

CHAPTER III.

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE AND THEIR TIME-
VALUES.

The systems and geological revolutions — Geological revolutions local, not
universal, 39. — Revolutions expressed by unconformity and disturb-
ance of strata — Appalachian revolution, 40. — Taconic revolution, 41. —
Acadian revolution — Appalachian revolution — Palisade revolution, 42.
— Rocky Mountain revolution, 43. — The division line between the Cre-
taceous and the Tertiary — Columbia River lava outflow, 44. — Glacial
revolution — Erosion of river canons as gauges of time duration — Con-
tinental value of revolutions as time-breaks in the history of North
America, 45. — Time-scale and the geological revolutions of the Amer-
ican continent — Revolutions made interruptions in the record, 46. —
Time-ratios, or relative time-value of the several systems, 47. — Ward's
estimate, 48. — Corrections and elements of uncertainty in these esti-
mates— Estimates of actual length of time highly hypothetical, 49. —
Systems the standard units of geological chronology — Geologic Eras
and Times and their names — Division of the Eras into Periods, 51. —
Period a recognized division of an era — Standard Periods and their
names, 52. — Use of the term Epoch in the time-scale — A comparative
time-scale for the study of the history of organisms, 53. — Importance
of a standard time-scale, 54. — Actual length of geological time, 55. —
Data upon which time-estimates are made — Physical and astronomical,
56. — Geological — Method of computing time from thickness of rocks,
57. — Errors arising from estimated values in the computations, 58. —
Errors affecting the values of actual, not relative, time-lengths — Vari-
ous estimates of the length of geological time, 61. — Average of the
estimates of only hypothetical value — Provisional units of the time-
scale assumed to be of equal value, 63.

CHAPTER IV.

STRATIFIED ROCKS— THEIR NATURE, NOMENCLATURE, AND FOSSIL

CONTENTS.

Common usage in classifying stratified rocks — Fossils of higher value than
strata for determining time-relations, 65. — The necessity of two scales ;
strata furnishing the data for the formation-scale, and fossils forming
the basis of the time-scale — Use of the terms Period and Formation,
66. — Strata parts of a geological formation ; fossils the marks of a
geological period, 67. — The Hemera of Buckman — The terms Age of
Reptiles, Planorbis Zone, etc., 68. — Nomenclature of the International
Congress of Geologists — Fauna and flora — Horizon — Zone and stratum
— Fades, 69. — Area, province, region — Geological range and geo-
graphical distribution — Variations and mutations — Development and
evolution — Initiation and origin, 70. — System — Geographical conditions
determining the local character of stratified rocks, 71. — Varying condi-

CONTENTS. Xt

tions of environment in relation to time estimates — Relative order of
deposits in relation to depression and elevation, 73. — Order of deposits
with a sinking land — Order of deposits with elevation of land, 74. —
Characteristic fossils, 75. — Summary, 76.

CHAPTER V.

FOSSILS— THEIR NATURE AND INTERPRETATION, AND THE GEOLOGI-
CAL RANGE OF ORGANISMS.

Fossils of vegetable and animal origin — Original material of fossils, 78. —
Various aspects of the original form represented — Preservation of fos-
sils, 79. — The majority of fossils are of marine organisms — Various
kinds of fossils enumerated, 80. — Fossils represent chiefly the hard
parts of organisms — Best and most perfectly adjusted organisms of
the time left their records — General laws regarding the occurrence of
fossils, 81. — Change of the forms with passage of time, and particu-
lar forms characteristic of particular periods of time, undeniable facts
of Paleontology — Inorganic things, on the contrary, unchangeable —
Fossils characteristic of particular periods of geologic time, 83. — Stony
corals, the Zoantharia — Numbers of genera of the Zoantharia recorded
for each era, 84. — Two types of the Zoantharia indicated by the two
maxima of genera in separate eras in the time-scale — Table of the
number of genera of Madreporaria making their first appearance in
each geological system, grouped in families — Evolution curve of a
group of organisms, 85. — Evolution curves of the various types of the
Madreporaria, expressing the rate of generic differentiation of each
type — Meaning of these evolution curves, 87. — Chronological value of
family groups of genera — The life-period of a genus, 88. — Organisms
express evolution in their geological history; a fundamental law — The
meaning of genus and species, 89. — The fossil coral, Favosites niaga-
rensis, as an illustration, 90. — Geological range and taxonomic ranks
of the characters, 92. — Table expressing the geological range of the
characters of the fossil Favosites niagarensis Hall, arranged according
to their taxonomic rank — Time-values of the characters of an individ-
ual differ according to their taxonomic rank, 93. — Stages of growth in
Ontogenesis, 94. — No successive stages of functional activity seen in
Phylogenesis — Contrast between the developmental stages of the in-
dividual and the succession of species, 95. — Evolution an organic pro-
cess, and not applicable to inorganic things — Fossils furnish the direct
evidence of evolution, 96. — Living organisms furnish direct evidence
of purposeful development, 97. — Fossils and geological biology — Hard
parts express both relation to environment and relation to ancestry,
98. — Kinds of hard parts of the animal kingdom preserved as fossils —
Protozoa, 99. — Ccelenterata, 100. — Echinodermata — Vermes — Arthrop-
oda, 101 — Molluscoidea, 104. — Mollusca, 105. — Vertebrata, 106. —
Summary, 109.

CHAPTER VI.

GEOGRAPHICAL DISTRIBUTION— THE GENERAL RELATION OF ORGANISMS
TO THE CONDITIONS OF ENVIRONMENT.

The importance of the study of geographical distribution, 112. — The nat-
ural conditions of environment : nomenclature — Natural-history prov-

XI J CONTENTS.

inces, 113. — Normal adaptation to conditions of environment — Specific
centres of distribution and varieties, 114. — The distinctness of the flora
and fauna of distinct provinces — The various classifications of natural-
history provinces, 115. — Marine organisms particularly important to
the paleontologist — Haeckel's classification of the marine conditions
of life — Walther's further analysis of conditions of environment, 116.
— Relations of organisms to time and to environment equally signifi-
cant, 117. — An explanation required for succession of species as well
as for adjustment of species — Evolution and adaptation both observed
facts, 118. — Ancestry and environment as causes of evolution — Differ-
ences of opinion respecting interpretations, not facts — Introduction of
causation into the discussion, 119. — Ancestry and Environment in
relation to the beginning of each individual — Definition of the terms
"Ancestry" and "Conditions of Environment," 120. — Two factors
producing the effects of evolution — Three views possible — First cause
of some sort essential to any complete theory of evolution, 121. —
Edward Forbes on origin of species and centres of creation — Reality
of specific centres not questioned; the fact variously interpreted, 122.
— Representative species, common descent, and migration of species —
Darwin did not deny the facts, but explained them differently from
Forbes — Forbes' explanation of the origin of species, 123. — The mean-
ing of evolution by descent — Distinction between Evolution and De-
velopment, 124. — Immutability or mutability of species, 125. — Muta-
bility of species the central thought in the new theory of the origin of
species — Two extremes of opinion regarding the mode of origin of
species by evolution, 126. — An unknown cause assumed to explain
origins by both Forbes and Lamarck, 127. — Conclusions, 128.

CHAPTER VII.

GEOGRAPHICAL DISTRIBUTION: SPECIAL CONSIDERATION: THE ADJUST-
MENT OF ORGANISMS TO ENVIRONMENT.

Resume, 129.— Gastropoda illustrate the law of relationship between
organisms and environment — Meaning of the classification of organ-
isms, 130. — Distinguishing characters of the class Gastropoda, 131. — •
Zones of environment in which Gastropods are distributed, 132. —
Reasons for selecting the Gastropods — Peculiarity of the divisions of
the Gastropods as to range of adaptation, 133. — Mode of existence of
the Glossophora, 135. — The zonal distribution of the Ctenobranchina,
136. — Genera of the Ctenobranchina characteristic of the several
bathymetric zones, 137. — Evidence of the adjustment of the morpho-
logical characters to environment — Law of the adjustment of organ-
isms to conditions of environment, 138. — Summary, 139. — Relation
between zonal adaptation and geographical range, 140. — Families
whose genera have a very wide range of adaptation, and restricted
adjustment only among the species — Great difference in the closeness
of adjustment of the characters of different taxonomic rank, 142. —
Species generally closely adjusted to particular conditions — Fresh-
water families; restriction in their distribution — Two closely allied
families separated in their distribution, 143. — Table of the Geological
range of the families Strombidse and Chenopodidae, 144. — Relation of

CONTENTS. Xlll

antiquity to distribution, 144. — Table of the Geological range of the
families Cerithiidse and Rissoidae — Distribution in relation to the
temperature of the waters, 145. — Tabulation of the facts, 146. — Table
expressing the relation between the differences in structure of the
Gastropoda and different conditions of environment — Summary of
results, 147.

CHAPTER VIII.

WHA T IS A SPECIES?— VARIOUS DEFINITIONS AND OPINIONS.

What are species ? Their numbers and importance, 149. — Definitions of spe-
cies— Tournefort — Linne — Buffon — De Candolle — Cuvier — Zittel, 150.
— The theory of mutability of species and evolution, 151. — Lamarck —
Etienne Geoffroy St. Hilaire — Anaximander — Philosophical import-
ance of the transmutation theory of the lonians, 152. — Antiquity
of the notion of evolution — Reality of species logically antecedent ta
the notion of specific mutability — The idea of species as immutable,
153. — A mutable species necessarily temporary — The question of the
mutability of species entirely distinct from that of the origin of
species, 154. — The fundamental tenet of the mutability school —
State of opinions when Darwin began his investigation of the origin
of species, 155. — New conception of the nature of species — Remarkable
evolution of thought started by Darwin's "Origin of Species," 156. —
Evolution theory of Biology and the uniformitarian theory of Geology —
Evolution and Development contrasted, 157. — Evolution the history
of the steps by which variation is acquired, not transmitted — A defini-
tion of Darwinism, 158. — The Lamarckian theory of evolution — Phylo-
genetic evolution, 159. — The fact of evolution established beyond con-
troversy; the real nature of evolution to be learned only by a study
of the history of organisms — What is an individual? 160.

CHAPTER IX.

WHAT IS AN ORGANISM? THE CHARACTERISTICS OF THE INDIVIDUAL
AND ITS MODE OF DEVELOPMENT.

Mutability of organisms a foundation principle of all evolution — Morpho-
logical similarity the characteristic of species, 162. — The definition of
an organism — Living and performance of physiological functions are
essential parts of the definition of an organism — A zoological specimen
in the museum as much a vestige of an organism as a fossil, 163. —
Living implies change, and change is incessant in a living organism —
An organism is an aggregate of cells — Tl^e organic cell the morpho-
logical unit, 164. — The three ways by which cell modification is accom-
plished— Metazoa characterized by Histogenesis, or the formation of
tissues, 165. — Histogenesis, Cryptogenesis, and Phylogenesis — Anal-
ogy between the cell aifd organism, and the molecules, elements, and
minerals of inorganic matter — The individuality of the organism, 166.
— Growth and reproduction of the Protozoa and of the Metazoa con-
trasted— Generation the fundamental function of an organism — Sum-
mary of the steps of progress in organic development, 167. — Growth-
Development — Evolution — Embryology, 168. — The functions of a

XIV CONTENTS.

metazoal organism; generation — Agamogenesis, 169. — Gamogenesis,
170. — The several stages of development in the higher organisms,
171. — The primitive tissues, Endoderm, Ectoderm, and Mesoderm,
172. — The special organs arising from primitive tissue layers — The
embryo stage, characterized by dependence and passivity, is not sub-
ject to individual struggle for existence, 173. — The stage from the
free existence of the individual to the maturing of its functions — The
cell an organism — Differentiation of the cell a mark of its organic
nature, 174. — Differentiation and specialization the marks of an
organism, 175. — The attainment of heterogeneity — Grand results of
ontogenesis, or development of the individual, 176. — Classification of
the functions of a Vertebrate, 177. — Are the laws of ontogenesis the
same as those of phylogenesis ? — The meaning of function, 178. —
Normal growth — Natural Selection, 179. — Definition of Ontogeny and
Phylogeny — The main features of development predetermined before
they begin, 180. — Slight possible effect of environment, 181.

CHAPTER X.

WHAT IS THE ORIGIN OF SPECIES?— THE PROBLEM AND ITS
EX PL AN A TION.

Variation and mutability essential presumptions in the discussion of origin
of species, 183. — Variability an inherent characteristic of all organ-
isms— The origin of form, not of matter — Definition of species whose
origin is sought, 184. — Meaning of " origin of species " — Development
of individual characters known and observed— The law of develop-
ment, 185. — No analogy between the origin and development of an
immutable species — Inorganic properties and organic characters com-
pared, 186. — The idea of mutability at the foundation of the discussion
of the origin of species — What is mutable? — A concrete example; its
characters symbolically represented, 187. — Spirifer striatus Martin,
var. S. Logani Hall, taken as the example, 188. — New species con-
ceived of as arising by a process of variable characters becoming
permanent, 189. — Characters of any particular specimen differ greatly
in antiquity, 190. — The majority of the characters of a so-called new
species have appeared before, 191. — Fixed characters those which are
transmitted unchanged in natural descent — Rank of characters, the
precision of their reproduction, and their antiquity, 192. — Plasticity of
characters — Origin of species from the physiological point of view —
Darwin's theory of the origin of species, 193. — Do characters become
of higher rank as they are transmitted ? — Evolution of genera and ac-
celeration and retardation, 196. — Growth-force or bathmism — The
origin of species still an open question, 197.

CHAPTER XI.

THE PRINCIPLES OF NA TURAL HISTORY CLASSIFICA TION: ILLUSTRA TED
BY A STUDY OF THE CLASSIFICATION OF THE ANIMAL KINGDOM.

Classifications in Natural History — Species and genus of Aristotle, 200. —
Scaliger's terms — The terms of Linne — Cuvier's perfection of the

CONTENTS. XV

nomenclature and the present usage — The classification of Cuvier, 201.
— Uniformity of usage of specific and generic names, 202. — Selection of
a standard classification — Differences of opinion regarding the rank of
the characters — Claus' and Sedgwick's definitions of the nine branches
of the animal kingdom — Protozoa — Ccelenterata, 203. — Echinoder-
mata — Vermes — Arthropoda — Molluscoidea — Mollusca — Tunicata —
Vertebrata, 204. — The classes of importance in Paleontology and
their known range in geological time — Species and genera of chief
use in tracing the history of organisms, 205. — The classes of the
Animal Kingdom and their geological range, 206. — Species of the
paleontologist — Varieties — Mutations — The history of organisms; the
two methods of its study, 207. — Embryos or fossils; the imperfection
of the evidence, 208. — Mature individuals, not embryos, used by the
Paleontologist — Differentiation attained during the first or Cambrian
Era, 209. — Nature and extent of the elaborations — Recurrence of char-
acters accounted for by descent, 211. — Modern zoology applicable to
the fauna of the Cambrian Era — Characters whose origin is traced
back to Cambrian time — Protozoa, Metazoa — Echinodermata — Anne-
lids— Arthropoda, 212. — Insignificance of characters of marine inverte-
brates evolved since Cambrian time, 218.

CHAPTER XII.

THE TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM.

Records of evolution expressed chiefly in generic and specific characters —
Course of individual development supposed to have been constant,
219. — Beginning of individual life and development — Hypotheses re-
garding the phylogenetic evolution of races, 220. — The undifferentiated
cell, 221. — Polarity — Antimeres and Metameres— Radiate structure,
bilateral symmetry, and actinimeres — Primary axis, 222. — Somites,
arthromeres, and diarthromeres of the Arthropods, 224. — Distinc-
tive characters of the Metazoa — Molluscan type of structure — De-
velopment of organs and their taxonomic rank and value, 225. — The
principle of Cephalization, 226. — Cephalization one of the expressions
of the general law of differentiation — Meaning of homology and ho-
mologous parts — Analogy and analogous parts, 227. — Differentiation
illustrated in the case of motor organs — Two directions in which differ-
entiation proceeds — Ciliary motion, 228. — Water-vascular system of
Echinoderms — Cilia in Molluscoidea and Mollusca — Skeletal parts —
Multiplication of like parts preceding specialization of their functions,
229. — Comparison between embryonic development and succession of
ancestors — Muscular motion or specialized motion, and locomotion,
230. — Differentiation of nervous system a concomitant of locomotion,
231. — Differentiation along the digestive tract — Differentiation of the
motory system into muscular and skeletal organs, 232. — Archetypal
structure — Cuvier's classification, 233. — Von Baer's embryological
classification — Fundamental divisions of classification discerned by
earlier naturalists, 234. — The polymeric type — The dimeric and mono-
meric types, 235. — The metameric and diarthromeric types — Meaning
of typical structures and types in modern Zoology, 236.

XVI CONTENTS.

CHAPTER XIII.
PHYLOGENESIS IN CLASSIFICATION.

Principles of classification illustrated by the Mollusca and Molluscoidea —
The author's philosophy reflected in his classification — Effect of theo-
ries of phylogenesis upon classification, 237.— Analytic and synthetic
method of classification, 238. — Mollusca and the Brachiopods as illus-
trations of evolutional history — Zittel's classification of the branch
Mollusca, 239. — Points of view of the embryologist and of the mor-
phologist, 240. — Embryological likeness of organisms whose mature
characters are diverse — Evolution not traceable between different
classes, 241. — General characters of Mollusca — Molluscoidea — Bryozoa
— Tunicata — Brachiopoda — The Mollusca (proper) — Lamellibranchs —
Gastropoda — Cephalopoda, 242. — Lankester's classification of the Mol-
lusca— The Coelomata — Description of the Mollusca — Digestive system
— Muscular, nervous, and motory systems — Differentiation of the
nervous system — Branches, classes, and subclasses of Mollusca, 246. —
Distinctive features of the Lankester classification, 251.

CHAPTER XIV.

THE ACQUIREMENT OF CHARACTERS OF GENERIC, FAMILY OR HIGHER
RANK; ILLUSTRATED BY A STUDY OF THE BRACHIOPODS.

Generic and specific evolution illustrated by the Brachiopoda, 253. — Bra-
chiopods thoroughly differentiated in early Paleozoic time — Many of
them extinct since Paleozoic time, 254. — Generic life-periods of the
Brachiopods, 254. — Climax of generic evolution at a definite period,
255. — Evolution curves of the Brachiopods — Table of the new gen-
era initiated— Its interpretation, 256. — Majority of characters of living
Brachiopods traceable to Cambrian ancestors, 258. — Perpetuation and
repetition of characters a common law of generation, 259. — Evolution
accounts for divergence, not for perpetuation or transmission, 260. —
Brachiopods ancient types and early differentiated — Laws of evolution
gathered from study of the early families, 261. — Genera making their
initial appearance in each era — Comparison of the rate of evolution of
generic, family, and ordinal characters — Evolution curves for the
several families, 262. — Conclusions from study of generic evolution
curves of the Brachiopods, 263.

CHAPTER XV.

WHAT IS EVOLVED IN EVOLUTION?— INTRINSIC AND EXTRINSIC
CHARACTERS.

Laws of evolution indicated by history of Brachiopods — Magellania fla-
vescens examined as an illustration, 265. — Evolution of the class char-
acters— Evolution of the ordinal characters, 266. — Calcified loops which
are subordinal characters were evolved between the Cambrian and
Silurian eras — Each case of evomtion a case of the appearance in
some individual of a character not possessed by its ancestors, 267. —
Evolution of fundamental characters relatively rapid, 268. — This rapid

CONTENTS. XV11

evolution difficult to account for by any working of natural selection —
What is evolved? — How does the evolution proceed? 269. — Intrinsic
and extrinsic development, and intrinsic and extrinsic characters, 270
— Example of an intrinsic character — Example of an extrinsic char-
acter, 271. — Characters early and rapidly evolved were chiefly intrinsic
characters — Application of the terms intrinsic and extrinsic to the
elaboration of machinery — Summary and conclusions, 272.

CHAPTER XVI.

THE MODIFICATION OF GENERIC CHARACTERS, OR GENERIC LIFE-
HISTORY.

Statistics of the^life-history of the spire-bearing Brachiopods (Helicopeg-
mata) — The rapid appearance of the different modifications of the bra-
chidium, 277. — Three families of the Helicopegmata, 279. — Geological
range of the families — Description of the structure of the brachidium,
280. — Significance of the facts — The loop of the Ancylobrachia and the
brachidium of the Helicopegmata, 282. — Relation of the jugum to the
primary lamellse, 283. — Relation of the primary lamellae to the crurse,
285. — The number of volutions of the spiral, 286. — Direction of the axes
of the spiral cones, 287. — The form of the loop, 288. — Characters of the
brachidium found to be good distinctive characters of genera — Plastic-
ity a characteristic of their early initial stage — Evolution of the char-
acters of the brachidium relatively rapid, 289. — Rate of initiation of
the genera of Helicopegmata — Table expressing the rate of expansion
of the family, subfamily, and generic characters of the Helicopeg-
mata, 290. — General law of rate of initiation of generic characters —
The life-periods of genera and the initiation of a new genus, 291 —
During the life-period of the genus its characters constant, 292. — A
culminating point or acme in the life-period of a genus — Summary of
the geological characters of a genus, 293.

CHAPTER XVII.

THE PLASTICITY AND THE PERMANENCY OF CHARACTERS IN THE
HISTORY OF ORGANISMS.

Races in Paleontology — Phylogeny of the race — Mutability and Phylogeny,
294. — The phylogenetic theory of evolution, 295. — Mutability the
fundamental law of organisms ; the acquirement of permanency sec-
ondary, 296. — Early plasticity succeeded by permanency expressed in
geological history — Pritchard's definition in which the constancy of
transmission of some peculiarity is made the criterion of species, 297.
— Permanency of characters in living forms coordinate with limitation
in distribution and breeding — Specific variability restricted with each
successive generation in fossil forms — Illustrations of the acquirement
of permanency of characters, 299. — The history of the Spirifers — The
permanent characters of generic or higher rank, 300. — Characters which
are plastic at the first or initial stage of the genus — The fixation of
plastic characters in a generic series — Spiral appendages — General
proportions of the shells — Delthyrium and deltidium — Hinge area —
Surface markings — Plication of surface and median fold and sinus —

XV111 CONTENTS.

Structure of shell — Surface spines, granulation, etc. — Special develop-
ment of the median septum, 301 — Evolution of extrinsic specific char-
acters comparatively slow, although their plasticity is greatest at the
initial stage— Laws of intrinsic and extrinsic evolution expressed in
variability and permanency of characters, 311. — Hall's analysis of the
genus Spirifer and classification of its species — Range of species of
Spirifer in American formations, 312. — Each type of Spirifer shows a
continuous series of species, 313. — Each of the chief types represented
at the initial period of the genus — Three epochs of expansion, with
slow and gradual change during the rest of the history of the genus,
314. — Characteristics of the life-history of Atrypa reticularis, 315. —
Considerable and continuous plasticity of the species — Nature and ex-
tent of the variations, 316. — Hall's comment on the variability of the
species, 317. — In the closing part of the life-period of the race the ex-
tremes of acceleration and retardation expressed — Summary, 319. —
Conclusions suggested by the study of Atrypa reticularis, 320. — The
initiation of the species of Ptychopteria, 322. — The law of progressive
evolution of Mammals, formulated by Osborne, 323.

CHAPTER XVIII.

THE RA TE OF MORPHOLOGICAL DIFFERENTIA TION IN A GENETIC SERIES,
ILLUSTRA TED BY A STUDY OF CEPHALOPODS.

The evidence furnished by the Cephalopods — Lankester's schematic Mol-
lusk, 325. — Supposed characteristics of the primitive mollusk — Differ-
entiation of the foot organ in mollusks, 327. — The structure of the
Cephalopods, 329. — Numerical rate of differentiation expressed in terms
of the initiation of new genera, 336. — Rate of differentiation of the sub-
order Nautiloidea, 337. — Mode of curvature of the Nautiloid shell —
Rate of initiation of the Orthoceratidae — of the Cyrtoceratidae, 339 — of
the Nautilidae — History of Trochoceras by species — General law of
evolution of shell curvature in the Nautiloidea — Rate of evolution of
new species in the American region, 340. — Hyatt's formulation of the
law of rapid expansion of differentiation at the point of origin of a
new type of organism, 341. — Summary, 342.

CHAPTER XIX.

PROGRESSIVE MODIFICATION OF AN EXTRINSIC CHARACTER;

ILLUSTRA TED BY THE EVOLUTION OF THE

SUTURE-LINES OF A M MONOIDS.

The Ammonoids illustrate the law of acquirement of differences by grad-
ual modification, 344. — Description of the characters of the Ammo-
noids, 345. — Two divisions of the Retrosiphonatae : Goniatites and
Clymenias, 348. — Quick evolution of the Clymeniidae — Classification of
the Goniatites, 349. — Differences in the sutures of the Ammonoidea
explained as various degrees of crimping of the edge of the diaphragms
— Classification of the types of sutures — (A) the Nautilian type of
suture — (B) the Goniatite type of suture — (C) the Ceratitic, Helictitic,
and the Medlicottian types of suture — (D) the Ammonitic type of sut-

CONTENTS. XIX

lire — (E) the Pinacoceran type of suture, 350. — Relation of order of
succession of initiation to order of ontogenetic development and evo-
lutional history — Order of the ontogenetic growth of these characters,
352. — Chronological succession of the characters, 353. — Rate of elabo-
ration of the various types of suture — Rapidity of modification of each
type soon after it was initiated, 354. — Summary of the laws of evolu-
tion of the suture-lines of the Ammonoidea, 355. — Evolution of the
suture results in the improvement of the structure of the shell, 357.

CHAPTER XX.

THE LAWS OF EVOLUTION EMPHASIZED BY STUDY OF THE GEOLOGICAL
HISTORY OF ORGANISMS.

Testimony of vertebrates— Remarkable and extreme evolution of the Mam-
mals in the Eocene, 359. — Synthetic types illustrated by Vertebrates of
the Mesozoic, 361. — Specialization of five fingers in Reptiles and its
relation to later specializations, 362. — Finger-bones and teeth as tests
of degree of differentiation — Laws derived from the study of the
teeth of mammals by Osborne, 363. — Method and purpose in the se-
lection of the evidence here set forth — Different kinds of evidence
borne by living and fossil organisms, 365 — Natural Selection seems rea-
sonable when based alone upon the study of living organisms — Every
species of organism that has flourished in the past the fittest for its
place and generation, 366. — The geological evidence does not empha-
size the importance of natural selection as a factor of evolution, 367. —
A statement of the laws of evolution emphasized by fossils, 369.

CHAPTER XXI.

PHILOSOPHICAL CONCLUSIONS REGARDING THE CAUSES DETERMINING
THE COURSE OF EVOLUTION.

What is the philosophy of evolution ? Statement of the case — The point
of view, 371. — The act of evolving as well as the order of events in-
cluded in the discussion — The course of the discussion, 372. — Darwin's
origin of species centres its interest in the search for causes — The evo-
lutional idea of creation, 373.— Evolution the mode of creation of or-
ganic beings — The properties of matter not evolved; either eternal
or created, 374. — Evolution does not apply to the mode of becoming of
chemical or physical properties of matter, but is the distinctive char-
acteristic of organisms, 375. — The evolutional idea an enlargement of
the conception of God as Creator, 376. — Evolution as an account of the
course of the history of creation a gain upon the older idea of arbi-
trary creation, but not a satisfactory substitute for creation— Con-
sideration of causation indispensable to a thoughtful study of na-
ture, 377. — Causes not discovered by observation, but discerned by the
reasoning mind, 378. — Ability to adjust the organization to conditions
of environment a chief element in the fitness for survival, 379. — The
philosophy of evolution: a summary, 380.

GEOLOGICAL BIOLOGY.

CHAPTER I.

THE HISTORY OF ORGANISMS. ITS SCOPE AND
IMPORTANCE.

Man an Organism among Organisms. — Man has been very
slow to grasp the fact that he is an organism among organ-
isms. Darwin was the first to speak with such loud em-
phasis as to thoroughly rouse the world to an appreciation of
the very intimate relationship man bears to the whole series
of organic forms of not only present but all past time. We
are apt to be offended by the bold statement that man is de-
scended from the monkeys, but, without insisting upon the
truth of this specific statement, the investigations of modern
science have demonstrated beyond controversy that the same
conditions of affinity and relationship which lead to the classi-
fication of animals into species, genera, or classes, and as con-
nected with each other by direct genetic descent, apply to
man as one of the organisms.

For want of a better name this relationship of man to
other organisms may be called his natural-history relationship.
Man is an organism among organisms, and it is this fact that
lifts the history of organisms out of the field of simple mor-
phological or physiological sciences into a place of direct
human interest. Man's origin and history is intimately asso-
ciated with the origin and history of other living beings in the
world.

Not only is there human interest in the subject of the his-
tory of organisms, but because of this interest there is a de-

2 GEOLOGICAL BIOLOGY.

mand for discussion of the facts themselves from a special
point of view.

The naturalist takes interest in the form and functions of
individual organisms from a scientific point of view ; they are
to him objects of interest in themselves. He classifies and
arranges them as favorite objects of knowledge. But the
general student, the active thinker, the busy worker in human
affairs finds the details of such studies irrelevant, and to him
the vital interest is in the questions concerning the relations
of organisms to the past and to himself.

More than this, the deepest interest of all attaches to the
philosophy which is involved in the proposition that man is
not so distinct from the dumb organic world around him as
was up to a few years ago universally believed to be the case.

History of Organisms and Man's Relationship to Living Things.
— If man has arisen from organisms that were not men ; if the
machinery of his vital organization is represented in less com-
plex form in other animals; if he may find his functions in
operation in simpler forms of life, and separated into their
elements in lower types, then he has in the organic world a
field of study of the greatest interest, which he cannot neglect
without ignoring knowledge that is, in a literal sense, vital to
his best interests as a man.

The study of the laws of organisms, their relations to each
other and to the conditions of environment, their antiquity,
their history, and the nature of those laws of adjustment
which are suggested by the words heredity and descent, varia-
bility, natural and unfavorable habitat, struggle for existence,
adaptation to environment, evolution, and many others which
have arisen within the last fifty years, is of more importance
than we ordinarily attach to the study of the curiosities of
natural history.

The Discussion not from the Zoological and Botanical Side.—
The approach to the study of organisms, from the zoological
or botanical side, presents great difficulty in the very immen-
sity of the subject. When we attempt to analyze the charac-
ters of a single animal, to classify animals and describe them,
the mere mass of detail — the abundance of the characters to
be distinguished — removes the subject from a place in a gen-

THE HISTORY OF ORGANISMS. 3

eral course of liberal education. Such a treatment of organ-
isms as may be sufficient for the illustration of their history
does not necessarily enter into an analysis of the structural
characters of any particular species. Hence, from the point
of view of a technical course of study in biology, this treatise
will seem quite superficial.

The Geological Aspect of the History of Organisms. — On the
other hand, there are characters distinguishing groups of or-
ganisms, evidence of which may be preserved in the rocks,
which are of far greater importance than the specific details in
indicating the relationship organisms bear to each other, to
the conditions in which they have lived, and to the place they
have occupied in the history of the life of the globe. Such
characters are those which will concern us here. In defining
our topic as geological biology, we are not proposing to inves-
tigate the anatomical organs and tissues of which particular
animals are made, but to review the facts and theories which
have led to the belief that each living animal and plant is but
the last of a long line of organisms whose remains can be rec-
ognized in more or less perfect fossils, and whose varying
characters can be traced back into the immense antiquity of
geological time.

Geological History not a Repetition of Like Events, but a Pro-
gressive Change of Phenomena. — If there were only repetition of
the same things, this would not constitute history. If differ-
ent things have succeeded each other, to ascertain the relation-
ship borne by those that follow to those that preceded them
becomes an important problem. We do not, at the outset,
assume to explain the causes, but geology makes the fact
clear that there has been a very elaborate history of the or-
ganisms that have lived on the earth. The question we pro-
pose to answer is, "What are the prominent laws expressed
in this history?"

The geologist observes that there has been a history for
the earth itself : the rocks, as geological formations ; the lands,
as parts of the crust above the surface of the ocean ; the sur-
face of the earth, as a whole, in all its complexity — all these
have come to be what they are through innumerable changes.
The geological conditions in the past have been associated

4 GEOLOGICAL BIOLOGY.

with the history of the organisms. It is proposed to examine
and note what have been the relations existing between organic
form and geological and geographical conditions and progress.
Investigation of the Laws of Evolution. — Evolution has been
discussed and applied in a thousand ways of late years, until
the word has become a kind of shibboleth of modern science.
It is proposed, in the following chapters, to ascertain what
the term really means in the one field in which it may be
properly and scientifically applied. For this purpose it is
necessary to use the methods of philosophy, as well as those

. of science ; to weigh the arguments and reasonings of natural-
ists, as well as to examine, analyze, and classify the facts of

I nature.

Old Notion of an Organism contrasted with the New. — Within
the last thirty years very great change has taken place in the
general ideas regarding the nature of organisms and their rela-
tions to each other. The old idea of an organism perpet-
uating its kind by generation, in which difference of kind was
at once evidence of difference of origin, has of late been almost
entirely replaced by the new idea in which there is not only
repetition by generation of the characters of its ancestors, but
.a constant slight and slow divergence from them, resulting, in
the course of many generations, in bringing about all the dif-
ferences of form which distinguish the various species of the
world, present and past. The new theory has led to an ex-

I haustive study of the relations which organisms bear to one
another and the interrelations existing between geographical

| and geological conditions on the one hand and the form of
organisms on the other.

Work of the Paleontologist. — While embryologists have been
tracing out in detail the changes experienced by the indi-
vidual in passing from the embryonic to the adult stage of
growth, and while the zoologist and botanist have been mi-
nutely examining and teaching the differences in structure
and function of the various parts of each animal and plant,
the paleontologist has been accumulating data to show the
order of succession of life, in the past, and thus has been
opening the way for the particular study of organisms in their
relations to time and space, their geological sequence, their

THE HISTORY OF ORGANISMS. 5

geographical distribution, and the various laws regulating
these modifications and adjustments. The paleontologist is ]
able actually to see the orderly succession of organisms in the
past, and he is constantly called upon to note the relation of
the several forms under his view to the environing conditions
of their life, and thus to interpret the history of the great races
of beings that have peopled the world.

Botanists and Zoologists observe Individual Characters. — The
development of the individual organism from the embryo to
the mature individual is familiar to us all in its general prin-
ciples. We know how the seed or the acorn grows to be-
come the flowering plant or the oak tree. We know that the
egg, by some mysterious process inside the shell, changes so
as to become the chick which cracks its way out, breathes
and develops into the crowing cock or the egg-laying hen.
In each of these cases the history is the history of an indi-
vidual organism. It is the history of a single organism, and
the science teaching about these phenomena is the science of
Embryology, and is concerned with the laws of individual
development.

Botany and Zoology, too, are mainly concerned with a
study of the morphology of the characters of the individual,
its form and structure, and particularly the analysis of its
organs and their functions, in their morphological relations,
the relations of the organs as they are combined for the func-
tions of life of the individual. What there is of history is
life-history of the individual, and what there is of study of
form is of the form of the parts, or of the whole as a complex
of such parts, of an individual organism. And what there is
of classification is classification to bring out the differences
existing between the component parts of separate individuals.
In these studies the individual organism is the highest unit,
and the investigations are conducted in each case as if there
were but one organism : comparisons are between its parts and
not with other organisms.

Paleontologists interested in the History of Species, of Races, and
of Groups of Organisms. — It is for the paleontologist to speak
of the history of races and communities of organisms, that is,
to look upon individual organisms as parts of some complex

GEOLOGICAL BIOLOGY.

whole, to look at organisms as related to each other in the com-
plex environment of the earth, the temporary world-surface^
and in the consecutive time-relations which are recorded in the\
geological strata making up the surface of the globe. In the)
life-history of the individual, or Embryology, we have the
body of the individual to bind together the various stages of
development. For this history the hours of the clock or
the days of the calendar are satisfactory time-divisions. The
relations of the various organs or parts to each other are
easily determined by noting the effect of artificial separation
or excision ; but we see no history of organisms until we
compare those now living with others that lived unmeasured
hours and days and even years ago. Comparison of living
species with living species only, shows us differences which
our classifications enumerate. While we might, theoretically,
guess that the present living organisms came from others not
like them, if we knew nothing of fossils this would be but a
mere vague fancy, and could never find a place in true sci-
ence. Paleontology, however, reveals to us a long series of "|
organic forms, and when we speak of their history we assume
that the series is connected genetically ; the time-relations we
read from the rocks, and in terms of subjacent strata. The
relationship must be determined by comparison of entirely
distinct forms ; we must learn of organisms from their fossilized
remains. These and many other facts must be presented
before we have the data for defining the successive steps of
the history.

Organisms and Environment. — Our subject, then, divides
itself into two grand divisions, organisms on the one hand,
and, to use a very comprehensive term, environment on the
other hand — living things, and the conditions under which
they have lived. The environment or conditions of life are
strictly included in the science of Geology, — for geography is
but the present final product of geological processes. When
we treat of Biology geologically and study the history of
organisms, we assume the truth of two propositions which
are not required in the study of the characters and the devel-
opment of the individual organism. The propositions are :
first, that long periods of time have elapsed separating the

THE HISTORY OF ORGANISMS. 7

periods of living of the several organisms under our investiga-
tion ; and second, that there is genetic affinity between the
organisms now living and those that have lived in the past.
We assume that series of organisms genetically connected
have lived during geological time.

Geological Formations. — It will be necessary to particularly
consider the nature of geological formations, for in them are
found the fossils, and from them is derived the evidence of
the history which we are to read. We must consider how the
formations were made, how the chronological scale is deter-
mined and what reliance may be placed in it. We must con-
sider the manner of deposition, and under what condition fos-
sils have been preserved ; we must examine into the perfec-
tion or imperfection of the record, what has transpired to
destroy the record, and hence how we can supplement the
record we possess. Hence, geological classifications must be
critically examined and analyzed. This will occupy the
earlier chapters.

The Organism. — The second step will be to learn what the
organism is and what it is not ; what is meant by species
and genera ; what is the nature of systematic classification ; the
meaning of generation, race, modification, struggle for exist-
ence, geographical distribution, and many kindred terms.

Races and their History. — This will bring us to the third
part of our subject, the specific study of races, their geologi-
cal history, and the laws to be gathered from their study.
The history of the organism may be viewed under two lights ;
as we consider the development of the individual as it passes
from the germ to the fully organized adult, or as we consider
one particular kind of organism as assuming the features
which now characterize it from some other different kind of
organism which preceded it. In the one case that which is
continuous in the history is the individual life which develops,
in the other case that which is continuous is the race which
evolves.

The Chronological Scale. — In any discussion of history the
first and essential element of fact to be established is a relia-
ble chronological scale by which to mark off the relations of
successive events or epochs of the history. In studying the

8 GEOLOGICAL BIOLOGY.

history of the development of the individual organism, as
artificial time-measures the clock or watch, or the regular
periods of day and night, satisfy the demand. When longer
periods are recorded, the seasons and years, with their arti-
ficial names, are sufficiently definitive. Human history deals
with still longer periods, marked by great events in the na-
tions : the rise or fall of a dynasty, the founding of a city, the
discovery of a continent, the living of some man of powerful
influence — these constitute landmarks by which to measure
the order of lesser events. These, as chronological measures,
are now easily applied, but in our studies in natural history we
soon pass beyond the reach of even such records. A very
few centuries back and human history ceases altogether;
therefore the time-scale for the history of organisms must rest
upon an entirely different kind of evidence. Another reason
renders the ordinary units of time useless for the study of the
history of organisms. The animals and plants associated with
the earliest known traces of man present only the most insig-
nificant amount of divergence from their living representa-
tives. In most cases the differences are not greater than
differences presented by the known descendants of common
ancestors within the memory of a single generation of men.
The period of human existence, however long or short that
may be, is too brief to record any but the more minute details
of those modifications of which paleontology teaches. It is
unnecessary to state that the records we are to study are
buried in the rocks. Everybody knows that the rocks must
be of considerable antiquity ; but when we pass beyond the
age of man, as an inhabitant of the earth, our ideas of time-
relations are necessarily vague ; even for scientific men these
time-relations, both their actual length, in terms of human
standard, and also their relative periods, are not matters of
simple arithmetical calculation.

Theories regarding the Length of Geological Time. — The the-
ories underlying the interpretation of the rocks are far more
important than at first would appear. The common notion,
up to a very few centuries, and in some quarters a few
decades ago, was that the antiquity of the inhabitants, and
the world itself, did not exceed six thousand years. We now

THE HISTORY OF ORGANISMS. 9

believe that the time that has transpired since the first organ-
isms lived upon the earth is measured by millions rather than
by centuries of years, " tens of millions and not millions nor
hundreds of millions," as Mr. Walcott maintains.* Sufficient
evidence appeared to have convinced the earlier geologists of
this statement ; but the evidence is not direct testimony to
the fact of the great antiquity of the earth and its inhabitants.
The fact that fossils are in the solid rocks and that they
are different from the shells or hard parts of any organisms
now living, were facts well known long before the notion
of six thousand years was considered inadequate for the
history of the earth. But the opinion that the fossils
were the remains of organisms of no great antiquity, arid
that they had been buried by some great flood, some
extraordinary cataclysm, was held to be sufficient to explain
the brevity of the assumed time ; and the differences between
the fossils and the living forms were mysteries which were
simply not explained at all until about the beginning of the
present century. The general belief that cataclysms are pos-
sible, that antiquity is the great reservoir for the remarkable,
the extravagant, the unscientific, or the unknown, has been,
and is to some extent now, the common excuse for mistakes
made in interpreting the laws of nature. In the study of
rocks, we need to learn how to use them as measures of the
time-relations of the fossil contents. An analysis of the
classifications which have hitherto been made to express the
chronological relations of rocks will show us what the facts
are, how these facts have been interpreted, and how far these
interpretations are at present satisfactory.

* Vice-Presidential Address, Section E, Am. Assoc. Adv. Sci., 1893.

CHAPTER II.
THE MAKING OF THE GEOLOGICAL TIME-SCALE.

The Heterogeneous Names now in Use. — A critical examina-
tion of the nomenclature applied to the several divisions of
the geological scale reveals a strange mixture of names, the
reason for which is not evident to modern students of the
science. In the list of system-names we find Carboniferous
.and Cretaceous, indicative of mineral characters, associated
with Tertiary and Quaternary, meaning rank in some unde-
fined order of sequence. The presence of these terms is no
less mysterious than the absence of grauwacke and old-red
sandstone, and primary and secondary, which were originally
included. Triassic is the name of another system and records
the threefold division of the system of rocks to which it was
applied ; and Devonian, the name of another, reminds us of
the county in England in which its rocks were first named.
Observing these things, one is tempted to call in question the
reliability of a systematic classification so heterogeneously
compounded.

Importance of a Systematic Classification. — Although the older
living geologists can remember back almost to the beginnings
of the science, those who now are beginning their study of
geology may find profit in examining the foundation prin-
ciples, and the systems which have been devised and have
led to the construction and belief in the present classification
— a classification the adoption and unification of which has
been thought worthy of the organization and continuance of
an international Congress of Geologists. It is needless to call
attention to the necessity of some systematic classification of
geological formations, but as a foundation for the scientific
study of the history of organisms there is need of a time-scale
running back into the past, the degree of accuracy of which

THE MAKING OF THE GEOLOGICAL TIME-SCALE. II

is known as well as the extent of its unreliability. In early
attempts to classify rocks the chronological element of the
scale was not considered, but by degrees the classification has
passed from a classification of rocks to a classification of
periods of time.

Ancient Notions of Geology. — The ancients in many respects
were keen observers ; they knew much about plants, animals,
physical and chemical phenomena, and astronomy. But,
with all their learning, there appears to have been no concep-
tion formed of an ancient history of the globe and its inhab-
itants prior to the earlier centuries of the Christian era. One
of the first geological phenomena to become generalized into
a theory was that of the formation of mountains by earth-
quakes, as cited by Avicenus in the tenth century. The
gradual change of relative level of land and sea, as seen in
the encroaching of the sea or the departure of sea from the
shore, gave rise to speculations regarding the great length of
time required for the lifting of the whole land by that means.
In the sixteenth century, Lyell reminds us, attention was
drawn to the meaning of fossils, and dispute arose as to their
nature. Leonardo da Vinci doubted the then current belief
that the stars were the cause of the fossil shells and pebbles
on the mountain-sides, and advanced the idea " that the mud
of rivers has covered and penetrated into the interior of fossil
shells at the time when these were still at the bottom of the
sea near the coast."*

Beginnings of a Scientific System of Classification. — By degrees,
as Lyell has described in such fascinating manner, one after
another the foundation principles were announced, discussed,
controverted, and finally, by their intrinsic truth, became estab-
lished. But it was not till nearly the beginning of the present
century that enough was known of rocks for the formation of
a general systematic classification of geological formations.
The belief in a limit of six thousand years for the formation
of the world was prevalent. Catastrophe was the universal
resort for explanation of phenomena not then understood.
And for geological purposes the Noachian deluge was an in-

* Lyell's Principles, p. 34.

12 GEOLOGICAL BIOLOGY.

dispensable agent for the scientific explanation of any ex-
traordinary phenomena. For these reasons inquiry did not
reach far into the antiquity of the geological ages. And the
first attempts at classification took little or no account of
actual time-factors in geology.

Lehmann's Classification according to Order of Formation. —
Lehmann * is generally credited with having first proposed a
classification of rocks on the basis of the order of their forma-
tion, as Primitive, Secondary, and a third class, the modern or
superficial rocks made by the deluge or ordinary river action.
Lehmann recognized also a direct relation of origin for the
Secondary from the Primitive rocks, and thus arose the begin-
nings of the geological time-scale. Lehmann described three
originally distinct kinds of rocks, or rock formations. The
volcanic were separated from the others because having no
particular connection with either in origin. The distinction,
however, between Primitive and Secondary was fundamental.
The Primitive was strictly the original, basal rock formed by
crystallization from chemical solution before organisms lived ;
and the Secondary rocks were of secondary origin, made out
of fragments of the older and always lying above them. In
the original classification of Lehmann, Secondary included all
the stratified rocks, as we now describe them, and in the
classifications for some years following Lehmann the term
Secondary was applied, though in a restricted sense.

Cuvier and Brongniart 's and R6bouPs Contributions. — Cuvier
and Brongniart f proposed the name Tertiary for the rocks
classified as Secondary by Lehmann, but lying above what is
now known as the Cretaceous system ; and Quaternary was
used by R£boul $ in 1833 for the rocks of superficial position
and of glacial or fluviatile origin. Thus the nomencla-

* J. G. Lehmann, " Versuch einer Geschichte von Floetzgebirgen, etc.,"
Berlin, 1766 (Kayser), 1756 (Poggendorf). French translation cited by Lyell
"Essaid'un Hist. Nat. des Couches de la Terre," 1759. See Lyell, "Princi-
ples," vol. i. p. 72, and Conybeare and Phillips, "Geology," p. vi and p. xlii.
Johann Gottlob Lehmann died in St. Petersburg, 1767.

f Cuvier and Brongniart, " Descr. Geol. des Environs de Paris, "ed. 2, 1822,
p. 9.

\ Reboul, "La Geologie de la Periode Quaternaire," 8vo, 1833. Morlot,
Bull. Soc. Vaudoise des Sc. Nat., iv. 41, 1854.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 13

ture of Lehmann, which was proposed originally to indicate
the derivation of the Secondary from the Primitive, was
expanded on the basis of stratigraphic succession, and we
observe the anomaly of a retention of two names (Tertiary
and Quaternary), formed on the principle of Lehmann's
terms, but his own terms, as well as his theory as a basis of
classification, entirely discarded.

Werner's Perfection of the Lehmann Classification. — Werner
(1750-1817) elaborated Lehmann's scheme and modified it.
He was the great teacher of geology at Freiburg, Germany,
in 1815, and left his impress upon the geologists of the time,
though he wrote little in the way of systematic exposition of
his theories of classification. He adopted Lehmann's Prim-
itiv Gebirge, but of the Secondary rocks he made a lower
class, which he called transition rocks (Uebergangsgebirge)\
they were stratified, contained none or but few fossils, and
were more or less oblique in position ; these characteristics
were observed in northern Europe, where he studied them.
The remainder of the original Secondary rocks he called
Floctsgebirgc, or flat-lying formations, and these were the
equivalents of Lehmann's Secondary in the classification of
the early part of the century. Later, the Wernerian school
called the formations above the Cretaceous neues Floetzgebirge,
to which, as they were studied in the Paris basin, Cuvier and
Brongniart, in the latter decade of the last century, applied
the name Tertiary, which still remains in the scheme. Wer-
ner called the looser, overlying, unconsolidated rocks ange-
schwempt Gebirge, or alluvial formations, which were after-
wards, as above stated, called Quaternary by Re"boul and
Morlot.

The classification of Lehmann, as perfected by Werner,
was then as follows :

German Names. English Equivalents.

IV. Angeschwempt Gebirge. Alluvial formations.

III. b. Neues Floetzgebirge. Tertiary "

a. Floetzgebirge. Secondary "

II. Uebergangsgebirge. Transition "

I. Urgebirge. Primitive '.'

14 GEOLOGICAL BIOLOGY.

These were the formations which made up the geological
series as then recognized. Volcanic rocks were looked upon
as local formations, and of small account in general classifica-
tion. But they came to be more deeply studied by Werner,
and his notion that trap was of aqueous origin led to much
controversy, and gave chief prominence to his views (the
Neptunian theory) and to that classification of rocks which will
be next considered. The rocks of igneous origin, although
sometimes interstratified with sedimentary rocks, do not enter
into the present geological time-scale, and for the present
purpose further consideration of their classification is unneces-
sary. There has always been a remnant of rocks at the base
of the scale, the consideration of which may be discarded
here, because it is chronologically known only as below those
rocks of which distinct evidence of their relative age is appar-
ent. The name Primitive has been changed to Primary, and
finally to Archaean, a name which was proposed by Dana,*
and is likely to be permanently retained for some of the basal
part of the series.

This first comprehensive classification of rocks may be
called the Lehmann classification. It was based upon a
structural analysis of the rocks in the order of their actual
positions. The nomenclature is applied on the theory of
relative order of formation.

Richard Kirwan and Geology at the Close of the Last Century. —
Richard Kirwan f claimed to be the first author to publish a
general treatise on Geology in the English language. Al-
though the book is written in a decidedly controversial spirit,
the author appears to have had a thorough acquaintance with
the various treatises in French, German, Latin, and English,
in which were expressed contemporaneous opinions regarding
geological science. He was a Fellow of the Royal Societies
of London and Edinburgh, member of the Royal Irish Acad-
emy, and of Academies in Stockholm, Upsala, Berlin, Man-
chester, and Philadelphia, and Inspector General of his
majesty's mines in the kingdom of Ireland. It is probable,

* Amer. Jour. Sci., vm. 213, 1874.
f "Geological Essays," London, 1799.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 1 5

therefore, that he presents a fair idea of the opinions which
underlay the Lehmann classification. According to Kirwan's
book the rocks were originally in a soft or liquid state, the
centre of the earth was supposed to be hollow, or the whole
earth was a solid exterior crust with immense empty caverns
within. The materials of the earth were then in a state of
fusion or solution, and by condensation, as time progressed,
the solids were crystallized out and deposited from the chaotic
fluid. The water contracted its surface and lowered upon it
by sinking into the interior cavities. With the deposition of
the primitive rocks from the chaotic fluid, the water became
purer. Mountains were conceived of as the local points of
original crystallization which drew to them, in the process,
the minerals from the general fluid. As the waters gradually
withdrew by evaporation and sinking into the interior caverns,
they became clarified and capable of supporting organic life.

Kirwan says:* "The level of the ancient ocean being
lowered to the height of 8500 or 9000 feet, then, and not
before, it began to be peopled with fish." (Under the name
fish he included shell-fish and all other petrifactions.) The
plains were formed of depositions from the water of argilla-
ceous, siliceous, and ferruginous particles, mingled with those
derived by erosion from the already protruding mountains.
All the rocks above the height mentioned, he observed, quot-
ing from testimony of numerous travellers, "are lacking in fos-
sils ; even the limestones are crystalline or ' primitive ' lime-
stones and marbles." These observations were cited in refuta-
tion of Button's " error " in claiming that all limestones were
derived from comminuted shells. According to some author-
ities, primitive mountains should include rocks of even less
height than 8000 feet, and the occasional presence of fossils
at a greater elevation was by them accounted for by their
transference to that elevation by the deluge.

Geological Mountains (Gebirge) and Formations. — This account
of Kirwan's will suggest the way by which the rock formation
first came to be called " Gebirge " or mountains. Rocks were
supposed to lie as they were originally formed, and thus in

* " Geological Essays," p. 26.

l6 GEOLOGICAL BIOLOGY.

classifying rocks the larger aggregates were naturally moun-
tain masses. As the conception of movements in the earth's
crust with folding and displacement came into the science,
the idea of classification and grouping of rocks was retained,
but that their grouping was based upon present massing above
the surface as mountains ceased to be accepted as truth. In
the German language the term " Gebirge" was retained, and
apparently with restricted meaning. Kirwan apparently trans-
lated the term directly into English as mountains. Formation,
however, took the place of mountain, as applied to rock classi-
fication, in the early part of the century.

The Formation of Sedimentary Rocks according to Werner and
his School. — In the following cut is illustrated the conception
of the Wernerian school of the mode of formation of the
rocks and the reason for the relative positions each kind occu-
pies. In the figure a a' a is the supposed fundamental basin
of primitive rocks crystallized out from the chaotic fluid as
described above by Lehmann, and these rocks were hence
named Urgebirge, or Primitive rocks. When the ocean

a

FIG. i. — Diagram expressing the supposed mode of formation of the several formations {Gebirge)
according to the Wernerians. (After Conybeare & Phillips.)

level had sunk to b b, deposition began and went on till the
rocks b b' b' b' b were formed, the Uebergangsgebirge or tran-
sition rocks of Werner, whose position is oblique because of
conformity to the sides of the original mountains as they
stood in the original seas. As the surface of the ocean con-
tinued to sink, the deposits were accumulated lower and
lower down on the mountain-sides, and more and more
nearly horizontal, c c'c'c and d d'd, which represent the
Floetzgebirge or flat-lying rocks; finally above the neues

7 'HE MAKING OF THE GEOLOGICAL TIME-SCALE. 1 7

Floetzgebirge (dd'cT) were deposited the loose-lying gravels
and soils of the valleys, e, of the rivers (alluvial) and of their
flood-plains (diluvial).*

Lehmann's classification, in so far as it goes, expressed
established facts of nature. There are Primitive, Secondary,
Tertiary, and Quaternary formations, but the theory that
they may be defined and determined by physical structure
and present relative position is only approximately true.
All crystalline rocks are not primitive, all the secondary rocks
are not merely consolidated fragments of primitive rocks.
Some of them are fully metamorphosed. All Tertiary rocks
are not unconsolidated, as the Tertiaries of California illus-
trate, and we now know that altitude above the sea, or rela-
tive position of the various formations, is by no means
uniform and forms no criterion for their determination.

Werner's Classification of Rocks by their Mineral Characters.
— The next important advance in the classification of rocks
was started by Werner and his pupils. It was a classification
based upon the mineral constitution of the rocks. As the
study of geology advanced Lehmann's classification was found
difficult to apply with precision, and it was found to be un-
natural in that rocks of apparently similar kind were dis-
sociated, while rocks of unlike character were brought into
the same class. And the mineral character and composition
of rocks was found to be an accurate means of defining them.
As the mineral characters became clearly understood, the
rock masses received their names from the chief minerals in
them, and finally the mineral nomenclature entirely super-
seded the nomenclature of Lehmann, and a second classifica-
tion arose in which the theory of the original order of forma-
tion of the rocks gave place to the actual sequence of mineral
aggregates, one after another, in examined sections of the
earth's crust. In this study of minerals Werner was a con-
spicuous leader, and the classifications at the beginning of
the present century were mainly his or adaptations of them.

* W. D. Conybeare and William Phillips, " Outline of the Geology of England
and Wales, with an introductory compendium cf the general principles of that
science, and comparative views of the structure of foreign countries," Part I.
p. xix.

1 8 GEOLOGICAL BIOLOGY.

Conybeare and Phillips' s Perfection of the Weraerian System.
— The form which the geological scale assumed in English
geological systems is seen typically in Conybeare and Phillips's
Geology of England and Wales (1822). Arranged in order
from above downwards, it is as follows :

I. Superior order. (Neues Floetzgebirge of Werner.)
II. Supermedial order. (Floetzegebirge of Werner.)

(1) Chalk formation.

(2) Ferruginous sands.

(3) Oolitic system or series.

/ N ( Red marie or New Red sandstone.

{ Newer Magnesian or conglomerate limestone.
III. Medial, or Carboniferous order.

(1) Coal-measures.

(2) Millstone, grit and shales.

(3) Mountain limestone.

(4) Old Red sandstone.

De la Beche. — De la Beche* carried out the system more
completely, calling the first, or superior order, Supercretaceous
group, and applying the terms Cretaceous, Oolitic, and Red
sandstone to three groups into which he divided the second
order, and giving the third the name Carboniferous group.
Below these he recognized Werner's Grauwacke group, for
what was the lower part of the original Uebergangsgebirge of
his earlier classification, and below this were the inferior
stratified or non-fossiliferous rocks, and the unstratified rocks.
All of the names, it will be observed, are names indicative
of mineral characters.

Maclure's Application of the System to American Rocks. — If
we turn back to the year 1817 we find the same Wernerian
system applied to the classification of North American rocks
by William Mackire.f The author writes : " Necessity dic-
tates the adoption of some system so far as respects the clas-
sification and arrangement of names. The Wernerian seems
to be the most suitable, first, because it is the most perfect
and extensive in its general outlines; and secondly, the

* "A Geological Manual," 3d edition, 1833.

f "Observations on the Geology of the United States of America," Phila-
delphia, 1817.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 19

nature and relative situation of the minerals in the United
States, whilst they are certainly the most extensive of any
field yet examined, may perhaps be found the most correct
elucidation of the general accuracy of that theory, so far as
respects the relative position of the different series of rocks." *
The classification there set forth is as follows (in the order
from below upwards) :

Class I. Primitive rocks.

Class II. Transition rocks — including (i) transition lime-
stone, (2) transition trap, (3) greywacke, (4)
transition flinty slate, (5) transition gypsum.
Class III. Floetz or secondary rocks — including (i) old red
sandstone, (2) 1st floetz limestone, (3) 1st
floetz gypsum, (4) 2d variegated sandstone,
(5) 2d floetz gypsum, (6) 2d floetz limestone,
(7) third floetz sandstone, (8) rock-salt for-
mation, (9) chalk formation, (10) floetz-trap
formation, (u) independent coal formation,
(12) newest floetz-trap formation.

Class IV. Alluvial rocks — including (i) peat, (2) sand and
gravel, (3) loam, (4) bog iron ore, (5) nagel
fluh, (6) calc tuff, (7) calc sinter.

Notice that in this classification the "coal formation" is
placed near the top of the secondary rocks, the "rock-salt
formation " near its middle, and the " old red sandstone " at
its base. Later investigations did not confirm Maclure's
opinion of the accuracy 01: Werner's system as applied to
American rocks.

Amos Eaton's Classification of the New York Rocks. f — Amos
Eaton's classification of the New York rocks is an elaboration
of the same system .

Principles involved in the Wernerian System of Classifica-
tion.— In each of these classifications, except in *a few cases
of the retention of distinctions based upon the structural anal-
ysis, the whole nomenclature and classification is based upon
mineralogical composition of the rocks. In the succeeding
progress of the science a great part of the nomenclature has
been replaced by other names composed on a different prin-

* "Observations, etc.," p. 28.

f As exhibited in his " Geological and Agricultural Survey of the district
adjoining the Erie Canal in the State of New York," Albany, 1824.

20 GEOLOGICAL BIOLOGY.

ciple, but many of the divisions here recorded are still re-
tained. This latter fact we may interpret to mean that dis-
tinctions based upon mineral or lithological characters are of
some real and permanent value in geological classification.
The history of development of this system from the first, or
Lehmann's system, shows that the linear order of the series
of formations in the list is based on the conception of a time-
scale and a natural order of succession of the several forma-
tions. The Wernerian classification in this respect was a
correct one for the rocks in Northern Germany for which it
was constructed. The English scale expressed the facts of
sequence, so far as known, for the English rocks, but the
attempt to fit either of them to the facts in North America
emphasized their imperfection. The fundamental error in
the Wernerian system was the assumption that the scale of
Northern Germany was a universal scale, or, expressed in
general terms, that the mineralogical constitution of a rock
bears some necessary relation to its place in the stratigraph-
ical series.

Fossils substituted for Minerals in classifying Stratified
Rocks. — The next step of progress in making the geological
time-scale arose from the study of fossils. Fossils had been
observed and recognized as organic remains for centuries
before Lehmann and Cuvier. Lehmann, and he not the first,
observed that Primitive rocks did not contain fossils, while
Secondary rocks contained some, and what are now called
Tertiary rocks contained them abundantly. But it was not
until fossils were closely studied, their characters examined,
and the species compared and classified that their importance
was realized.

Cuvier and Brougniart. — Cuvier and Brongniart are gener-
ally credited with being the first to establish the scientific
importance of fossils.* In 1796 Cuvier had called attention
to the fact that elephant bones discovered by him in the
Paris basin were different from the bones of living species.
In thus drawing a distinction between living and extinct
animals, as implying present and past groups of living beings,
the foundation was laid, not only of Palaeontology, but of the

* "On the Mineral Geography and Organic Remains of the Neighborhood of
Paris," 1808.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 21

whole field of investigation into the history and evolution of
organisms. Cuvier and Brongniart, applying their methods
of analysis to the rocks of the Paris basin, succeeded in clas-
sifying them into strata, and in defining the separate strati-
graphical divisions in terms of the contained fossils. The
Paris basin rocks, being found to lie above the Cretaceous
rocks of France and England which represent the top mem-
ber of the secondary formation of the Lehmann classification,
were named Tertiary to indicate their geological importance
and their relative position in the geological scale. These
naturalists did not, however, perfect the geological classifica-
tion which their biological studies suggested.

William Smith and Lyell — William Smith in England * em-
phasized the value of fossils as means of identifying strata in
different regions, and others had some part in the elaboration
of the principle involved, but Lyell, more than any one else,
perfected the scheme of classification of geological formations
on the basis of their fossil contents.

Lyell's Classification of the Tertiary into Eocene, Miocene, and
Pliocene. — The first attempt to use fossils as the fundamental
basis of a classification of geological formations was made by
Lyell in the classification of the Tertiaries of England. In
the second edition of his " Elements of Geology," published
in 1841, we find him saying: "When engaged, in 1828, in
preparing my work on the Principles of Geology, I conceived
the idea of classing the whole series of Tertiary strata in four
groups, and endeavoring to find characters for each, expressive
of their different degrees of affinity to the living fauna." f A
mathematical comparison was made between the proportion-
ate numbers of recent and of extinct species in the several
divisions of the Tertiary rocks of England. The result is
given in the following table : %

Period.

L-li«y. R^Spede,

Number of Fossils
compared.

Post- Pliocene,

Freshwater. Thames Valley,

99-100

40

Newer Pliocene,

Marine Strata ne^r Glasgow,

85- 90

1 60

Older Pliocene,

Norwich Crag,

60- 70

III

Miocene,

Suffolk, red and coralline Crag,

20- 30

450

Eocene,

London and Hampshire,

I- 2

400

"Tabular View," 1790, and in unpublished maps and sections of the first
and second decades of this century.

f p. 280. \ Copied from his " Elements," 2d ed., vol. I. p. 284.

22 ^GEOLOGICAL BIOLOGY.

In the nomenclature here proposed Eocene is derived from
the Greek ^S", dawn, and xaivos, recent; Miocene from
jjieiov xairos, less recent ; Pliocene from n\eiov xaivos, more
recent ; and the definite meaning of the nomenclature and the
classification is to signify that the strata called Eocene contain
the first traces of the fauna now living, the Miocene strata
a small proportion of the living species, the Pliocene and
Post-Pliocene more and still more of the living types, and
that the whole of the Tertiary is distinguished from the
Secondary and all older beds by containing some representa-
tives of the faunas now living.

In this earliest attempt to estimate time-relations by bio-
logical data, Lyell, like his contemporaries, considered species
to be sharply defined natural groups, and therefore it was
that the relations between a fossil fauna and its recent repre-
sentatives could be expressed in mathematical terms, indicat-
ing the number of identical species. The principle underly-
ing the classification, however, was of a deeper nature, and
concerned the orderly succession of faunas and floras in time.
Extension of the Lyellian System by Forbes, Sedgwick, and
Murchison. — From the application of this method of time-
analysis to the Tertiary beds, it was extended to an analysis
of the whole series of geological formations on the basis of
their organic remains, and the Lyellian classification took the
place of the older Lehmann classification as follows :
In place of Tertiary we have Cainozoic.
" " Secondary " Mesozoic.
11 *' Transition " Paleozoic.
« « « primitive " Azoic.

This latter classification and nomenclature was gradually
built up, and mainly by English geologists, as the Lehmann
and Wernerian classification was largely elaborated by
German and French geologists.

Edward Forbes proposed to divide the known faunas and
floras into two great groups, Neozoic (modern) and Palaeozoic
(ancient). The two terms Palaeozoic and Protozoic were pro-
posed about the same time. Palaeozoic by Sedgwick, for the
formations known to be fossiliferous, extending from his
lower Cambrian upwards to include Murchison's Silurian sys-

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 2$

tern, and Protozoic was a provisional name proposed for pre-
Cambrian rocks which might be found to contain fossils.*

In his " Silurian System," Murchison proposed Protozoic in
the following words: " For this purpose I venture to suggest
the term ' Protozoic rocks/ thereby to imply the first or
lowest formations in which animals or vegetables appear." f

Without entering into the delicate question of apportion-
ing the honors due to each of these great English geologists,^:
it may be said that in this early usage of the terms, the dis-
tinction between Protozoic and Palaeozoic was ideal — and in
later developments Paleozoic has been retained for that
lower great division of the scale containing distinct remains
of organisms, with the Cambrian system at the bottom. To
show the connection with the older nomenclature, it may be
noted that Paleozoic is equivalent to Primary fossiliferous,
and in the "Silurian System" Azoic was applied to the
Primitive rocks of the Lehmann system.

Phillips's Scheme. — John Phillips, in 1841, proposed to ex-
tend this method of classification to the whole geological series;
and as his scheme was apparently the first complete classifica-
tion constructed on this basis, it is offered as it appeared in
" Palaeozoic Fossils of Devon and Cornwall." §

Proposed Titles depending on the OrHina v Titl*.

Series of Organic Affinities. lmary 1

{Upper = Pliocene Tertiaries.
Middle = Miocene Tertiaries.
Lower = Eocene Tertiaries.
{Upper '= Cretaceous system.
Middle = Oolitic system.
Lower = New Red formation.

( Magnesianlimestoneformation.

Palaeozoic strata : -<

Upper? ,

( Carboniferous system.

Middle? Eifel and South Devon.

T , Transition strata.

Lower = •< .

Primary strata.

* Sedgwick, Proc. Geol. Soc., vol. n. p. 675, London, 1838.
•f Murchison, "Silurian System," p. n.

\ See American Journal of Science, vol. xxxix. p. 167, 1890.
§ London, 1841, p. 160. See also Penny Cyclopaedia, articles "Geology,"
" Palaeozoic Rocks," " Saliferous System," etc.

|| The terms are founded on the verb Ca'ca or £<WG>, to live ; combined with.
recent ; /icSog, medial or middle ; and itttkaioS, ancient.

GEOLOGICAL BIOLOGY.

Joseph Le Conte proposed Psychozoic, on the same prin-
ciple, for the latest geological period in which man has
appeared.*

Chronological Succession included in Lyell's System. — Lyell
proposed to make, on this basis, a geological time-scale, and
he applied the term Period to each of the several divisions of
the scale. Thus we find in his Geology, f second edition, pub-
lished in 1841, a recognition of the time element in classifi-
cation, without, however, the adoption of the biological
nomenclature. He gives a table " showing the order of
superposition, or chronological succession, of the principal
European groups of fossiliferous rocks." Under the heading
" Periods and Groups" we find the following:

j A. Recent.

IB.
re.

I D.
1 E.

IF.

G.
H.
I.

I. Post-pliocene Period

II. Tertiary Period

III. Secondary Period

IV. Primary Fossiliferous
Period :

K.
L.

M.

N.
O.

P.

Q.

Post-pliocene.

Newer Pliocene.

Older Pliocene.

Miocene.

Eocene.

Cretaceous group.

Wealden group.

Oolite, or Jura Limestone
group.

Lias group.

Trias, or New Red Sandstone
group.

Magnesian Limestone group.

Carboniferous group.

Old Red Sandstone, or De-
vonian group.

Silurian group.

Cambrian group.

Later Lyell adopted the biological nomenclature, and was
prominent among geologists in developing and elaborating the
idea of the successive appearance of new types of organisms
coordinate with the progress of geological time.

Dana's Elaboration of a Geological Time-scale. — Dana was
the first to classify and teach the facts of geology from a purely

* See Le Conte, "Elements of Geology," first edition, New York, 1878.

f Lyell, " Elements of Geology," second edition, London, 1841, vol. n. p. 178.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 2$

historical point of view. In 1856* he wrote: "Geology
is not simply the science of rocks, for rocks are but incidents
in the earth's history, and may or may not have been the
same in distant places. It has a more exalted end — even the
study of the progress of life from its earliest dawn to the ap-
pearance of man ; and instead of saying that fossils are of use
to determine rocks, we should rather say that the rocks are
of use for the display of the succession of fossils. . . . From
the progress of life geological time derives its division into
ages, as has been so beautifully exhibited by Agassiz."

Referring to the nomenclature he used in the classification
of American geological history he speaks of having adopted
for the subdivisions of the Paleozoic the names given by
the New York geologists; but, he adds, "I have varied
from the ordinary use of the terms only in applying them to
the periods and epochs when the rocks were formed, so as to
recognize thereby the historical bearing of geological facts."
The nomenclature proposed by Dana in 1856 is given in the
following table :

I. Silurian Age.
1. Lower Silurian.

( ist epoch. Potsdam sandstone.

I. Potsdam Period. •< 2d " Calciferous sand-

/ rock.

ist epoch. Chazy limestone.

2. Trenton Period. -

2d " Birdseye.
3d " Black River.

4th. " Trenton.

" ist epoch. Utica Shale.

2d " Hudson River Shale

3. Hudson Period.

(Hudson River shale and

Blue limestone of Ohio in

parts of the West).

2. Upper Silurian.

I. Niagara Period, i Ist ePoch; Oneida conglomer-
( ate, etc.

2. Onondaga Period . . ist epoch. Gait limestone, etc.

3. Lower Helderberg Period, etc.

II. Devonian Age.

I. Oriskany Period . . j ISt fPOch' f Oriskany sand'
( stone, etc.

* American Journal of Science, vol. XXII. pp. 305 and 335.

26 GEOLOGICAL BIOLOGY.

2. Upper Helderberg )

Period ° }• ist epoch. Schohane grit, etc.

3. Hamilton Period. ... 1st epoch. Marcellus shales, etc.

4. Chemung Period 1st epoch. Portage, etc.

5. Catskill Period.. . iCatS™ "? sandstone a"d

( shales, etc.

III. Carboniferous Age.

1. Subcarboniferous ) . ^

Period f ist epoch. Conglomerates, etc.

2. Carboniferous Pe- )

riocj }• ist epoch. Millstone grit, etc.

3. Permian Period, etc.

This classification was further elaborated in his manual,
the first edition of which appeared in 1863,* and it has become
the standard classification for American geology. Here we
find the larger divisions, called times : I, Archean ; II, Palae-
ozoic; III, Mesozoic; and IV, Cenozoic times. The Palaeo-
zoic time is classified into ages, viz. : The age of Invertebrates,
the Cambrian and Silurian; the age of Fishes, the Devonian;
the age of Coal Plants, the Carboniferous. The Mesozoic is
called the age of Reptiles. The Cenozoic time includes the
age of mammals and the age of man.f

Each of the ages is subdivided into periods and epochs, in
which the stratigraphical groups and formations form the
basis, and the particular faunas and floras of each constitute
the data of determination for the time-divisions.

The following chart shows the modifications in the nomen-
clature through which the classification now in use has grown
out of the classifications of earlier authors :

* James D. Dana, " Manual of Geology; treating of the principles of the
science, with special reference to American Geological History," ist edition,
1862 ; 2d edition, 1874 ; 3d edition, 1880 ; 4th edition, 1895.

f In the article of 1856 the following periods were named (i.e., Triassic,
Jurassic, Cretaceous, Tertiary, and Post-tertiary), but divisions into epochs
were in this paper proposed only for the latter. The divisions of the Post-
tertiary were the Glacial Epoch, the Laurentian Epoch, and the Terrace Epoch.
Quaternary has been substituted, in the manual for Post-tertiary, and Champ-
lain epoch for Laurentian.

In the last edition (1895) Era has taken the place of Age in the former
editions, a Cambrian Era has been recognized in addition to Lower Silurian,
and Carbonic Era has been substituted for Carboniferous Age ; the name
Carboniferous being applied to the formations included under the terms Coal-
measures and Millstone grit of the early classifications.

1 J 5

7^7^ MAKING OF THE GEOLOGICAL TIME-SCALE. 27

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28 GEOLOGICAL BIOLOGY.

The distinctions upon which the above divisions are based
are primarily stratigraphical, and we have still to seek a time-
classification on a purely biological basis for the whole geo-
logical series.

Biological Classification of Oppel. — One of the earliest at-
tempts at systematic classification upon a purely biological
basis was made by Dr. Oppel in classifying the Jurassic
formations on the basis of the successive Ammonites charac-
terizing the beds.* Oppel divided the lower part of the
Jurassic system (the Lias) into 14 zones or beds, characterized
successively from below upwards by their dominant fossil
forms, chiefly ammonites.

Thus the successive zones were those of: i, Ammonites
planorbis ; 2, A. angulatus ; 3, A. Bucklandi ; 4, Pentacrinus
tuberculatus ; 5, A. obtusus ; 6, A. oxynotus ; 7, A. raricos-
tatus ; 8, A. armatus ; 9, A. Jamesoni; 10, A. ibex ; n, A.
Davczi ; 12, A. margaritatus ; 13, A. spinatus ; 14, Posido-
nomya Bronnii. Later classifications, elaborations or re-
visions of Oppel's system, have been made by Wright, in
1860; Judd, 1875; Tate and Blake, 1876, etc. This method
of classification recognized the principle of temporary con-
tinuance of species and of associated faunas ' and it has been
applied with greater or less success all through the geological
scale of formations for the definition of the lesser divisions.

As early as 1838 the importance of the biological evi-
dence in determining the time-scale was clearly enunciated
by Murchison, who wrote in the introduction to the Silurian
System, " that the zoological contents of rocks, when coupled
with their order of superposition, are the only safe criteria of
their age." \

Geological Terranes and Time-periods Contrasted. — The making
of the geological time-scale has now been traced far enough
to clearly demonstrate the fact that the ordinary classification
of geological formations, as found in our text-books, includes
two distinct series of facts: (i) geological terranes, arranged
stratigraphically and classified by their positions relative to

* A. Oppel, "Die Juraformation, Englands, Frankreichs und des siidwest-
lichen Deutschlands " (1856-1858).
f " The Silurian System." p. Q.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 2$

each other and by their lithological characters ; and (2) chrono-
logical time-periods, which may be locally marked by the
stratigraphical division-planes, but which depend, fundamen-
tally, upon biological evidence for their interpretation and
classification. Gilbert * has concisely expressed the impor-
tant fact of the purely local nature of the division-planes
separating the formations stratigraphically into stages, series,
systems, or groups in the words: " There does not exist a
world-wide system nor a world-wide group, but every system
and every group is local." "The classification developed in
one place is perfectly applicable only there. At a short dis-
tance away some of its beds disappear and others are intro-
duced ; farther on its stages cannot be recognized ; then its
series fail, and finally its systems and its groups."

If we accept the correctness of this statement, it is evi-
dent that geological terranes and the stratigraphical division-
planes by which they are marked, although indicative of
time succession, present nothing in themselves to indicate
the particular place they occupy in a time-scale. Even were
the age of a particular stratum in one section accurately de-
termined by other means, there is no stratigraphical or litho-
logical mark upon the rock stratum by which the correspond-
ing age can be recognized in another section. This is not
meant to imply that it is impossible to trace a stratum or
formation from one section to another in the same general
geological province, for in such case it is a process of tracing
with slight interruption the continuity of the one terrane.
But when we pass from one basin to another the physical
continuity is broken, and the stratigraphy and lithology were
made on a separate basis. Hence we reach the conclusion
that the perfecting of the geological time-scale must be
wrought by the means, primarily, of organic remains. Chro-
nological time-periods in geology are not only recognized by
means of the fossil remains preserved in the strata, but it is
to them chiefly that we must look for the determination and
classification of the rocks on a time basis.

* G. K. Gilbert, "The Work of the International Congress of Geologists."
Proc Am. Assoc. Adv. Sci., August, 1887, vol. xxxvi. p. 191.

3O GEOLOGICAL BIOLOGY.

United States Geological Survey Definitions of Formation and
Period. — This principle is clearly enunciated in the rules
adopted by the United States Geological Survey for the
direction of the Survey.* ''Among the clastic rocks there
shall be recognized two classes or divisions, viz. : structural
divisions and time-divisions." "The structural divisions
shall be the units of cartography, and shall be designated
formations. Their discriminations shall be based upon the
local sequence of rocks, lines of separation being drawn at
points in the stratigraphic column where lithologic characters
change. . . . The time-divisions shall be defined primarily
by palaeontology and secondarily by structure, and they shall
be called periods" (p. 65). We have thus reached the stage
in the making of the geological time-scale at which the ideas
of the geological formation and the geological period have
become thoroughly differentiated. The geological period as
a time-unit is primarily defined by the characters of the fossil
remains in the rock, so that the elaborating further and mak-
ing more precise the geological time-scale must come from a
direct study of the life-history of organisms as recorded in
the stratigraphical formations.

The classification of time-divisions made on this principle
by the United States Geological Survey is expressed in the
Tenth Annual Report as follows :

Period. Letter Symbol. Color used in Mapping.

Neocene N Orange

Eocene E Yellow

Cretaceous K Yellow, Green

Jura-Trias J Blue, Green

Carboniferous C Blue

Devonian D Violet

Silurian S Purple

Cambrian C Pink

Algonkian A Red

English Usage. — The English geologists maintain the dom-
inance of the systems as the basis of classification, and deal
with the geological formations as prime factors, considering
the periods as secondary and as dependent upon the forma-

* Report of the Director in the Tenth Annual Report, 1890, pp. 63-65.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 31

tions. In geological text-books and in other geological liter-
ature of America, when Silurian, Jurassic, or Cretaceous is
used alone, the Silurian, Jurassic, or Cretaceous Period is
meant. In English literature, however, system is generally
understood when not otherwise specified.

Geological Systems the Standard Units of the Time-scale. — The
final result of these attempts to arrange chronologically the
geological formations is found in the standard classification of
the systems. The systems were originally groups of success-
ive rock-formations; their limitation was therefore deter-
mined, in the first place, by the extent of the rocks in the
particular region where they were first defined. Hence the
series of formations constituting an original system is in each
case a standard of reference, and its general application is
accomplished by determining its equivalent formations in
other regions.

The time-periods are the periods represented by these
systems; hence the periods of time-duration receive the
names of the systems which were formed during the periods.
The expression, the Cambrian Period, means the period of
time during which the Cambrian system of rocks was forming,
or the period in which the Cambrian faunas and floras lived.
It is all-important to know what formations make up these
standard systems ; for only as other rocks contain the same
faunas or floras can they be identified as of equivalent age,
and therefore as belonging to the same system. The real
time-indicators are, therefore, the fossils, although the rock-
formations which held the fossils give us the names for the
chief divisions of the time-scale.

THE GEOLOGICAL SYSTEMS.

Cambrian System.— The CAMBRIAN SYSTEM was defined by
Sedgwick, and the name was applied to formations studied in
North Wales. In the original definition of the system (1835),
in a paper by Sedgwick and Murchison,* the extension of the

* " On the Silurian and Cambrian Systems, exhibiting the order in which
the older sedimentary strata succeed each other in England and Wales."
British Assoc., August, 1835.

32 GEOLOGICAL BIOLOGY.

system was too low, including rocks later recognized to be
older than the Cambrian system (the " Lower Cambrian
group " of 1835), and too high, in that the " Upper Cambrian
group " of this first paper was claimed also by Murchison in
his original Silurian system ; and in fact the Upper Cam-
brian of Sedgwick is the same stratigraphically with the
Lower Silurian of Murchison, as at present used. The Cam-
brian system includes the " Middle Cambrian" of the 1835
paper, which is composed of the following formation, viz. :
Longmynd, Harlech, Menevian, Lingula flags, and Tremadoc,
The rocks at first were believed to contain no fossils ; later,
fossils were found, and were more fully elaborated in Bohemia
by Barrande, and defined by him as the " first fauna." In
later correlations, and in other countries this first or primor-
dial fauna of Barrande has been the distinguishing evidence of
the Cambrian period of time.

The Cambrian system in North America includes three
divisions: the earliest, or lowest, the (i) Georgian group, typi-
cally represented in the shales and limestones of that name in
Western Vermont, and containing a fauna characterized by
the presence of the Olenellus, a genus of Trilobites. The
second or middle division is the Acadian group, typically
seen in the form of shales and slates in Eastern Massachusetts,
in New Brunswick, and in Newfoundland, and containing the
Paradoxides fauna, or the fauna with the genus Paradoxides.
The third division is the Potsdam group, and is typically
represented in sandstones about the base of the Adirondack
mountains, and contains the genus Dicellocephalus.

Ordovician System. — The ORDOVICIAN SYSTEM is a name
proposed by Lapworth, in 1879, as a substitute and compro-
mise for the Upper Cambrian of Sedgwick and the Lower
Silurian of Murchison, both of which covered the same inter-
val, and the original usage of which in current geological
literature the geologists of the two schools have, since the
death of the authors, strenuously maintained. The standard
series of rocks are in Wales and Western England, and are the
Arenig, Llandeilo flags, and Bala or Caradoc. The fauna is
the " second fauna " of Barrande, and the standard system in
North America includes the Calciferous group, typically

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 33

represented around the borders of the Adirondacks, the
Chazy, also at the eastern part of the Adirondacks ; and the
Trenton, expressed typically at Trenton Falls and on the
western slopes of the Adirondacks, and extending southwest-
ward.

Silurian System. — The SILURIAN SYSTEM, as now re-
stricted, is the Upper Silurian of Murchison as defined in
1835 to 1838 and later. It includes typically the Mayhill,
Wenlock, and Ludlow formations, as defined in the Silurian
system of Murchison, of Western England. The fauna is
characterized as the " third fauna" of Barrande. It is typi-
cally represented in North America by the Niagara, the
Salina, and the Lower Helderberg groups of New York State.

Devonian System. — The DEVONIAN SYSTEM is a name, also,
first proposed and defined by Sedgwick and Murchison in
1838. The typical rocks were found in North and South
Devonshire, England. The limits of the system, strati-
graphically, were not so definitely fixed as in the previous
cases, the system having been founded originally on the dis-
tinction of the fossils, which by Lonsdale were determined as
constituting a group intermediate to the Carboniferous and
the Silurian faunas. The fossils from which the original de-
termination was made were from the limestones of Plymouth
and Torbay, South Devonshire. Later investigations have
shown them to be of Mesodevonian age. The Devonian sys-
tem was originally intended to include the rock series from
the top of the Silurian to the base of the Carboniferous.
The lowest member of the system in South Devonshire is the
Foreland sandstone, and the highest are the Pilton beds,
near Barnstaple, North Devonshire. There are what are
known as Lower, Middle, and Upper Devonian faunas, and
recent investigations have led certain European geologists*
to set the lower limit of the Devonian system low enough to
include part of what in North America are called Lower
Helderberg group faunas. In America the standard Devo-
nian system comprises the Oriskany sandstone, the Cornifer-

* See Kayser, " Die Fauna der altesten Devon-Ablagerungen des Harzes "
(Berlin. 1878), and papers on the Hercynian question.

34 GEOLOGICAL BIOLOGY.

ous, the Hamilton, and the Chemung, including the Catskill
group, all typically represented in New York State.

Carboniferous System. — The CARBONIFEROUS SYSTEM, as
now limited, was first defined by Conybeare in 1822.* In
his original grouping he included with the Coal-measures the
Millstone grit; the Carboniferous or Mountain limestone, and
the Old Red sandstone of England, typically represented in
north England in the Pennine range, and not fully repre-
sented in any other one section in England. In England the
Permian was regarded as a distinct system by Murchison, and
as lying unconformably upon the lower strata ; but the Per-
mian fauna and flora both have closer affinity with those of
the Coal-measures below than with the later Mesozoic types,
and on paleontological grounds the Permian is now classified
as the upper group of the Carboniferous system. In North
America the standard rocks of the Carboniferous system are
the Mississippian series, formerly called Lower or Subcar-
boniferous, of the Mississippi valley, having for its lowest
member the Kinderhook or Chouteau formation, and for its
upper member the Kaskaskia or Chester limestones and
shales. The middle member of the Carboniferous system is
the Coal-measures and underlying conglomerates, typically
represented in Pennsylvania ; but in the western part of the
continent it is not coal-bearing, but consists of massive marine
limestone. The upper member is typically seen overlying
the Coal-measures in Kansas and Nebraska and farther west-
ward and southward ; it contains a marine Permian fauna,
and is represented in Pennsylvania and Virginia by a plant-
bearing series terminating the Coal-measures.

The Post-paleozoic or Appalachian Revolution. — The chrono-
logical division-line between the Carboniferous system and
the Triassic is a very important one, both geologically and
palaeontologically. In America the point is indicated by the
Appalachian revolution. It constitutes the division between
the terranes of the Palaeozoic and the Mesozoic times in the
history of organisms. After the close of the Carboniferous

* Conybeare and Phillips, " Outlines of the Geology of England and Wales,"
London, 1822.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 35

in North America the great part of the eastern half of the
United States was raised permanently above water, and is
therefore, except at its margins, devoid of records of later
marine life. A border of a few hundred miles on the east and
south contains Mesozoic and Cenozoic deposits and their
characteristic fossils ; but the larger part of the areas covered
by Triassic, Jurassic, and Cretaceous deposits are found west
of the 97th meridian, or of a line running from the western
border of Minnesota to the western shore of the Gulf of
Mexico. The disturbances recorded in this elevation of the
quarter of a continent were felt in other parts of the world,
and occasioned great shifting of marine conditions of environ-
ment, causing migration or extinction of great numbers of
organisms, and opening up new regions with new conditions
to which the organisms of the Mesozoic were rapidly ad-
justed.

Triassic System. — The TRIASSIC SYSTEM was first defined
by Alberti in 1834.* The rocks which were grouped together
to constitute the Trias system are the Bunter sandstone,
overlaid by a middle calcareous 'member, the Muschelkalk,
followed by sandy shales, and the Keuper; and they are well
represented in central and southern Germany.

This system is poorly represented in eastern America — so
poorly that the United States Geological Survey proposes to
join the Trias and Jurassic of America into a common group,
calling it the Jura-Trias system, distinguished by a common
continuous fauna and flora. In the Rocky Mountain region
there are thick deposits, mainly sandstones, with few fossils,
which are intermediate between the Permian, or closing for-
mation of the Paleozoic age, and the Cretaceous formations ;
but it is difficult to determine in particular cases whether the
rocks should be classed with the European Triassic or Jurassic
systems. In California Dillerf has recently described fossi-
liferous Triassic terranes containing typical Triassic marine
faunas.

* " Beitrag zu einer Monographic des Bunter sandsteines, Muschelkalkes,
und Keupers," Stuttgart u. Tubingen, 1834.

f " Geology of the Taylorville Region of California." Bull. Geol. Soc.
Am., vol. HI. pp. 369-394, July, 1892.

36 GEOLOGICAL BIOLOGY.

Jurassic System. — The JURASSIC SYSTEM was originally
applied by Brongniart, in 1829,* to the Jura limestone of the
Jura Mountains and the associated formations — the Lias, thin,
regular-bedded argillaceous limestones, known in many places
in Europe and England, and the Oolitic rocks, or Oolite, so
named on account of its resemblance to the roe of fish (oon,
egg, and lithos, stone ; roe-stone).

This Jurassic system is rich in its Ammonite faunas in
Europe. In America the system is not characteristically
represented, but in Texas, in the Rocky Mountain area and
in California are seen typical exhibitions of the Triassic and
Jurassic systems of the American type.

Cretaceous System. — The CRETACEOUS SYSTEM is an expan-
sion of the " Chalk formation " to include the system of rocks
associated with it ; the Chalk of the shores of the British
Channel was described in literature under that name before it
became established as the name of a geological division. The
Dutch geologist J. J. d'Omalius d'Halloy described as ter-
rain cretace' the third division in his geological classification
of the secondary strata of northern Europe, in an essay on
the geological map of Holland, etc., in i822.f

His classification of the secondary rocks was as follows:
I, terrains peneens (todte-liegende of the Germans); 2, ter-
rains ammoneens (the Jurassic); 3, terrains cretace'; 4, masto-
zootique (the Tertiary of others) ; and 5, pyroide (for the rocks
having igneous origin). Fitton, in 1824,^: grouped together
into a continuous series the rocks which were afterward recog-
nized as constituting the typical Cretaceous system, but he
did not name them at the time. The typical Cretaceous rocks
of England and Europe were the Wealden, the lower Green-
sand, the Gault, the upper Greensand, terminating with the
Chalk. In North America our standard has been determined
by comparison of contained fossils, and the typical Cretaceous

* " Tableau des Terrains qui composent l'6corce du globe," p. 221.

f " Observations sur un essai de carte geologique des Pays-Bas, de la
France, et de quelques contrees voisines": Memoires, etc., Namur, 1828, p. 23.

\ " Inquiries respecting the geological relations of the beds between the
Chalk and the Purbeck limestone in the southeast of England": Ann. of
Phil. His., vol. vn. p. 365, 1824.

THE MAKING OF THE GEOLOGICAL TIME-SCALE. 3/

system is found along the Atlantic and Gulf borders in beds
of clay, sands, green sands, and chalky limestone containing
typical Cretaceous fossils.

Tertiary System — The TERTIARY SYSTEM was first defined
and the name specifically applied by Cuvier and Brongniart to
rocks of the Paris Basin.* The system so named included
the fossiliferous rocks lying superior to the Cretaceous system,
the upper member of the Secondary. As now understood, it
includes the three divisions, defined by their fossils, and
named by Lyell, Eocene, Miocenr, and Pliocene, typically
exhibited in France and England in several places. The three
divisions of the typical Tertiary of eastern North America are
the Alabama, Yorktown, and Sumpter formations. For the
West the series consists of Laramie, Wahsatch, Green River,
Bridger, Uinta, White River, and Niobrara beds, some of
which are fresh- water deposits, f

Quaternary System. — QUATERNARY SYSTEM was applied by
Morlot, in 1854, to the rocks or geological material lying
above the typical Tertiary deposits, f But the term " Quar-
ternaire " was used by Reboul as early as i833.§ The system
is divided into the Pleistocene and Recent, and is distinguished
by the presence of traces of man. In North America the
deposits so named are classified as Glacial, or drift, from the
evidence of glacial agency in their arrangement, Champlain, or
Diluvial and Alluvial, or materials distributed by waters of
melting glaciers and by river action, and the River Terrace, or
Recent Period.

Fossils the Means by which the Age of a System is Determined.
— These systems, although actually arbitrary groupings of the
stratified rocks of particular regions, have come into common
use as the primary divisions of the rocks whenever chrono-
logical sequence is considered. In describing any newly dis-
covered fossiliferous strata in any part of the earth, the first
step to be taken, in giving them a scientific definition, is to

* " Descriptions g6ologiques des environs de Paris," 2d ed., 1822, p. 9.
f See for details and nomenclature of the subdivisions of the systems, in
this and other cases, Dana's " Manual of Geology," 4th ed., 1895.
\ Bull, Soc. Vaudoise de Sci. Nat., iv. p. 41.
§ " La Geologic de la Periode Quarternaire," 1833.

3 GEOLOGICAL BIOLOGY.

assign them to one or other of these systems upon evidence
of the fossils found in them. The character of the rocks
themselves, their composition, or their mineral contents have
nothing to do with settling the question as to the particular
system to which the new rocks belong. The fossils alone are
the means of correlation. It thus happens that each geolog-
ical system, which is a local aggregation of strata, of particular
composition, structure, and thickness, becomes a standard ot
chronological period and duration by virtue of the fossils
which it contains. The fossils are characteristic of some
particular period in the history of organisms, and the strata
containing them were deposited during that period.

CHAPTER III.

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE AND
THEIR TIME-VALUES.

The Systems and Geological Revolutions. — The systems,
although they are arbitrarily limited and classified, rep-
resent certain grand events in the history of the earth.
Without explaining how the series of stratified rocks came
to be divided into these particular ten systems, it may be said
that their retention as the great units of geological classifica-
tion and nomenclature is mainly due to the relatively sharp
boundaries which each system exhibits in its typical locality.
The systems thus serve as known and definite standards of com-
parison in the construction of the time-scale, as the dominance
of nations, or the dominance of dynasties, in each case serves
as a time-standard for the discussion of ancient human history.
As the period of each dynasty in ancient history is marked by
continuity in the successive steps of progress of the country,
of the acts of the people and of the forms of government,
and the change of dynasties is marked by a breaking of that
continuity, by revolutions and readjustment of affairs, so in
geological history the grand systems represent periods of con-
tinuity of deposition for the regions in which they were
formed, separated from one another by grand revolutions
which interrupted the regularity of deposition, and disturbed,
by folding, faulting, and sometimes by metamorphosing them,
the older strata upon which the succeeding strata rest uncon-
formably and constitute the beginnings of a new system.

Geological Revolutions Local, Not Universal. — Geological rev-
olutions were not universal for the whole earth ; from which
it results that these typical systems and their classification
are not equally applicable to the geological formations of all

39

4° GEOLOGICAL BIOLOGY.

r lands. It is important also to note that the geological revo-
; lution was not a sudden catastrophe, but the culmination of
I .slowly progressing disturbances bringing the surface of the
' region concerned ultimately above the level of the ocean, the
ocean-level being a pivotal point in geological rock formation.
The area whose surface is below the sea-level may be accu-
mulating deposits and making rocks, but so soon as the region
is lifted above the surface it becomes a region of erosion,
destruction, and degradation. Whenever, therefore, in the
oscillations of level, any particular part of a continental mass
of the earth's crust passes permanently or for a long geologi-
cal period of time above the sea-level, a great event in geo-
logical history has culminated. In case the elevation is only
temporary the event is marked by unconformity, or a break
in the continuity of the formations; when it is permanent,
the geological record for that region ceases, except so far as
fresh-water deposits in lakes may continue independent rec-
ords. Hence it is that these periods of revolution are of such
importance in the history of the continents, and constitute
the most satisfactory marks for the primary classification of
geological history.

Revolution Expressed by Unconformity and Disturbance of
Strata. — The natural geological system is theoretically a con-
tinuous series of conformable strata. A geological revolution
is expressed by unconformity and more or less disturbance
and displacement of the strata from their original position.
The grander revolutions are also recorded in the permanent
elevation of mountain masses or extensive continental areas
above the level of the sea, and thus out of the reach of later
strata accumulation.

Appalachian Revolution. — The most widely recognized revo-
lution in geological time, since the close of the Archaean, sep-
arates the Carboniferous from the Triassic system. In Amer-
ican classification, following Dana's usage, it may be called
the Appalachian revolution. It terminated the series of for-
mations which, with only minor interruptions, had been
continuously accumulating in the Appalachian basin from the
early Cambrian period onward. It left above the sea-level
not only all the Appalachian region, but the great part of the

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 4!

eastern half of the continent, extending westward beyond the
Mississippi River to a line running irregularly from Texas to
western Minnesota. This revolution produced the Alle-
gheny Mountains and those flexings and faultings which are
still recognized in the line of lesser ridges extending from
Pennsylvania to Georgia. In England, northern Europe,
and northern Asia like disturbances took place at the same
general period of time. In Australia, southern Africa, and
South America the indications are that the revolution was not
so extensive, if it took place at all at the same time. The
probabilities are that while it was almost universal for the
northern hemisphere, it was mainly confined to this half of the
earth. The Appalachian revolution was not limited to a brief
geological period, but, beginning near the close of the coal
measures of the east, it did not become effective in the region
of Kansas and Nebraska till the close of the Permian. The
wide extent of the disturbance of strata and, consequently,
of records at this point in the time-scale has led to making here
a primary dividing-point of the scale, marking off Paleozoic
from the following Mesozoic time. Several lesser, more or
less local, revolutions have left their permanent marks in the
grander structure of the rocks or in conspicuous geographical
features of the restricted region of the continental area.

Although revolutions of the same kind, and perhaps pro-
ducing greater effects upon the final condition of the crust,
may have occurred previous to the deposition of the Cambrian
system, as time- marks only those revolutions which occurred
after fossils appeared in the rocks, and in stratified rocks, are
here noticed ; and their names and the particular events re-
corded are those affecting the history of the North American
continent.

laconic Bevolution. — The first of these was the Taconic
revolution, which separated the (Lower Silurian) Ordovician
from the (Upper Silurian) Silurian, in the eastern part of
North America. The elevation, disturbance, and metamor-
phism of the rocks of the Taconic mountain range along
western New England, and extending from Quebec on the
north to New Jersey, stand forth as monuments of this event.
The Cincinnati uplift, extending from the western part of

42 GEOLOGICAL BIOLOGY.

Ontario, Canada, into Tennessee, marks a contemporaneous
disturbance. Evidence of the same revolution is seen in un-
conformability between Ordovician and Silurian rocks in Nova
Scotia and New Brunswick. The revolution is not sharply
distinguishable in the rocks of the more southern or western
regions.

Acadian Revolution. — The second of these lesser revolutions
is expressed most sharply in elevation and unconformity ter-
minating the Devonian formations of Maine, New Brunswick,
and Nova Scotia, and may therefore be called the Acadian
revolution. In the continental interior it may be indicated
by the remarkable thinning out of the Devonian rocks toward
the southwestward. In Tennessee, Alabama, and Arkansas
they are represented by a thin sheet of black shale, a few feet
thick, or by but little more than a line of separation between
the rocks of the Silurian below and the Carboniferous beds
resting scarcely unconformably upon them. This seems to
indicate an elevation of the region still further south, toward
the close of the Devonian, sufficient to produce extensive
erosion, uncovering the lower Silurian rocks, which were
again depressed to receive the marine deposits of the early
Carboniferous period upon their eroded surfaces.

Appalachian Revolution. — The Appalachian revolution
closed the Paleozoic time and left the great part of the east-
ern half of the continent above sea-level. It forms the
natural interval between the Carboniferous and the overlying
system, whatever that may be. Its characteristics have
already been described (p. 40).

Palisade Revolution. — A revolution which affected the
rocks along the eastern border of the continent during or
closing the period in which the Triassic sandstones were
being deposited may be called the Palisade revolution. It is
expressed by the trap ridges in the Connecticut valley, the
Palisades and other similar tracts distributed inside the coast
from Nova Scotia to North Carolina, and by the uptilting
and in some cases faulting of the underlying red sandstone
and shale, and the resulting unconformity with the succeeding
formations. The evidences of the revolution are not widely
extended, nor is the time-relation of the termination of the

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 43

revolution sharply defined, but it is sufficiently so to form a
natural boundary-line separating the Triassic- Jurassic from the
Cretaceous. After this point of time there occurred nothing
in the eastern half of the continent which deserves the name
or rank of a geological revolution, except the glacial revolu-
tion which is defined further on. The western part of the
continent is conspicuous for the late occurrence of its geological
construction, which was chiefly after the Triassic ; along the
western coast the Sierra Nevada revolution marked the same
general interval of time recorded by the Palisade revolution
of the East. These events on the opposite borders of the
continent are alike at least in preceding the Cretaceous and in
terminating the formations which are of Jura-Triassic age.

Rocky Mountain Revolution. — The Rocky Mountain revolu-
tion, which resulted in the elevation and disturbance of all the
rocks in the region of the Rocky Mountains, and extended
from them to the border ranges, is distributed along the time
from the close of the Cretaceous to the Miocene, or possibly
later. It is altogether probable that the actual length of
time taken in elevating, tilting, and disturbing the strata,
after the last marine deposits of the pre-Laramie formations,
which resulted in the permanent adding to the continent of
its western third, was not longer than that consumed in the
various events terminating the Paleozoic and making into
permanent land the great mass of the eastern half of the
continent.*

This Rocky Mountain revolution resembles the Appa-
lachian revolution in extending over and affecting a large
area of the continent, in its general upward-lifting of that
area, which process extended over a long period of time, and
in the .great accumulation of coal or lignite which was asso*
ciated with the gradual emergence of the continental mass
above the sea-level. Another feature in which the two revo-
lutions resemble each other is the wide extent of the disturb-
ances recorded. The elevation of the mountain ranges, from
the Pyrenees eastward to the Himalayas and to the islands

* See further regarding this revolution Dana, "Manual of Geology," 4th
ed.t 1895, p. 875, etc., paragraph on " Post-Mesozoic Revolution: Mountain-
making and its results," also pp. 932-939.

44 GEOLOGICAL BIOLOGY.

beyond, took place chronologically at the same general
period, and that this series of disturbances may have affected
the whole of the northern hemisphere is further suggested by
the occurrence of gigantic erratic blocks of granite in the
midst of Eocene strata in the neighborhood of Vienna and
other places : Vezien * has suggested that an ice age is indi-
cated by them.

The Division-line between the Cretaceous and the Tertiary. —
This Rocky Mountain revolution marks the period of the
second great break in the life of the geological ages. The
Mesozoic time began with the close of the Appalachian rev-
olution, and closed with the elevation of the marine Creta-
ceous beds above ocean-level. In our classification the
division-line between the Cretaceous and the Tertiary was
arbitrarily placed at the top of the chalk formations conspicu-
ously developed on both sides of the British Channel. The
difficulty, which American geologists have found in drawing
the precise line to separate the Mesozoic from the Cenozoic,
[has resulted from the change in the character of life of the
j beds in the western interior from marine to brackish, fresh-
water, and land types. This change was incident to the
Rocky Mountain revolution, which had already begun and
was slowly lifting the whole region while the fresh-water
sediments were being laid down. Several stages may be
marked in this grand revolution, but the facts connected with
them are not so well developed as to serve for general purposes
of classification of the time-scale. The amount of elevation
produced by these epeirogenic movements after the deposit
of the marine Cretaceous, in the western half of our continent,
is estimated to have been not less than 32,000 or 35,000 feet.f
Columbia River Lava Outflow. — At the close of the Miocene
a great outflow of lava in the northwestern part of the United
States took place, and continued with interruptions through
the Tertiary into the Quaternary time. About the Columbia
River, where it cuts through the Cascade range, the basalt is
over three thousand feet thick, and the outflows cover a vast
extent of territory, estimated at 150,000 square miles. This

* Rev. Sci.. vol. xi. p. 171, 1877.

f G. M. Davvson, Am. Jour. Sci., vol. XLIX. p. 463.

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 45

was incident to the vast earth disturbance which raised, to the
amount of at least five thousand feet, a large part of the west-
ern half of the continent. The long line of volcanoes along
the western coast of the two Americas had their origin in the
same general period of time.

Glacial Revolution. — There was, still later, a revolution
which has left little record in the way of disturbance or dis-
cordance of strata, but was of particular importance in life-
history, as it introduced the recent period or the age of man.
It constituted the combination of events marking the glacial
epoch. In general, it consisted geologically of oscillations of
the northern lands, for the northern hemisphere, and was
associated with the accumulation of ice upon the surface and
its continuance as a great ice-sheet for a long period of time.

Erosion of River Canons as Gauges of Time Duration. — Some
of the more definite estimates of the length of geological
time are based upon the rate of erosion or gorge-cutting of
rivers, and the period so measured dates back to the last
uncovering of the river channels, coincident with the northward
withdrawal of the ice-sheet. Standard examples of such esti-
mates of the length of geological time are those made re-
garding the cutting of the Niagara River gorge, the retreat of
the falls of St. Anthony from Fort Snelling to their present
position, and the cutting of the caftons of the Yellowstone
and Colorado rivers. But the unsatisfactory nature of these
estimates is shown by the fact that different authors reach
such divergent results from the same data. The time taken
in the cutting of Niagara gorge is estimated by Upham to
be 6000 to 10,000 years, by Spencer (1894) to be at least
32,000 years.

Continental Value of Revolutions as Time-Breaks in the History
of North America. — The above revolutions are selected, not as
the only revolutions interrupting the regular course of sedi-
mentary formation of stratified rocks, but as chief examples of
such interruptions in the North American column of deposits.
All along the course of geological time there are evidences to
show that there were constant oscillations of the relations be-
tween land- and ocean-level, and at some localities these oscilla-j
tions were passing across the datum-plane of the ocean surface. I

46 GEOLOGICAL BIOLOGY.

Wherever this happened, on one side rocks were forming, and
on the other erosion and degradation were obliterating them
as time-records. The Appalachian and the Rocky Mountain
revolutions constitute the two grander revolutions. The first
closed the Paleozoic life period, the fossils being chiefly
marine until the Devonian, and being associated with marine
forms up to the close of the Carboniferous. The deposits are
distributed across the continent, with local interruptions.
After the Appalachian revolution the eastern half of the con-
tinent, except its Atlantic and Gulf borders, became perma-
nently above the sea-level. The period between the Appa-
lachian and Rocky Mountain revolutions is the period of the
Mesozoic life. In the faunas and floras of this period, land
and fresh-water species take a prominent part. The marine
life is distributed over the western half of the continent and
along a narrow line of formations on the Atlantic and Gulf
borders. After the beginning of the Rocky Mountain revo-
lution, the deposits of marine origin and their faunas were
distributed on the marine borders of the continent as it now
is, and fresh-water and land deposits were accumulated over
the plains and plateaus of the western half (with few excep-
tions) of the continent.

Time-scale and the Geological Revolutions of the American Con-
tinent.— Thus the grander revolutions recorded in the devel-
opment of the American continent break up the geological
time-scale, as expressed in the systems of stratified rocks, into
a few natural sub-divisions, as may be illustrated by the dia-
gram on the opposite page :

Revolutions made Interruptions in the Record. — In the' use of
the time-scale for the study of the history of organisms, the
places marked by the revolutions are those in which are found
the grander interruptions to the continuity of the record.
They may represent periods of great relative magnitude.
They do represent periods of marked change in the faunas
and floras over extensive regions. Between the grander in-
tervals of revolution the records of life-history are relatively
continuous. There were series of successive faunas or even
sub-faunas in which were expressed the general features of
the evolution of life on the globe. The species preserved

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE.

and known present but a very imperfect representation of the
species that were living ; but of those preserved in one forma-
tion there are generally found in the succeeding formations1
representatives of the same or closely allied genera ; so that,
for the kinds of organisms whose remains are best preserved,
the record is fairly continuous for the grander rock-systems • 0<
in terms of the generic, and in some cases of the specific'
characters.

Cenozoic

Mesozoic <

Paleozoic*

Glacial revolution

Rocky Mountain
Revolutions

Palisade revolution

Appalachian «
Acadian "

Taconic •«

? Pre-Cambia'an •*

? Archaean

FIG. 2.— Diagram representing the order of succession from below upward of the formation of the
geological systems in North America and the approximate time at which the grander revolu-
tions eroded and disturbed the already made deposits.

Time-ratios, or the Relative Time-value of the Several Sys-
tems.— While the conditions of deposition for a particular
region remained relatively constant and uniform, the strata
were accumulated in successive beds one upon another; and
thus the thickness of the deposits of the same kind, with pro-
portionate thickness for deposits of different kinds, constitutes
a scale of definite time-value ; a foot of deposit representing
a period of time, and the relative time-separation of two
faunas is represented by the thickness of the strata between
them. It was on this principle that the time-ratios of Dana
were estimated. The maximum thickness of the known

48 GEOLOGICAL BIOLOGY.

strata of each geological system was taken as a means of de-
ducing the relative duration of their formation, as was first
done by S. Houghton. The limestones were assumed to
represent five times the time-value that is represented by the
other sedimentary deposits per foot ; or, in other words, every
foot of limestone was estimated as equivalent to five feet of
other sedimentary deposits in making up the time-ratios.
Dana* estimated the time-ratio for the several geological
periods to be as follows :

Quaternary ................. T)/~

^ , . \ \ Cenozoic I.

Tertiary ..................... f j

Cretaceous .................. I }

Jurassic ..................... i£ V Mesozoic 3^.

Triassic .................... . I )

Carboniferous ...... ........... 2 ^|

Devonian .................... 2

Silurian (Upper) ............. i^ ^ Paleozoic 12^.

Ordovician (Lower Silurian)... 6

Potsdam ... ................ I

Ward's Estimate. — Lester Ward, in the fifth annual report
of the United States Geological Survey, has proposed to ad-
just these proportions as follows :

( Quaternary-Recent
3« -< Miocene-Pliocene
( Eocene
Cretaceous

Jura-Trias

f Permo-Carboniferous ...........

Devonian ....................

Silurian ......................

1^ Cambrian .....................

thus forming nine divisions of equal length.

I

* Dana's "Manual of Geology," 30! edition, 1874. In the latest edition, 1895,
these estimates are revised and the following remark is made : "There is great
doubt over conclusions based on this criterion [i.e., maximum thickness], because
thickness is dependant so generally on a progressing subsidence — no subsidence
giving little thickness, however many the millions of years that may pass. But
as it is the only available method, it is still used," p. 716; also see beyond on
p, 49-

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 49

Corrections and Elements of Uncertainty in these Estimates.
— Since Dana's estimate was published additions have been
made to the known thickness of the Cambrian rocks of North
America, which may lengthen the Cambrian ratio to 5 in the
above table, and duplications of thickness due to confusion
in regard to the Quebec group may reduce the Ordovician
(Lower Silurian) to 5, and the Cretaceous ratio may be some-
what enlarged. The Tertiary estimate in Dana's ratios
assumes the thickness to be of less (-J) time-value because of
the increased rate of deposition due to transportation of
rivers. This and many other factors enter in to complicate
the time- value of thickness of strata ; and it must be granted
that the thickness of the sediments is the prime factor in
determining these time-values of the geological scale.

In the last edition, 1895, of the " Manual," Dana ex-
presses the following opinion: "The evidence at present ob-
tained appears to favor the conclusion that the relative duration
of the Cambrian and Silurian, the Devonian and the Carbon-
iferous eras, corresponds to the ratio 4$- : I : I, or perhaps 4 :
I : I, the ratio hitherto adopted; and for the Paleozoic,
Mesozoic, and Cenozoic, 12 13 : i." However, the condi-
tions of deposition, the fineness or coarseness of the clastic
fragments, the abundance or rarity of supply of materials, and
other variable conditions must be taken into consideration in
an accurate reduction of thickness of strata into length of time.
Errors, also, whose value is almost impossible of estimation,
arise from the intervals between strata, particularly those
where unconformity exists. However, after all these uncer-
tainties are weighed the time-ratios formed on this general
basis are of great importance in studying the history of organ-
isms, and the value of accuracy in the time-scale is a sufficient
reason for calling attention to the points in which greater
accuracy may be attained by further investigation.

Estimates of Actual Length of Time Highly Hypothetical. —
It is doubtful if it is possible with our present knowledge to
reach an estimate, in years or centuries, of the actual length
of geological time which is within 100 or perhaps 200 per
cent of the truth. We may accept Dana's estimate of a.t

SO GEOLOGICAL BIOLOGY.

least 48,000,000 of year 3, or Geikie's of from 100,000,000 to
680,000,000. We find at one extreme the ancient theory of
6000 years, and at the other McGee's possible maximum of
7,000,000,000 years. The rate of accumulation of sediment
over the bottom of the sea may vary between the limits of
one foot in 730 years and one foot in 6800 years, as pointed
out by Geikie, the figures being based upon the estimated
proportion between the annual discharge of sediment in cubic
feet and the area of river basins in square miles, in the case
of the rivers Po and Danube. The estimate of 680,000,000
of years, quoted above, is dependent upon the assumption
that the total thickness (maximum) for the sedimentary de-
posits is not less than 100,000 feet, and that the average rate
of accumulation was not more rapid than that now going on
at the mouth of the Danube, based upon Bischof s determina-
tion of the amount of sediment and matter in solution in the
Danube at Vienna. It may be a query worth considering
whether the estimates based upon the examination of the
amount of suspended and dissolved matter in river water are
not likely to err in the direction of too small amount of mat-
ter by reason of the abnormal precipitation along the course
of the river incident to the presence of salts and acids put into
the river by man. If the rate of the river Po were taken, the
length of time would be 73,000,000 of years instead of
680,000,000.

The actual length of time in years, however, is of less
importance to the geologist than the relative length of time
for each of the eras, and these latter, the time-ratios of Dana,
are deducible from the physical thickness, and size of constit-
uent particles, of sedimentary rocks. Relative thickness is
certainly one of the elements in the determination of the time-
values of the geological formations, and the fields for investi-
gation, along which greater accuracy is to be reached, include
the problems of the rate of accumulation of muds, sands, and
pebble beds, and of the formation of limestones, in relation
to each other and under varying conditions, and the detection
of the marks in the strata recording the conditions incident
to the varying rates of accumulation. In making estimates
of time, as represented by thickness of deposits, there should

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 5 1

also be considered the effects of elevation or depression of
the interior of land masses upon the amount of detritus
carried down to the sea borders, there to be made into sedi-
ments. With all the errors of estimation there is, however,
a real value to the time-ratios of Dana, and to legitimate cor-
rections deduced from study of the same facts, which cannot
be denied ; the principle of time-ratios may be used as a
" working hypothesis" until something better is devised.

Systems the Standard Units of Geological Chronology. — In
the preparation of a universal time-scale for the history of
organisms, systems are the actual facts in nature which are
accepted everywhere as standard units of chronology.

Geological Eras and Times and their Names. — Whatever may
have been the actual length of time occupied in the making of
any one of them, or however much the estimates of the rela-
tive value of each may differ, it is certain that each system in
its particular region represents that particular part of the
geological time-scale during which the fauna and flora whose
remains it contains lived.

This portion of time may be called the life era of the
organisms which make up the fossil fauna and flora of each
system.

The names of these systems may then be applied directly
to the eras, and we thus have in the time-scale ten eras,
viz., the Cambrian era, the Ordovician era, the Silurian era,
the Devonian era, the Carboniferous era, the Triassic era, the
Jurassic era, the Cretaceous era, the Tertiary era and the
Quaternary era, including the present_time. In a time-scale
we know of the eras before the Cambrian only as precanibrian,
i.e., those that were earlier than the Cambrian.

The first five of these eras are classed together as Paleozoic
time, the next three eras are called Mesozoic time, and the
Tertiary and Quaternary constitute Cenozoic time.

Division of the Eras into Periods. — Fossils have been col-
lected and studied with different degrees of precision for the
several eras and in different parts of the world, but, taking
the present stage of knowledge of fossil organisms, the paleon-
tologist is able to distinguish about twenty different successive

52 GEOLOGICAL BIOLOGY.

fauna-floras, or sets of organic species, which can be recog-
nized wherever on the face of the globe they are found.

The time-duration of each of these fauna-floras may be
called a period, and the successive periods thus distinguished
constitute the divisions of the eras which at present are recog-
nizable in each of the continents with greater or less fulness.

Locally greater precision in classification has been attained,
but differences arising from adjustment of the organisms to
conditions of environment, and in living species expressed in
geographical distribution, make it doubtful if we are able to
correlate fossil faunas or floras, the world around, with greater
precision than to recognize the marks of the same period in
each district.

Period a Recognized Division of an Era. — For the present,
also, it seems more likely to conduce to real progress of knowl-
edge to consider the periods to be divisions of the standard
era, rather than absolute units of time-duration, dependent
on their own criteria alone for definition.

In naming them, therefore, the subdivision of the era into
early, middle, and later divisions is preferable to the adoption
of separate distinctive names, and each continent or geologi-
cal province will then be free to adopt its own interpretation of
the local limits and marks of the period in its series of strata.

Standard Periods and their Names. — There are already de-
fined such divisions in the several eras, as follows : in the Cam-
brian era, an early or Eocainbrian period, a middle or Meso-
cambrian period, and a later or Neocambrian period. Walcott
has called the faunas of these periods the Olenellus, the Para-
doxides, and the Dicellocephalus faunas.

In the same way the Ordovician era is made up of the
Eoordovician, or period of the Calciferous formation of
American geology, and a Neoordovician period, or the period
of the Trenton group of North America.

The Silurian era is composed of the Eosilurian period
(Oneida, Medina, Clinton, Niagara), and a Neosilurian period
(Salina and Lower Helderberg).

In the Devonian era are the Eodevonian period (Oriskany,
Corniferous), Mesodevonian period (Hamilton) and Neodevo-
nian (Chemung). In the Carboniferous era are the Eocarbon-

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 53

iferovs period (Mississippian or Subcarboniferous), Meso-
carboniferous period (coal measures), and Eocarboniferous
(Permian).

In the Tr lassie and Jurassic eras no divisions have been
defined which can be recognized in other continents than
where described ; hence the periods are equivalent to the eras,
one period for each.

The division of the Cretaceous into Eocretaceous and
Neocretaceous periods is fairly well recognized in several con-
tinents.

In the Tertiary era Eocene is the first period, and the
Neocene period includes the Miocene and the Pliocene.
And finally the Recent period may be regarded as geologically
the time of the living of the fauna associated with man.

Uce of the Term Epoch in the Time-Scale. — The term Epoch
may be appropriately applied as an expression for the time-
duration of each local formation : thus we may speak of the
epoch of the Iberg limestone of the Hartz ; of the Psammites
of Condroz in France; of the Marwood beds of England; of
the Dominik slates of Russia; of the Chemung of eastern
North America; of the Lime Creek beds of Iowa. These
are each of them well-defined formations in separate regions,
each having a distinct geological structure, thickness, and
relative stratigraphic position, and the period of each is neo-
devonian ; but the faunas, although distinctive and constitut-
ing the means of determining the geological age, are not alike ;
and in time-values it is not possible to say that one is or is
not the exact equivalent of the other. An epoch, upon this
basis, would be a definite division of a period, distinguishable
in the history of the organisms of a restricted region, but not
of universal application.

With the present means of correlation it is impossible to
attain a greater degree of precision, in comparing the fossil
fauna-floras of widely separate regions, than . to distinguish
the periodsby their characteristic species.

A Comparative Time-scale for the Study of the History of Organ-
isms.— The tabulation of these facts and nomenclatures pro-
duces a standard geological time-scale for use in discussing
the history of organisms.

54

GEOLOGICAL BIOLOGY.

A GEOLOGICAL TIME-SCALE, PREPARED FOR THE COMPARATIVE
STUDY OF THE LIFE-HISTORY OF ORGANISMS.

Times. Eras. Periods.

Percentage of the
Whole Scale.

Time-ratios
according to —

II

•o

Walcott.

1!

PALAEOZOIC. MESOZOIC. CENOZOIC.

65# 20* I5<*

f QUATEKXARY.... 5* Recent .. \ "y'gSScene } ..

r XT I Pliocene )

5%

M

i

2

i

3

12

16

5%

%

i

TERTIARY 10* j " 1 Miocene f •
L Eocene

&

i

(( Neocretaceous

5%
&

i

i

5

CRETACEOUS ID* <
( Eocretaceous

JURASSIC 5/t Jurassic

&

JA

i

&

i

r C Neocarboniferous

5*

2

i

12

5$

st

5%

2

•

5*

Eodevonian

5*

&

*K

i

&

&

6

ORDOVICIAN xojf •<

5%

&

i

i

Mesocambrian

&

5%

T.00%

Total

l6Jr£

?

9

>

19
p

PRECAMBRIAN

The periods are taken as the smallest divisions of time which can be uni-
versally recognized, and hence it is assumed that they are units of equal length.
This assumption probably exaggerates the length of the more recent periods.

Importance of a Standard Time-scale. — For the comparative
study of the history of organisms this time-scale may be used
irrespective of estimates of actual length of time represented
*by each period.

The division of the eras into twenty successive periods is

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 55

a scheme which is actually recognized in the classification of
the geological formations throughout the world, where the
criteria of classification are the fossils contained in them.
Geologists dealing with distinct series of strata have named
the individual members of the series differently for different
regions of the earth. Therefore, as the systems are made up
of formations presenting local features, of stratification, of
petrographic composition, of structure, and of thickness, which
are given local names, the fossil fauna-floras representing each
one of the periods are found in formations which have different
names in separate regions.

In using such a scale it becomes necessary to correlate
the faunas of formations having different names. While the
formation names may well be retained, in the discussion of
the time-relations of organisms it is essential to use a uniform
scale of time-divisions expressed in a single series of names :
the scale and names above given supply us with such a
standard time-scale.

Actual Length of Geological Time. — That geological time is
immensely long, as compared with any human standards, all
modern geologists admit ; but as to how much time, in cen-
turies or years, has elapsed since the beginning of the series
of sedimentary rocks, opinions greatly differ. A few facts
may be mentioned to illustrate what is meant by great length
of time in terms of geological work accomplished :

(1) Since the close of the Cretaceous Period the greater part
of the mountain elevation along the southern part of Europe
and extending to the extreme southeastern part of Asia was
accomplished ; and the Himalayas were raised, so that at least
16,000 feet thickness of their mass is composed of marine
strata of Tertiary or earlier era.

(2) The large part of the Rocky Mountain region was
under marine water in the Cretaceous time. Since the
close of the Eocene, or beginning of the Middle Tertiary,
as Captain Button estimates, the region of the Colorado
canons has been elevated approximately 10,000 or 11,000
feet, and 10,000 feet of erosion has taken place. G. M. Daw-
son estimates the total amount of elevation which has taken
place since Cretaceous time, in British Columbia, to have been

56 GEOLOGICAL BIOLOGY.

32,000 to 35,000 feet. There are now canons from 5000 to
6000 feet deep, excavated entirely since the Eocene period.

(3) It is believed that all the lava outflows in the North-
west, which cover 150,000 square miles along the Columbia
River and the neighboring states, and through which the
Columbia has cut a channel, in some cases, from 3000 to
4000 feet deep, were erupted and laid down since Miocene
Tertiary time.

(4) Niagara River gorge, from the falls down to the whirl-
pool, and thence to the cliffs of the lake at Lewiston, it is
estimated, was cut out since the retreat of the glacial ice from
the surface of the northern part of the continent, and this is
believed by many geologists to represent closely the length of
time since man first appeared upon the earth. The gorge is
7 miles long, one fourth of a mile wide below, narrower
above the whirlpool, and varies from 200 to 500 feet in depth.*
The length of time required for its excavation is estimated to
have been from 10,000 to 32,000 years. Taking Dana's gen-
eral estimate of relative length of time, it is seen that the
time since the Cretaceous is not over one sixteenth of the time
from the beginning of the Cambrian, and that the length of
Quaternary time is not over one third that of the Tertiary.
Whatever be the actual length of time taken for these and
similar geological processes, it is evident that the same forces
working at the same rate would require but the extension of
time to include the whole history of the earth.

Data upon which Time-estimates are Made. — Although we
cannot go into full particulars respecting the theories proposed
to determine the time-limits and extent of the geological ages,
a few of the prominent attempts may be cited. The principal
data upon which the theories have been based are as follows :

(i) Physical and Astronomical. — Estimates from the earth's
heat, its rate of cooling, and the radiation of heat into space.
(Kelvin.)

Estimates from influence of tidal friction, and thence to
the length of time since the moon was separated off from the
earth. (Darwin, G. H.)

*See J W. Spencer, "The Duration of Niagara Falls:" Am. Jour. Sci.,
vol. XLVIII. p. 455. December, 1894,

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. $?

From the rate of the sun's loss of its stores of heat. (Tait.)
From other physical data. (Croll and others.)
(2) Geological. — (a) Calculations based upon the estimated
thickness of the geological deposits of the total series of strat-
ified rocks and the estimated rate of accumulation of deposits
along the shores of continents at the present time. (Hough-
ton, Dana, Croll, Wallace, Lyell, Humphreys and Abbott, etc.)
(b) Calculations based upon rate of erosion since the
retreat of the glacial cover at the close of the Tertiary era.
(Dana, Lyell, Hall, Gilbert, Winchell, etc.) ; and general
estimates and sundry hypotheses as to the time since the
glacial age. (Geikie, McGee, Croll, Prestwich, Wright,
LeConte, and others.)

Method of Computing Time from Thickness of Rocks. — The
elaborate report of Humphreys and Abbott on the " Physics
and Hydraulics of the Mississippi River " furnishes the kind of
evidence required for making the kind of calculations mentioned
under (20) above — that based upon the rate of deposition, or
formation of deposits, at the mouth of rivers. The amount
of silt borne down and deposited by the Mississippi River
annually is estimated by Humphreys and Abbott to be equal

to a mass with I square mile base and 241 feet deep,

the earthy matter pushed along.. 27 " "

or a total of sediment I mile square by 268 " "

But upon Humphreys and Abbott's estimate, and distributing
the sedimentary deposit along the coast for a distance of 500
miles, and giving the strip 100 miles width (or spread it out
for 1000 miles, and make it 50 miles wide), assuming the area
of distribution of the product of erosion of the whole river
to be 50,000 square miles, — on such assumptions the deposit
in 6000 years would reach a depth of approximately 32 feet,
or 53 feet in 10,000 years; or, if we put it in round num-
bers, 50 feet in 10,000 years. The thickness of sediments
for the Devonian era is, according to Dana, 14,300 feet of
clastic sediments and 100 feet of limestone; estimating the
100 feet of limestone to be equivalent in time-ratio to 500
feet of ordinary fragmental sediment, we thus obtain in terms
of fragmental sediments a total of 14,800 feet. Reducing

5 GEOLOGICAL BIOLOGY.

this 14,800 feet of thickness of sedimentary deposits into time-
equivalent, on the basis of the above rate of formation of sedi-
ments, we have 2,960,000 years for the duration of the
Devonian era. If now we assume the Devonian to be ap-
proximately 10$ of the whole time-duration from the base of
the Cambrian to the present, the total time-duration would be
29,600,000, which is a little over one half the estimate sug-
gested by Dana, viz., 48,000,000 years since the beginning
of the Paleozoic time — Paleozoic 36,000,000, Mesozoic
9,000,000, and Cenozoic 3,000,000.*

Forshay's estimate makes the amount of annual deposit
964 instead of 268 feet on a base I mile square in I year's
time, which is about four times as rapid accumulation as the
estimate of Humphreys and Abbott, and the effect upon time-
duration expressed by rock-thickness would be to reduce the
time one fourth, making the Devonian 740,000 instead of
2,960,000 years long. This would bring the age of the earth,
as a solid globe, nearer to the estimate of Clarence King
(24,000,000 years), to which Lord Kelvin gave approval as
lately as March, 1895^

Errors arising from Estimated Values in the Computations. —
According to this estimate we notice that there are several
important data which are assumed, and not observed or known.

(i) The thickness of the deposits themselves. Forma-
tions, as may be noticed, vary greatly in thickness for even
the few localities or regions of America in which they have
been studied. We find that the maximum thickness of the
North American Paleozoic series is given as 55,000 feet, the
general thickness of these deposits in the Appalachian region is
40,000, and in the interior of the continent it varies from
6000 to 3500. Since this estimate was made, Walcott has
claimed for the Cambrian 7000 feet of fragmental rocks and
200 of limestones ; in the Rocky Mountain province 10,000
feet of fragmental and 6000 feet of limestones, which, reduced
to time-ratios (-f for limestone), gives, instead of (7000 -f-
1000 =) 8000, (10,000 -f- 30,000 =) 40,000, or five times the

* See " Manual of Geology," 3d edition, p. 591.
\ See Nature, vol. LI. pp. 438-450.

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 59

time-duration expressed. The maximum thickness of the
whole series is estimated to be about 100,000 feet, or 20
miles.*

Samuel Houghton estimated that the time represented by
the intervals between the strata, when deposition was not
going on at the locality where the strata are examined, was as
great as that recorded by them. This will fully make up for
the error from overrating the maximum thickness. Measur-
ing the greatest thickness recorded on the earth for each of
the various formations, Houghton estimated the aggregate to
be 177,200 feet.

Upham proposes to increase this figure to account for
undiscovered strata, and places the total maximum thickness
of stratified rocks at 50 miles, or 264,000 feet. Thus, re-
garding thickness, we have estimates ranging from 100,000
to 264,000 feet. It may here be stated that the average thick-
ness of the total known strata of the world is somewhere
near 80,000 feet.

(2) Another element entering into the question of rate of
accumulation of deposits is the rate of removal of mineral
substances carried from the continents into the ocean in solu-
tion (see Dana, Geikie, and others).

Mr. Reade estimates that the soluble minerals removed
from England and Wales in this way, mainly Calcium and
Magnesium Carbonates and Sulphates, would equal I foot
removed from the whole surface of the area in 12,978 years.
Prestwich estimated I foot in 13,000 years for the area of
drainage of the Thames, and for the world an average of 100
tons per square mile annually, with an assumption that the
amount removed mechanically is six times as great, or total
(600 -|- 100 =) 700 tons per square mile.

Houghton, f adopting the estimate of the rates of denuda-
tion of river-basins required to lower the entire rain-basin a
thickness of one foot to be as follows :

* See Walcott, " Geologic Time, as indicated by the Sedimentary Rocks of
North America": Proc. A. A. A. S., vol. XLII. 1893, pp. 129-169.
f See Nature, vol. xvin. 1878, pp. 266-268.

60 GEOLOGICAL BIOLOGY.

Ganges 2358 years.

Mississippi 6000 "

Hoang-Ho 1464 "

Yangtse-Kiang 2700 "

Rhone 1528 "

Danube 6846 "

Po 729 "

found the mean rate to be 3090 "

From this table he concluded that " atmospheric agencies
are capable, at present, of lowering the land-surfaces at the
rate of I foot per 3000 years; but since the sea bottoms
are to the land surfaces in the proportion of 145 to 52, the
rate at which (under present circumstances) the sea bottoms
are silted up, that is to say, the present rate of formation of
strata, is I foot in 8616 years. If we admit (which I am
by no means willing to do) that the manufacture of strata in
geological times proceeded at ten times this rate, or at the
rate of I foot for every 861.6 years, we have for the whole
duration of geological time, down to the Miocene Tertiary
epoch, 861.6 X 177,200 = 152,675,000 years. The coeffi-
cient 177,200 is the total number of feet of maximum thick-
ness of all the known stratified rocks."

In this same paper Houghton expresses, in concise terms,
the following conclusion, viz. : " The proper relative measure
of geological periods is the maximum thickness of the strata
formed during these periods."

If this sediment be distributed over a strip 30 miles wide
and 100,000 miles long — the estimated coast border of depo-
sition, amounting to an area of 3,000,000 square miles, or
ly1^ of the land area, on this area the accumulation will be
nineteen times as fast as estimated for the whole area, or I
foot in about 158 years. Assuming this to be a more cor-
rect estimate of the actual depositing-ground, Wallace, taking
Houghton's estimate of 177,200 for the total maximum
thickness of the stratified rocks, gets for the time-period of
the deposition of their thickness, approximately, 28,000,000
of years.

(3) The proportion between fragmental sediments and

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE, 6l

limestones is an uncertain quantity, and the rate of deposi-
tion of limestones is a matter of vague estimation.

Errors Affecting the Values of Actual, not Relative Time-lengths.
— But allowing that the various data are quantities of only ap-
proximate values, in making the estimates the errors are of
such a nature that they do not materially affect the time-ratios.
These time-ratios, it must be remembered, are the reliable
facts that we get from the computation ; whether the total
time be 48,000,000 or 480,000,000, the probability is that
the proportions derived by this method of calculation are
correct to the degree of accuracy of our knowledge of the
facts themselves.

By comparing the three series of values, assigned upon
this principle to the several divisions of the time-scale, by
Dana, Walcott, and the author, as tabulated in the above
scheme (p. 54), it will be seen, reducing them to percent-
ages, that there is a general agreement in the results.

The percentages for the three grand divisions are, accord-
ing to the three computers, as follows :

Dana.

Walcott.

Williams.

Average of the
three estimates.

Cenozoic

6.25

10.526

1C

10-4-

Mesozoic ....

18.75

26.315

20

1
21 +

Paleozoic ....

75.00

63.156

65

68-

100.00 99.997 ioo 99 -f-

Various Estimates of the Length of Geological Time. — Many
estimates, varying greatly in amount, have been made as to the
total length of time represented in the formation of the pres-
ent stratified crust of the earth. The extremes are seen in
McGee's estimate * that the demands of evolution and the facts
of geology warrant the assumption that 7,000,000,000 years
have passed since the earliest fossiliferous rocks were formed,
and twice as long, 14,000,000,000, since the earth began its
planetary form, and in the old conception, on the other hand,
which was supposed to be interpreted from the Bible record,
of 6000 years from the beginning of creation to the present
time. Both of these are probably far outside the limits of fact.

* Am. Anthropologist, October, 1892, vol. v. pp. 327-344.

62 GEOLOGICAL BIOLOGY.

Sir Archibald Geikie, the Director of the Geological Sur-
vey of Great Britain, has expressed the opinion that the for-
mation of all stratified rocks of the earth's crust required be-
tween 73,000,000 and 680,000,000 of years.*

Sir Wm. Thomson (Lord Kelvin), on the basis of radiation
of heat from the surface, and the present underground tem-
perature of the earth, estimated that the time since the con-
solidation of the crust is between 20,000,000 and 400,000,000,
and that all geological history showing continuity of life must
be limited within some such period of past time as 100,000,000
years, f

A more recent estimate made by Clarence King gave ap-
proximately 24,000,000 for the same period; this estimate
has recently been approved by Lord Kelvin, after the debate
arising from Prof. Perry's criticism of the validity of Kelvin's
primary assumptions. J Geo. H. Darwin estimated, from the
rate of retardation of the earth's rotation by tidal friction,
that not over 57,000,000 years have elapsed since the moon
separated off from the mass of the earth ; and Prof. Tait,
from these and other physical grounds, estimates not over
10,000,000 years for all the geological work on the surface
of the earth. Houghton's estimate from erosion gives
28,000,000 for the deposition of the rock strata; Wallace
accepts approximately the latter estimate.

Dana's estimate, as we have seen, is 48,000,000 years.
Upham's § estimate, based upon glacial phenomena, finds
Glacial and Postglacial time to be 30,000 to 40,000 years,
Quarternary 100,000, and thence, by estimating the relative
length of the faunal life periods, Tertiary 50 or 100 times
longer than the ice age, or 2,000,000 to 4,000,000 years;
this brings the mean approximately to the same figures given
by Dana.

* Address before British Association, in 1892. See Nature, August 4, 1892,
vol. XLVI. pp. 317-323.

f Address before the Geol. Soc. of Glasgow, February 27, 1868. See
" Popular Lectures and Addresses of Sir Wm. Thomson (Baron Kelvin)," vol.
II. p. 64.

\ Nature, vol. LI. pp. 224, 341, and 582. See also Lord Kelvin's reply,
pp. 227 and 438.

§ Am. Journal, of Science, vol. XLV. pp. 209-220.

THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 63

To these may be added Prestwich's* estimate of the divi-
sion of the 30,000 or 40,000 years of the Glacial and Post-
glacial period into 15,000 to 25,000 years for the former, and
8,000 to 10,000 for the latter. This estimate approaches the
amount derived from the rate of erosion of the Niagara River
gorge, and the retreat of the falls of St. Anthony. f

Mr. C. D. Walcott \ thinks the Mesozoic and Cenozoic
are in relation to the Paleozoic proportionately longer periods
than as estimated by Dana (that is, 1,3, 12 for the Cenozoic,
Mesozoic, and Paleozoic times respectively).

Walcott suggests the following as probably nearer the
truth: Paleozoic 12, Mesozoic 5, Cenozoic (including the
Pleistocene) 2. He places the estimated duration of these
geologic divisions of time as 17,500,000 years for the Paleo-
zoic, 7,240,000 years for the Mesozoic, and 2,900,000 years
for the Cenozoic, or 27,650,000 years for the time since the
beginning of the Cambrian. He further estimates that the
Algonkian was not over 17,500,000 years, and the Archaean
not over 10,000,000 years more.

Average of the Estimates of only Hypothetical Value. — Ex-
amining the estimates from all these various sources, of the
length of time required to account for the deposition of all the
stratified rocks in which the geological record of the history
of organisms is preserved, we reach the conclusion that an
average of opinions lies somewhere between 25,000,000 and
75,000,000 of years from the beginning of the Cambrian to the
present time. Although it should be held as an extremely
hypothetical belief, the probabilities are considerable that the
time represented is within these limits rather than outside
them either way.

Provisional Units of the Time-Scale Assumed to be of Equal
Value. — But so long as the estimated value of the time-lengths
in geology must be considered highly hypothetical, it may be

* " Geology," vol. n. p. 534.

f See, further, papers by Gilbert and by Spencer on the length of time repre-
sented by the erosion of Niagara Falls; and, regarding the St. Anthony Falls
estimate, see Winchell, vol. n., "Final Report of Geology of Minnesota;" and
a r/sumt/ of the subject in Dana's " Manual," 4th edition, pp. 1023, etc.

| " Geologic Time, as Indicated by the Sedimentary Rocks of North Amer-
ica": Proc. Am. Ass. Adv. Sci., vol. XLII. 1893, pp. 129-169.

64 GEOLOGICAL BIOLOGY.

as satisfactory in dealing with the time-scale to discard them
altogether, and to consider the divisions as units which, added
together, make up the total duration of time from the foot of
the scale to the top, or to present time.

Adopting this plan, each of the periods in the time-scale
on page 54 may be considered as a unit of time of uncertain
length, but of definite position in the scale ; and the several
periods may, until evidence is found for a closer estimate, be
considered to be of equal value. This makes the time-ratios
to approach nearly the estimate made by Walcott, dividing
the whole scale from the base of the Cambrian into 20
such units and assigning 13 of them to the Paleozoic, 4 to
the Mesozoic, 3 to the Cenozoic time. Walcott's values were
19 units, and 12, 5, and 2 for the Paleozoic, Mesozoic, and
Cenozoic times respectively. In the scale here adopted there
is one probable exaggerated error, i.e., the more recent
units were probably relatively shorter than the more ancient
units which are represented of equal length.

The time-scale as provisionally adopted is as follows : Di-
viding the total time represented by the faunas and floras
from the earliest Cambrian to the present time into one hun-
dred units, there are found to be twenty distinguishable and
pretty universally recognized biological life-periods, which
for convenience may be assumed to represent equal periods of
time, each period representing one twentieth or five per cent
of the whole. There are three of these periods in the Cam-
brian era, two in the Ordovician, etc. ; therefore the eras rep-
resent in percentages : the Cambrian, 15$; Ordovician, 10$;
Silurian, 10$; Devonian, 15$; Carboniferous, 15$; Triassic,
5$; Jurassic, 5$; Cretaceous, 10$; Tertiary, 10$; Quater-
nary and Recent, 5$. Paleozoic time is thus 65$, Mesozoic
20$, Cenozoic 15$ of the whole.

These estimates, for the purpose of measuring the rela-
tive duration of organic forms and thus the progress of the
history of organisms, have a rough approximation to the truth
according to the cumulative evidence from all sides at present
before us, but they must be accepted as provisional estimates
to be perfected by evidence which will come with the prog-
ress of knowledge.

CHAPTER IV.

STRATIFIED ROCKS— THEIR NATURE, NOMENCLATURE,
AND FOSSIL CONTENTS.

The Common Usage in Classifying Stratified Rocks. — As de-
fined on a previous page, geological systems are the primary
units of the time-scale ; they are also the grand divisions
made in classifying stratified rocks. When terms indicating
lapse of time are applied to these divisions, the meaning is
lapse of time during which the system was forming. There
is a Carboniferous period only as it is the unknown lapse of
time during which certain strata included in a Carboniferous
system were forming. The limits of that time are determined
only by the unknown points of time when the first and the
last strata of the system were laid down. The thickness and
kind of rock, or other phenomena, may give us a clue to the
possible duration measured between the two points, but it is
a mistake to imagine that we know anything of the particular
geological time, period, era, or epoch at which a particular
stratum was made, except as indicated by the fossils which
record the age. The laying down of a particular sandstone
at a particular place marked a definite point in time, though
we may not know in terms of years, or centuries, or millions
of years, how long ago it was, and it is the stratum, and not
the period, that is definite.

Fossils of Higher Value than Strata for Determining Time-
relations. — According to general usage the fossils are not sup-
posed to be the time-indicators, but the stratum is supposed
to be the indicator of the age of the fossil. This common
usage is defective, in that fossils, when considered as the re-
mains of races of organisms regularly succeeding one another,
record the steps of progress made in their evolution and may

65

66 GEOLOGICAL BIOLOGY.

thus become independent sources of information regarding time-
succession. From this point of view we find fossils to be the
marks of the stages of progress in life-histories upon the earth,
and the strata then serve, as the sand in the hour-glass, to
measure the length of the time-intervals spanned by the life of
particular species, genera, or families, or of faunas or floras.

The Necessity of Two Scales; Strata Furnishing the Data for the
Formation-scale and Fossils Forming the Basis of the Time-scale.
— This new point of view will lead to the separation of the
time-scale from the formation-scale, and the making of a dual
nomenclature and classification. The fossils, independent of
the thickness or succession of the strata holding them, have a
definite time-value, as indicated by the classification of the
scale into Paleozoic, Mesozoic, and Cenozoic times, and the
Eocene, Miocene, and Pliocene divisions of the Tertiary, pro-
posed by Lyell.

The extension of this method of dividing the time-scale
results in the formation of a pure time-scale, based upon the
stages in the life-history of the fossil faunas, quite independ-
ent of the formations of any particular section, although
adopting the systems, arbitrarily, as well-known and recog-
nized units of universal distribution.*

Use of the Terms Period and Formation — In treating of his-
torical geology we speak of the age of invertebrates, the age
of fishes, the age of coal plants, etc., but the application of
time-designations to the rocks or formations is always per-
plexing and often leads to confusion of ideas. The terms
Silurian, Cretaceous, Permian, Trenton, or Miocene were
names of rock formations before they could be applied to the
periods of time in which the formations were made. This
double usage was introduced as early, at least, as 1828, when
Lyell proposed to divide the Tertiary formation into "four
groups or periods to which they belonged," calling them
Eocene, Miocene, older Pliocene, and newer Pliocene. Al-
though the science demands two classes of designations, a
time-scale and a formation-scale, it certainly will tend to

* See p. 52 also " Dual Nomenclature in Geological Classification, "Journal
of Geology, vol. n., February-March, 1894, pp. 145-160.

STRATIFIED ROCKS. f

clearness, and definiteness of thought and language, to retain
the nomenclature now in use for the classification of rock
formations and to apply names derived from the fossils to the
time-divisions, since the fossils are the means by which the
time- divisions are recognized.

A geological formation, made up of clastic fragments of
other rocks, has in itself nothing by which to determine its
time-relations ; it is only its position, geographical and strati-
graphical, in relation to underlying or superimposed strata
that indicates its relative time-relation; when considered
abstractly, or irrespective of its position, it loses its time-indi-
cating characters.

Strata Parts of a Geological Formation, Fossils the Marks of a
Geological Period. — It is not scientific, therefore, to speak of a
rock or stratum as belonging to a particular period, the rock
belongs to a formation. The fossil imbedded in it, however,
does belong to the period, is characteristic of the period, and
thus, in nomenclature, it is actually taken as the mark of the
time-division. Just as we speak of the Chemung group, as
the name for the upper Devonian rocks of New York State,
so with like propriety we may say the disjuncta epoch, or the
epoch of the Spirifera disjuncta and the fossils associated
with it; and for the same reason. The application of Che-
mung to the group is appropriate, because one of the most
typical outcrops of the rocks so named is along the valley of
the Chemung River, at Chemung Narrows, in southern New
York. Not that it is not exhibited elsewhere, and not that
it is all exhibited at Chemung Narrows, but the group of
rocks of which the cliffs at Chemung are a good example
is appropriately and distinctly defined by the name. So to
call the epoch the disjuncta epoch is appropriate, because the
Spirifera disjuncta is a characteristic shell in the fauna of the
epoch, and the designation disjuncta as a specific name is
permanently applied to those characteristics of the genus
which are peculiar to the closing part of the Devonian age, in
all regions from which the fossil has been obtained ; and al-
though not the only fossil, and this one not always present,
still it may be used whenever found as indicative of the time-
epoch which is so named.

68 GEOLOGICAL BIOLOGY.

The "Hemera" of Buckman.* — Buckman has recently pro-
posed the term hemera (r//jepa, a day) to indicate a time-
division of this nature. He writes: " The term ' hemera ' is
intended to mark the acme of development of one or more
species. It is designed as a chronological division, and will
not therefore replace the term * zone ' or be a subdivision of
it, for that term is strictly a stratigraphical one. . . .
Successive ' hemerae ' should mark the smallest consecutive
divisions which the sequence of different species enables us to
separate in the maximum development of strata. In attenu-
ated strata the deposits belonging to successive hemerae may
not be absolutely distinguishable, yet the presence of succes-
sive hemerae may be recognized by their index species, or
some known contemporary ; and reference to the maximum
developments of strata will explain that the hemerae were not
contemporaneous, but consecutive."

Again he writes: " Our present 'zones' give the false
impression that all the species of a zone are necessarily con-
temporaneous; but the work of Munier-Chalmos in Nor-
mandy, and my own labors in other fields, show that this is
an incorrect assumption. The term ' hemera ' will therefore
enable us to record our facts correctly ; and its chief use will
be in what I may call ' palaeo-biology.' " f

The Terms Age of Reptiles, Planorbis Zone, etc. — The no-
menclature at present in use in geological classification, it
will be seen, is a nomenclature for the classification of forma-
tions, and is applied to the time-classification for want of a
better. We have in use names for a few of the grander di-
visions of time properly chosen, as the ages of man, of
mammals, of reptiles, etc., and in a few cases subdivisions
of the finer kind have received names after the same plan, as
the planorbis zone and the angulatus zone, before referred to
in the classification of the Ammonite beds of the Jurassic.
The selection of time-designations by this method can only
come through careful study of the characteristic fossils on the

*S. S. Buckman, "The Bajocian of the Sherborne District: Its Relation
to Subjacent and Superjacent Strata": Q. J. G. S., vol. XLix. p. 481, November,
1893.

f L. c., p. 482.

STRATIFIED ROCKS. 69

basis of their succession in chronological sequence. Although
the relative position of the strata is the only infallible mark
of time-sequence, it is the fossils in the strata that are the
only infallible marks of time-periods.

Nomenclature of the International Congress of Geologists. — In
general usage the time-designations have been applied directly
to the formations, as in the nomenclature proposed by the
International Geological Congress, where the formation-names
stage, series, system, group, have their corresponding time-
names age, epoch, period, era. In a similar way various other
terms, which apply to the strata of formations, have their
corresponding terms for the fossils of such formations.

Fauna and Flora — A particular bed, stratum, or forma-
tion is said to have its fauna or flora, in the same way as a
particular geographical region or province has its fauna or
flora. A particular rock stratum marks a particular faunal
horizon, as the Tully limestone may be called the horizon of
the Cuboides fauna. We find an admirable definition of
fauna in the Century Dictionary: il Fauna, the total of the
animal life of a given region or period ; the sum of the ani-
mals living in a given area or time." Flora is used similarly
for the plants of a region or period.

Horizon. — We find under the word horizon an equally apt
definition of that term. A geological horizon is defined as "A
stratum, or group of strata, characterized by the presence of
a particular fossil, or a peculiar assemblage of fossils, not
found in the underlying or overlying beds."

Zone and Stratum. — The term zone is applied in geology to
the stratum or the strata in which a particular fauna or flora is
distributed. In some cases authors speak of the zone of a
particular species ; but whether a single species, or that one
and other associated species, be taken as the distinguishing
marks of a geological zone, the difference between a zone and
a stratum is found in the distinction that the zone is charac-
terized by continuity of the same life and the stratum by
continuity of the kind of stratified deposit.

Facies. — The term fades is used in a particular sense in
geology to apply to the particular composition or condition
of a formation in a given region ; for instance, the Hamilton

7° GEOLOGICAL BIOLOGY.

formation in western New York is calcareous and finely argil-
laceous; in eastern New York the same formation is arena-
ceous and flaggy; although representing the same formation,
one may be called the argillaceous or calcareous facies, and
the other the arenaceous facies, of the Hamilton formation.

Area, Province, Region. — Again, the terms area, province,
region, when applied geologically, refer to the geographical
districts in which there was greater or less uniformity in the
kind and succession of sedimentation for a given geological
period. Thus, the Appalachian province or the Mississippian
province may be spoken of. These same terms when used in
zoology or botany refer to the districts which, separated by
more or less sharp physical boundaries, are characterized by
distinct faunas or floras.

Geological Range and Geographical Distribution. — A conven-
ient distinction may be drawn in the usage of the terms range
and distribution, which are now almost synonymous. In
speaking of the separation of species, or genera, or faunas, or
floras, when separated in space, distribution will be used ; when
separated in time, range. Thus, according to Ulrich, the
Vitulina fauna of the Middle Devonian may be said to have a
distribution limited to South and North America and Africa;
its range is Lower and Middle Devonian.

Variations and Mutations. — Waagen has proposed to dis-
tinguish the changes of form observed on comparing the same
species from different places. When the specimens compared
belong to the same geological horizon, but come from the
same or different geographical areas, the differences of form
are called variations ; when the specimens come from different
geological horizons, thus representing time-range, the differ-
ences of form are called mutations.

Development and Evolution. — Another analogous distinction,
which is explained more fully elsewhere, is observed in the
restriction of the term development to the processes of expan-
sion of characters of the individual in ontogenetic growth,
and the term evolution to the changes expressed in the indi-
viduals succeeding each other in phylogenetic succession.

Initiation and Origin. — Another distinction, in the way of
greater precision, is in the use of the term initiation in place of

STRATIFIED ROCKS. /I

origin, when speaking of the first appearance of a new type
of structure in the geological formations. It is difficult not
to associate some idea of causation with the terms origin and
originate, but the term initiation refers simply to an incoming
or a beginning to appear, leaving other questions open for
discussion.

System. — This is the name for one of the larger geological
divisions, but there is no uniform rule for its application.
Originally, as proposed by Murchison, system was applied to
a series of rocks continuously exposed in some geographical
region. Thus, Silurian system was the series of rocks exposed
in Wales and western England at one time inhabited by the
Silures. The Devonian system was the series of rocks exposed
in south and north Devonshire ; Permian system, certain fos-
siliferous rocks first thoroughly studied in Perm, Russia; etc.
The term system was afterwards adopted as a name for a
large and prominent series of stratified rocks, as Carbonif-
erous system, Tertiary system, etc.

Systems have been arbitrarily determined, and the list as
given, including those in which fossils have heretofore been
found, is as follows : Cambrian, Ordovician, Silurian, Devo-
nian, Carboniferous, Triassic, Jurassic, Cretaceous, Tertiary,
and Quaternary or Recent, or including Recent. These, as
has been said, are arbitrarily fixed, and there is in each case
a typical system expressed in the rocks of a particular region.

These systems are applied with an approximate degree of
uniformity in all countries, although arbitrarily ; and era is
the time-designation which is applied to indicate the lapse of
time during the formation of the rocks of a single system.

Geographical Conditions Determining the Local Characters of
Stratified Rocks. — There are a few particulars, regarding the
way in which these rocks were formed and their present
condition and order, which help to explain the conditions
under which the organisms lived in the past, and may ex-
plain why we have full records in some cases, very little rec-
ord in others, and in many cases very sparse and greatly
broken records of the life-histories we are seeking to read.

The stratified rocks, as already stated, are the result of
water-action: First, erosion from already formed rocks; sec-

72 GEOLOGICAL BIOLOGY.

ond, transportation of the fragments by water; and, in the
transportation, third, separation of fine from coarse and
further rounding of the individual grains; fourth, sedimenta-
tion under water in layers or strata. The materials for each
stratum have gone through these various processes of water-
action. The result is that the present characters of the strata
have been determined by (a) the nature of the source of
materials, (b) the rate., direction, and force of the activity of
the water, and (c) the relations of the bottom of the ocean
to the surface, or the depth of the water. Each of these
three conditions is variable and generally is the same for only
a limited area. To illustrate : We know from observing the
phenomena of an ocean beach that the beach material where
the shores are low and composed of soil is made up of the
wash of the shore. If a large river empties in the vicinity,
the shore is made up of fine silt and mud ; if, on the other
hand, the shores are hard rocks, the beach is composed of
coarse pebbles and gritty sand, the result of the disintegra-
tion of the rocks themselves. If we examine the shore ma-
terial of Florida, where calcareous rocks alone are exhibited,
we find the sand composed of broken shells and corals. This,
when filled by deposited calcite carried into the interstices
in solution and hardened, becomes a calcareous rock, called
coquina, and finally a compact limestone.

Again, if we examine the materials lying on the beach
at high tide and those on the bottom out to a depth of a
hundred fathoms, we find that the coarse pebbles and boulders
are distributed along the line of most violent wave-action
near shore, then gravel, and further out only fine sand, and
finally only the finest silt appears. This sorting is entirely
co-ordinate with the change in violence and rapidity of normal
motion of the water in waves and currents. The more rapid
and violent the motion of the water, the larger the particles
moved and transported by it, and, hence, the farther out
from its source the material is borne, the finer and less in
amount will be the resulting deposit.

For all fragmental material the land surface, where it
comes in contact with water in motion, may be regarded, in a
general sense, as the source, and, in a general way, distance

STRATIFIED ROCKS. 73

from such source determines the relative size of the particles
making up the sediment. The source may be far up in the
interior of the continent where river erosion or lake erosion
is eating away the land, or it may be on the ocean-shore,
but in general it is true that local geographical conditions
are fundamental in determining the lithological character of
geological formations.

Varying Conditions of Environment in Relation to Time-
estimates. — The conclusion from these observations is that all
sedimentary rocks may be supposed to have been formed
within about a hundred miles of the shore from which the
sediments were derived. This theory is supported by the
deep-sea soundings, which show very small amount of mate-
rial accumulated on the bottom of the present ocean at great
distances from land. From these considerations we turn to
our classification of formations, and see why it is that we
cannot expect to find uniformity of details in either the
structural or stratigraphical order, or in the lithological
composition of the formations, (i) At the same time there
may be in process of formation a limestone, a sandstone, a con-
glomerate, and a mud-shale, and all may be forming within
a relatively short extent of coast. (2) In the same period
of time the thickness of material accumulated may greatly
vary ; while an inch of limestone may be deposited in one
place, a hundred feet of sandstone may be formed in another.
Thus the limestone of one locality may be represented by a
sandstone in another, and a thousand feet of strata in one
place may be represented by a hundred or less in another not
far distant.

Relative Order of Deposits in Relation to Depression and
Elevation — Another series of facts may be considered in this
place. The shore-lines do not remain constantly the same for
the accumulation of sediments. The simple fact that there
are marine fossils in rocks above the level of the ocean is
evidence that there has been a change in the relative level
of land and ocean surfaces ; there has been an elevation of the
land surface. Since the conditions of sedimentation vary
with the distance from shore-line, a particular series of these
conditions extending from shore-line out into deep water will

74 GEOLOGICAL BIOLOGY.

be bodily shifted seaward by elevation, and landward by de-
pression of the continental border.

Order of Deposits with a Sinking Land. — Other conditions
remaining the same, for instance, on a shore with land to
the westward and ocean to the eastward, a gradual continu-
ous depression of the land would result as follows : The shore-
line would gradually retreat westward ; at each spot the water
would gradually become deeper and further off shore ; and,
considering only this one law of sedimentation, the deposits
forming at each spot would gradually become finer and finer
with the progress of time ; so that finally it would happen
that the deposit forming directly over the place where the
shore-line was at the outset would be the very fine silt peculiar
to deep water far out from shore, the same which at the
beginning of the period was being deposited only at a distance
off shore.

To compare the sections taken at three localities we would
get the following results, seen in Fig. 3 :

West. fca^j1::- nll „ East.

PIG. 3. — Three different sets of deposits formed during the same periods of time at three points,
i, 2, and 3, separate from each other, with a sinking of the land as the sediments are
accumulated.

In which the section at I would exhibit a series of deposits, one
overlying the other (a b c), presenting the same differences of
sedimentation that would be exhibited on comparing the first
deposits in the several sections (a a' a"). It is likely, too,
that the general character of the fossils would correspond, but
as a matter of age the deposits of like character in the three
sections (a" b' c) would represent consecutive periods, instead
of the same period of time.

Order of Deposits with Elevation of the Land. — If we sup-
pose a gradual elevation to take place, instead of depression,
then the shore-line would advance gradually seaward, — east-
ward in the supposed case, — and the first locality (i) would

STRATIFIED ROCKS.

7S

cease to receive deposits, and would be eroded away by the
action of the waves and partly redistributed over the other
deposits, while the one farthest out (3) would receive first the
finer deposits (a"), then still coarser (£"), and finally the shore
conditions would prevail and their appropriate sediments
would be deposited (<:"). The following would result :

West.

East.

FIG. 4. — Three sets of deposits formed under the same conditions as those of Fig. 3, except that
the land was gradually rising during the accumulation of sediments. In both figures the
coarser sediments are represented by open dots, the sands by fine dots, the coarse muds by
heavy horizontal lines, the finer muds by similar finer lines.

There would be a reversal in the order of the sediments ;
also a change in the relative thickness of the three sections ;
and number 3 would be the thicker section. Although gravel
might appear at the top of each section, it would represent a
later period in section 3 than in section I, and all the period
represented by b" and c" of the third section would be repre-
sented in the first section by an hiatus or line of erosion. It
is essential to assume that such oscillations, upward or down-
ward, were taking place constantly during the accumulation
of the sedimentary deposits now called stratified rocks, and
the above analysis exhibits the nature of the perplexities
which must arise in a precise study of the relations of the for-
mations of different regions to each other.

Characteristic Fossils. — In a general way fossils are charac-
teristic of the age of the systems, but actually the systems
represent great lengths of even geological time ; and in many
cases this time is long enough to include the beginning, the
luxuriant abundance, and the extinction of a whole genus
or a family of organisms. Such generic groups have had
their stage of beginning, have spread about the earth, and
during their distribution and adaptation to the various con-
ditions of environment have become specifically modified, so
that each of the systems is marked by the presence of cer-

76 GEOLOGICAL BIOLOGY.

tain genera which are characteristic of the fauna and flora
for a long period, and thus serve as arbitrary marks of
these great periods. The individual continuing beyond a
certain specific zone in one section does not interfere with
the general law that there are grand divisions of time which
are characterized by peculiar types of organisms.

Although we cannot, in the present state of knowledge,
draw sharp lines which shall be universal, between the for-
mations, or between the several species represented in them,
it is convenient to recognize these systems, and in each
country the lines can be arbitrarily fixed, and the sub-divi-
sions locally recognized.

SUMMARY.

Reference has been made to the difference between the
history of the organism (Ontogeny) and the history of organ-
isms (Phylogeny). It has been shown that there is a natural
history of the development of the individual, and that there
may be a history of organisms as a whole — a history in which
all the species of the same kind are but as a unit in a great
complex of organic life which has evolved with the geological
ages. In this latter history time and the conditions of en-
vironment have played very important parts; but ordinary
time-scales are practically useless, because they are not divided
into long enough periods, and because they do not reach back
far enough. A special time-scale was needed. This has
been constructed by an analysis of the classification of rock
formations. In this analysis we have seen that progress of
science is as much a progress of ideas as it is an increase of
known facts ; that the accumulation of confirmatory facts has
followed rather than preceded the formulation of speculative
theories; that the theories about the earth have dominated
in each proposed scientific classification of facts, and thus in
the formulated science of each period.

The result of the analysis emphasizes a few laws, which
may be stated as established, regarding the chronological as-
pects of the rocks.

First. There is a natural succession in the original forma-

STRATIFIED ROCKS. /7

tion of rocks. There are certain rocks that are relatively
primitive ; these are crystallized and compact, as granites and
gneisses. There are other rocks that are of sedimentary ori-
gin ; these are secondary in original formation ; they are made
of fragments of rocks, and are in stratified form, and lie upon
primitive rocks whenever the two are in contact. There are
still other geological formations that generally are not in com-
pact form, but are composed of loose fragments, sand, and
fine mud, or soil, and naturally lie above the others.

A second law established by experience is that (with ex-
ception explained by later disturbance) for the sedimentary
rocks natural order of superposition indicates relative chrono-
logical order of formation ; viz., in any given case of two
stratified rocks the underlying rock is the more ancient.

A third law is that the mineral character of any particular
stratified rock bears no necessary relation to its age. As, for
instance, rocks of the same composition, structure, and color,
but coming from separate geographical regions, may be of
entirely different geological ages.

Fourth. It is an established law that there is some definite
relationship between the characters of the fossils and the rela-
tive geological age of the rocks in which they occur. This law
is formulated in the classification Paleozoic, Mesozoic, Ceno-
zoic, applied to the respective geological formations in their
chronological order.

In accordance with these laws a classification of forma-
tions has been formed (Cambrian, Ordovician, Silurian, De-
vonian, etc.) in which the relative antiquity of the systems is
expressed. This constitutes the formation-scale, and it is
based upon the series of strata, lying one upon another, com-
posed of sedimentary materials of various kinds forming sand-
stone, limestone, shales, and conglomerites, etc., originally
nearly horizontal in position, but now variously tilted and
folded. In such rocks the fossils are found from which the
time-scale proper is constructed. The recognizable units of
this time-scale are the periods, characterized by fossil fauna-
floras, whose characteristic species may be distinguished the
world over and thus form the marks of the standard time-
scale for the study of the history of organisms.

CHAPTER V.

FOSSILS— THEIR NATURE AND INTERPRETATION, AND
THE GEOLOGICAL RANGE OF ORGANISMS.

Fossils of Vegetable and Animal Origin. — Having explained
the nature of the series of geological formations, their classi-
fication into systems, the value of these as reservoirs of in-
formation regarding the history of organisms, we next inquire
into the nature of the fossils, which are preserved in them and
furnish the records of the separate lives whose history we
would trace.

Fossils are any traces of any organisms which, having
been buried in rock-forming muds, are preserved to tell of
the life of the dead organisms. Vegetable fossils are remains
of plants, leaves, stems, wood, fruit, nuts, or resin, gum, or
carbonaceous matter, coal or bitumen, or oil or gas. Animal
fossils are remains of animals, their foot-prints, tracks, trails,
cases formed of particles of sand, as of the Caddis-worm, etc.,
skeletal or dermal hard parts, bones, teeth, spines, scales,
shells, or corals, and secretions of various kinds, formed during
life for protection or defense, or offensive weapons, or ex-
cretions, when of sufficient hardness to resist destruction, as
the coprolites of fish and reptiles, preserving, in some cases,
evidence of the shape of the intestinal canal (spiral) through
which they passed.

Original Material of Fossils. — The original materials were as
various as the hard parts now formed by living organisms.
The great majority of the known fossils were originally com-
posed of calcic carbonate, calcic phosphate, chiton, bone, silica,
or, in the case of plants, bituminous matter. In some cases
the whole animal may be preserved, as in the case of insects
in amber, or the fossil elephants in the ice of northern Si-
beria, which have furnished abundant store of ivory to enter-

78

FOSSILS— THEIR NATURE AND INTERPRETATION. 79

prising explorers ; or in the case of minute organisms buried
in the muds, the softer or destructible parts may decay
and pass away as gases or in solution. Generally, however,
fossils are but fragments or parts of the original structures
united during the life of the organism. Again, the origi-
nal substance of the fossil, when removed by solution after
fossilization, may be replaced by other mineral substance
brought in from without by infiltration. Or the mineral may
be molecularly changed or replaced ; an example is fossilized
wood, in which the grain and structure of wood is preserved
but silicified. This replacement may be by Silica, Calcite,
Pyrite, Marcasite, Siderite, and rarely some other minerals.

Various Aspects of the Original Form represented. — In the
fossil condition the form may differ from that we are accus-
tomed to see in the corresponding part of a living organism.
Thus a fossil snail-shell may be simply a fossil shell, that
is, the shell itself buried in the rock. Or it may consist of
the impression of the shell now removed, in which case it
may be the reverse or cavity over the exterior of the shell, or,
in case of flat shells, like clam-shells, similar impressions of the
inner surface ; or the cavity may be again filled with detrital
matter, forming a cast of either the inner or outer form of the
shell or object fossilized : in the former case it would be
called a mould ; in the latter, a cast.

Preservation of Fossils. — Fossils may have been covered un-
der various conditions and at various places; and the fossils
themselves are the best indication of the conditions. The
fossils may consist of land species alone, or types of organ-
isms adapted to live in air and not in water; but in order to
be preserved it is almost universally necessary that the part
fossilized be covered from the air : first, because atmospheric
conditions are extremely destructive to any substances exposed
to them, even quartz or glass suffering more or less by con-
tinuous exposure. The protection by soil will preserve the
more insoluble matters, but here again decomposition and
solution of any substance that can be decomposed or dissolved
will take place with slower or faster rapidity. Entire exclu-
sion from air and from circulation of acidulated and alkaline
waters is the condition under which the more perfect fossils

8O GEOLOGICAL BIOLOGY.

were preserved, and this condition is found only under
muds, in marine conditions; in the bottom of lakes or in
river bottoms fossilization may take place, but the fossils
are then liable to some change of composition. Fossils pre-
served under the most favorable conditions, by long-contin-
ued pressure and the slight circulation of fluids in rocks,
suffer change after their formation, particularly in the way of
assuming a crystalline structure.

The Majority of Fossils are of Marine Organisms. — From the
above remarks it is evident that the larger proportion of fossils
must be those preserved under the surface of the ocean ;
next will be found those buried in land basins covered by
fresh water; and only very rare will be the cases of fossils
otherwise preserved. Hence marine organisms will naturally
present in the rocks the fuller records of their history : fresh-
water or brackish-water species will be recorded less perfectly ;
and the organisms normally living under land or air condi-
tions will be recorded in fossils very imperfectly at the best.
The great majority of even the hard parts of such organisms
must be destroyed before reaching the position of a safe burial-
place, and our studies will be directed by this law of preser-
vation. Marine organisms, and largely marine invertebrates,
will be selected as illustrations of the laws of the history of
organisms, because the records regarding these are fuller than
regarding any other kinds of organisms.

Various Kinds of Fossils enumerated. - - To the question
" What are fossils?" the concise answer is: Fossils are
traces of organisms buried in the rocks. A full definition
would be a descriptive treatise on Paleontology. As to their
forms, fossils are as various as are organisms. A useful analy-
sis, however, may be made of their composition. Fossils
are composed (A) either of the original materials of the organ-
ism which made and left them ; they are then strictly remains
of dead organisms, or of parts of them. Or (B) fossils may be
casts or moulds in the rocks where these structures were origi-
nally buried and afterwards removed. (C) The filling of the
cavities thus formed constitutes other kinds of fossils, (i)
The cavity may be filled by mineral matter carried in by infil-
tration and redeposited ; (2) the cavity may be filled through

FOSSILS— THEIR NATURE AND INTERPRETATION. 8 1

molecular replacement of the mineral of the original structure
by some other mineral, as calcite, silica, pyrite, etc. ; (3) the
cavity may be filled by detrital matter washed into the cavity
from outside. (D) The original substance may be changed in
molecular constitution, or even in chemical composition,
losing a part of its elements, or gaining other elements-; thus,
a piece of wood may become coal, or a shell may become
crystalline calcite, or aragonite. (E) Finally, the fossil may
consist of traces left in the sediments while the animal was
alive, as footprints or other marks of organic activity.

Fossils represent chiefly the Hard Parts of Organisms. — An
important generalization may here be made regarding all
fossils. Fossils represent organisms, but almost universally
they represent the hard parts of living organisms ; hence
the most valuable lessons to be learned from fossils must be
derived from the study of the hard parts of organisms.
These hard parts are the parts which have attained definite
and fixed form during the life development of the individual.
Soft parts, or organs, are adjustable to changing exterior con-
ditions, but its hard parts are already adjusted, and, there-
fore, they are an expression of the working adjustment of
the species, to the conditions of its environment, at the partic-
ular time in which it lived.

Best and most perfectly adjusted Organisms of the Time left
their Records. — The history of organisms, which we particu-
larly trace in the study of fossils, is not the history of imper-
fect organisms struggling toward perfection, but it is the
history, for each age and epoch, of the perfected adjustment
of the organisms of the time to the particular conditions of
environment in which they lived. They did not die before
their time, overcome by the mythical fittest who are said to
survive in the struggle. They were the fittest, and died natu-
ral deaths, having provided before they gave up the struggle
for their progeny to succeed them. The hard parts record
the history of adults which had endured the struggle, and
thus represent the royal line of succession for the geological
ages.

General Laws regarding the Occurrence of Fossils. — There
are certain general laws, concerning the occurrence of fossils

82 GEOLOGICAL BIOLOGY.

and the relations which their specific forms bear to the place
they occupy in the geological scale, which indicate a definite-
ness in the order of their succession, quite independent of the
evidence furnished by the stratigraphic succession of the rocks
themselves; and it is this testimony of the fossils, pure and
simple, as mere physical forms, upon or in the rocks, that con-
firms and helps to complete the chronological scale indicated
by the successive geological systems.

Pictet* announced a number of propositions setting forth
the more prominent of the laws of occurrence of fossils and
their relations to time and place. In the " Handbuch der
Palseontologie " Zittel \ has condensed and culled them so
carefully that we there have concisely formulated in a few
sentences the chief facts regarding their occurrence.

They are as follows: (i) All stratified sedimentary rocks
(with the exception of metamorphic rocks) enclose, more or
less richly, fossils, and thus prove that the earth, for an im-
measurable length of time before the appearance of man, was
inhabited by organisms.

(2) The fossils of the oldest and deepest strata represent
extinct species, and for the most part extinct genera ; only
in the more recent strata are found forms which are identical
with those now living. The deeper down we penetrate in the
series of strata the more divergent are the fossils from the
forms now living; and, on the contrary, rising from the earli-
est to the more recent formations there is a continuously
increasing resemblance to the present creation.

(3) The different fossil faunas and floras follow each other
the world over in the same regular sequence ; the formations
stratigraphically nearer to each other contain the most similar
fossils, and those most separated in age present the greater
differences.

(4) Constant change characterizes the evolution of the
organic creation. Species of one geological formation are
either completely or partly replaced by other species in the
next superimposed strata.

* Pictet, Fran£ois Jules, " Traite de paleontologie : ou, hist. nat. des ani-
maux fossiles, etc.'' 1853-57.

f Zittel, " Handbuch der Palaeontologie," vol. I. pp. 17, 18.

FOSSILS— THEIR NATURE AND INTERPRETATION. 83

(5) Each species, like the individual, has a certain shorter
or longer life-period, after which it perishes, never to reappear.

(6) From these principles it arises that the approximate age
of a stratum may be determined by the degree of similarity
of its fossils to the forms of the present time. The fossils
contained in the strata are the means of determining the
equivalency (that is, likeness of age) of the strata themselves,
and in general, identical fossils indicate contemporaneity of
the enclosing strata.

Change of the Forms of Fossils with Passage of Time, and
particular Form characteristic of Particular Periods of Time,
undeniable Facts of Paleontology. — Thus it appears that what-
ever we make out of fossils, whether we consider them stones
or organisms, however we account for their origin, whatever
relation we conceive them to bear to each other, the fact is
startlingly vivid to the paleontologist that the form of a fossil
is intimately associated with the time in which it appeared on
the earth ; that the morphological characters assumed by fos-
sils have been gradually and incessantly changing from the
beginning of the world.

Inorganic Things, on the contrary, Unchangeable. — This is
contrary to the law in respect of every inorganic thing. The
chemical composition of things and the chemical properties
are the same so far back as we can trace them and to the
most distant star in space. Minerals in the Archaean ages,
before any fossils had appeared, crystallized out into exactly
the same forms which they assume to-day. We know of not
the least fluctuation in the laws of physics for all time.
Indeed, it is by dependence upon the absolute certainty and
uniformity of these laws that the astronomer is able to calcu-
late the position, the size, and the orbit of some unknown
and unseen planet, and directing his telescope to the place
where it should be, to discover it there.

Fossils characteristic of Particular Periods of Geologic Time.
— The morphological combination of characters, which we call
a fossil (as a Trilobite or an Ichthyosaurus), has its definite
relationship to geological time, and each form is characteristic
of a particular period of time. A fossil becomes the unmistakable
mark of the age of the rock in which it is enclosed: the Trilo-

84 GEOLOGICAL BIOLOGY.

bite is characteristic of the Paleozoic, and the Ichthyosaurus
is characteristic of Mesozoic time, as truly as man is charac-
teristic of recent time.

Stony Corals: the Zoantharia. — In order to emphasize and
illustrate this law of the intimate connection between organic
form and time, the statistics regarding the great order of the
stony corals (the Zoantharia) may be chosen.

For the convenience of those who may have no special acquaintance with
the scientific nomenclature of systematic Zoology, a few facts regarding the prin-
ciples of classification and nomenclature are here offered. The classification of
animals is based primarily upon differences in form, structure, and function.
On this basis zoologists have classified animals under nine chief divisions, called,
I, Protozoa ; 2, Ccelenterata ; 3, Echinodermata; 4, Vermes; 5, Arthropoda;
6, Molluscoidea ; 7, Mollusca ; 8, Tunicata ; 9, Vertebrata. (Claus.) Each of
these divisions is called a Branch (Phylum or Subkingdom) of the Animal King-
dom, and each is characterized by a distinct type of organic structure.

Under each of these chief divisions the animals are associated by their
greater degrees of likeness, and are separated by their lesser differences, into
subdivisions, called respectively, from higher to subsidiary rank, Classes, Orders,
Families, Genera, and Species. The Ccelenterata are thus at present known
under four Classes, viz., Spongia, Anthozoa, Hydrozoa, Ctenophora.

The class Anthozoa (coral animals) is subdivided into two orders, Alcyonaria
and Zoantharia. The order Zoantharia is subdivided into three suborders: The
Antipatharia^ the Actinaria, and the Madreporaria. The first two of these
suborders develop no hard parts that have been recognized in a fossil state, and
therefore we cannot speak of their historical relations. The Madreporaria are
the polyps which secrete stony corals, and of their calcareous skeletons great
numbers have been found in the rocks; many massive beds of limestone consist-
ing mainly of them or their fragments.

The Madreporaria, or stony corals, have been classified in two groups of
families, the most characteristic feature separating them being the arrangement
of the septae in one of them in fours or multiples of four, Tetracoralla, and in
sixes or multiples of six in the other group, Hexacoralla.

Numbers of Genera of the Zoantharia recorded for each Era. —
There are several thousand species of stony corals described,
but for the present purpose it is sufficient to note that there
are 448 genera of Zoantharia already described and recog-
nized. (Zittel.) That is, there are 448 different combina-
tions of form of the stony corals, which are sufficiently sharply
defined and constant in their character to be classed under
distinct genera. If we only note the numerical relation of
these genera to the successive geological periods of time, the
law above referred to becomes at once apparent. In the
Lower Silurian 4 genera are reported by Zittel ; 5 genera have

FOSSILS— THEIR NATURE AND INTERPRETATION. 8$

been since reported in the Cambrian for America alone by
Walcott ; the Silurian has 54 genera, Devonian 39, Carbonif-
erous 34, Triassic 17, Jurassic 84, Cretaceous 1 10, Tertiary
125, and Recent 132 genera.

Two Types of the Zoantharia indicated by the Two Maxima of
Genera in Separate Eras in the Time-scale. — In this series of
numbers of genera there are two maxima, one in the Silurian,
one in Recent time. This is explained by another fact : the
Order (Zoantharia) is divided, as above stated, into two bio-
logical groups, distinguished by a marked difference in the
numerical arrangement of radiating divisions of the body.
The first group, the Tetracoralla, has 8 1 genera ; the second
group, the Hexacoralla, has 367 genera. With the exception
of a single genus, all the 81 genera of Tetracoralla are con-
fined to the Paleozoic. The Hexacoralla are mainly later
than the Paleozoic.

These statistics for the Madreporaria, arranged in tabular
form, produce the following table (the figures in each column
opposite each family expressing the number of genera of the
family which made their first appearance in the geological
system corresponding to the letter at the top of the column) :

TABLE OF THE NUMBERS OF GENERA OF MADREPORARIA
MAKING THEIR FIRST APPEARANCE IN EACH GEO-
LOGICAL SYSTEM, GROUPED IN FAMILIES.

C.

O.

S.

D.

Cr.

T.

J.

K.

Ty.

Q.

R.

Tetracoralla 81 genera

5

4

42

10

19

o

O

o

o

i

Hexacoralla 367 genera

17

ii

22

72

75

8T

78

3

X4

8

O

8

O

i

i

4 Pocilloporidae

2

i

i

T

2

y

6

7

6

jo

g

Ig

I

i

13

«i

44

33

78

2

o

I

I

6

2

4

4

i

o

2

i

13

17

14

Total Madreporaria 448 genera

5

8

59

21

26

22

72

75

81

79

Evolution Curve of a Group of Organisms. — These statistics
may be so arranged as to express in graphic form the rate of
generic differentiation.

For this purpose a table is constructed, composed of a
series of ten perpendicular columns, each one, from left to

86 GEOLOGICAL BIOLOGY.

right successively, representing the successive geological eras,
Cambrian, Ordovician, Silurian, Devonian, etc. The name of
the era is indicated at the top of each column by its initial let-
ter. The length of time of each of these eras is represented
roughly by the width of the spaces between the separating
lines, according to the time-scale described in Chapter III.
Thus, starting from the lower left-hand corner, the abscissa
represent time-extension from the beginning of the Cambrian
era.

Drawing an horizontal line from this point across the base
of the several columns, the distance above this base-line or
the ordinate expresses the degree of differentiation in terms of
units of genera (or of species, as the case may be) appearing in
each era.

Thus, by connecting together the points representing the
amount of differentiation (the ordinate) for each geological
era (the abscissa), we produce a curve representing the rate
of generic differentiation for the particular order or class
(as the case may be) under consideration. This curve may be
called the evolution curve. In the following table are repre-
sented the evolution curves of the Madreporaria, and those of
several divisions and families of the Madreporaria, based upon
the statistics before us.

Construction of the Diagram. — This diagram was constructed as follows:

Extension laterally represents time - duration, beginning at the left-hand
lower corner with the base of the Cambrian; the total length of geological time
thence to the present is made to cover 100 spaces. The several geological
system-eras are represented in their estimated proportionate lengths, thus:
15$ is given to the Cambrian, io# to the Ordovician, 10$ to the Silurian, 15^
to the Devonian, and 15^ to the Carboniferous; to the Triassic 5*, Jurassic 5*,
Cretaceous io#; and the Tertiary and Quaternary together are given 15$,
io# to the former and 5$ to the latter.

This, it will be seen, assigns to the Paleozoic, Mesozoic, and Cenozoic,.
respectively, 65^, 20$, and 15*, or 13, 4, 3 as the time-ratios, Dana's revised
estimate (1895) being 12, 3, I, and Walcott's estimate stands 12, 5, 2, as ex-
plained in the third chapter.

Vertical lines are drawn to separate off the time-scale into periods with these
proportions. Vertical extension of the curved lines represents the number of
new genera of each period. The curve m running highest is the curve of generic
differentiation for the Order Madreporaria, and is compiled from the lists of
genera in Zittel's "Handbuch," with some corrections based upon facts appearing
since its publication, and the geological range there assigned to them, with a
rearrangement of the genera of the first family of the Hexacoralla, Favosi-
tidae, into Favositidae and Poritidae.

FOSSILS— THEIR NATURE AND INTERPRETATION. 8/

The differentiation curve is formed by making a vertical scale and placing
the point representing the differentiation for each period above the base-line by
the number of divisions corresponding to the number of new genera initiated
during the period. In the same way separate differentiation-curves are formed
for the genera of several of the families: thus / is the curve for the Favositidze; a,
the curve for Astraeidae; b, for the family Turbinolidae.

Paleozic Time

Ordovician Silurian

Carboniferous

in-

Mesozoic Cenozoic

U8\ /Trl. Jur. gretaceouk /Tertiary Qy.K

e

\J

m

FlG. 5. — Evolution curves of the families of the Madreporaria. The vertical lines represent the
points of time separating the several geological eras of which the names are at the top of the
chart. The horizontal lines represent, by tens, the number of new genera first appearing in
each era. The curved lines represent the rate of differentiation of each family type in
number of genera first appearing in each successive era. mm' evolution curve for the whole
Madreporaria, tt' for the Tetracoralla, hh' for the Hexacoralla, ff for the Favositidas, aaf
for the Astraeidae, bb' for the Turbinolidae.

Meaning of these Evolution Curves. — This diagram illustrates
the following points : The curves express the rate and the de-
gree of differentiation of generic form expressed in the subor-
der Madreporaria in geological time. This law for the whole
group is expressed in curve m. The irregularities of the
curve suggest at once that it is compounded of at least three
independent curves, of which the nodes are at the close of the
Silurian, Jurassic, and Tertiary, and this suggestion is verified
by examination of the taxonomic classification. There we

88 GEOLOGICAL BIOLOGY.

find that the systematists, studying the structure, have divided
the genera into 13 families, grouped in two divisions, Tetra-
coralla (2), Hexacoralla (n), and the curves for each of these
is separate. Thus we find that the curve of differentiation
for the genera of the Favositidae (curve ff of the diagram),
which begins with the Ordovician and ends with the Paleozoic,
accounts for the main features of the Paleozoic part of the
differentiation of the whole suborder. Although other families
have their beginnings in the Paleozoic, it is only with a few
genera.

If we examine the curve for the genera of the family As-
traeidae (aar on the diagram) it is evident that the chief dif-
ferentiation for the early Mesozoic was within this family.
This family and the Fungidae will nearly fill out the total
differentiation-curve. The third irregularity in the curve is
again explained by the late culmination and differentiation of
the family Turbinolidae, bbr , which shows its first genus in the
early Mesozoic (Lias), but presents 17 new genera as late as
the Tertiary.

Chronological Value of Family Groups of Genera. — Thus it
appears that groups of genera are not only families according
to the taxonomist (that is, genera naturally grouped together
because of the likeness of their general structure), but the
genera composing them are naturally associated together by
the time of their initiation among the organisms of the world ;
and the simple tabulation of the time-relations of the genera
of an order reveals, by the irregularities of the curve of differ-
entiation, that the order is made up of several families having
separate evolution-curves or separate life-histories.

The Life-period of a Genus. — The numbers thus given do not
refer to the same genera repeated, but in large measure to
different genera for each system. Without going into details,
this may be illustrated by the following statement : Of the
genera above tabulated 182 are peculiar to a single geological
system, 89 are found in only two contiguous systems, 40 have
a range of three systems, and only 9 range through four sys-
tems; or, to express the fact in proportionate numbers, the
life-period, or geological range, of f of the known 448 genera
of Stony-corals is not greater than that of a single geological

FOSSILS— THEIR NATURE AND INTERPRETATION. 89

system ; -J- of the genera have a life-period of one or two sys-
tems length ; -fa of them lived only through two periods and
into a third ; and only 9, or ^ , continued existence for more
than the length of three geological systems.

Organisms express Evolution in their Geological History ; a Fun-
damental Law. — These statistics are chosen only as a conven-
ient illustration of a general law, which might be illustrated
by any other group of which we have the facts. Without
stopping to ascertain what the particular nature of the forms
is, it is evident that divergence of organic form is intimately
associated with lapse of time. We do not require to see every
form that has lived on the earth to distinguish the working of
this law ; but the few imperfect evidences, as well as the fuller
particulars we know respecting some of the better preserved
organisms, emphasize the presence of the law whenever we
examine the facts. Thus we are led to conclude that mor-
phological differentiation (evolution) is as^ characteristic of the
history of organisms in geological time as organic growth (de-
velopment) is characteristic of the history of the individual
organism in its lifetime.

The Meaning of Genus and Species. — We have been speaking
of combinations of form which are defined as classes, orders,

FIG. 6. — Favosites niagarensis^ Hall. Original figures of the fossil coral from the limestone on-
Goat Island, in Niagara River : a, fragment of the coral showing the ends of the corallites ; b.
a magnified view of two corallites, showing the dissepiments and the perforations of the walls ;
c, end view of the corallites, showing the walls and perforations. (After Hall.)

genera, or species, and of genera as living at a particular time,
and having a particular range, and differing one from another.
In the study of fossils we do not actually see species and

90 GEOLOGICAL BIOLOGY.

genera, or classes, or subkingdoms ; but we see only certain
shells, or impressions, or marks on the rocks : we say these
fossils represent animals that have lived, and we give them
particular generic and specific names. To take an example,
we find a specimen in the Niagara limestone, illustrated in
the accompanying figure. (Fig. 6.)

The Fossil Coral, Favosites niagarensis, as an Illustration. — It
was named Favosites niagarensis by Hall, which means that
its generic characters are those of the genus Favosites, its
specific characters those of the species F. niagarensis, and
that it was described by the paleontologist James Hall. It is
a fossil coral (Actinozoan), of the order Zoantharia.

Analysis of the elements of form, which must be observed
in classifying the specimen, will reveal somewhat more dis-
tinctly what is meant by saying that organic form and lapse
of time are intimately associated. We notice, in the first
place, that the fossil is made up of a large number of polygo-
nal calcareous tubes attached together by their outer faces.
This peculiar structure is the evidence for placing it in the
order Zoantharia. Living corals (Zoantharia) secrete calcare-
ous tubular bases, in and upon which each Zooid is supported,
and in living corals these corallites are aggregated in the same
manner as in the specimen before us. The radially sym-
metrical structure of the corallites is sufficient evidence that
the specimen belongs to the subkingdom Coelenterata, and
we know of the existence of this subkingdom in the first or
Cambrian period.

The continuous, hard, calcareous skeleton shows the fossil
to be a Madreporarian, the structure of whose soft parts we
assume to have been that of living Madreporarians, and there-
fore to be one of the class Anthozoa which is characterized as
"polyps with oesophageal tube and mesenteric folds, with in-
ternal generative organs (no medusoid sexual generation)."
The septa, which are rudimentary in the species before us
(see Fig. 8), are twelve, and this character distinguishes the
specimen from the subclass Tetracoralla, in which the septa
are grouped in multiples of 4, and from the order Alcyonaria,
which has 8 tentacles; and they show it to be an Hexactinia
(or Hexacoralla), in which the septa are six or some multiple

FOSSILS— THEIR NATURE AND INTERPRETATION. 9 1

of six. True Hexacoralla have not been discovered below
the Ordovician, or second geological period.

The diagram Fig. 7 illustrates the fundamental elements
of a coral (Hexacoralla).

,s

FIG.

FIG. 9.

basal plate ;
mesoglcea ;

FlG. 7.— Diagram of the structure of a coral : ap — exotheca ; hs = mesentery ; fp = 1
ss = septa ; parts in white = calcareous skeleton ; shaded = ectoderm ; black =
dotted = endoderm. (After McMurrich.)

FlG. 8. — Diagram of an end view of a single corallum of Favosites, showing the rudimentary sep-
ta M, the dotted lines indicating the probable arrangement of the mesentery and the position
of the mouth opening o,

FlG. 9. — Diagram of two chambers of a corallum of Favosites with the perforated walls, and the
transverse dissepiment or tabulae, tt' ', separating the chambers.

The calcareous tube or support of each animal (polyp) is
the corallum, the wall (af) is the theca, the longitudinal parti-
tions (ss) are the septa. The septa radiate toward the centre
and are in multiples of 4 in the forms called Tetracoralla and
in multiples of 6 in the forms called Hexacoralla.

The characteristics of the Hexacoralla cup are expressed
in the specimen before us, the Favosites niagarensis ; the
septa are, however, in only rudimentary condition, appearing
in the fossil forms only as faint ridges or rows of spinous pro-
jections on the inside of the tubes, as in the diagram Fig. 8.
The transverse partitions (see Fig. 6, b) are basal plates, con-
structed as the corallum grows upward for the animal to rest
upon, and are called tabulae or dissepiments. The Favosites
are characterized by the prominent development of tabulae,
from which character the corals of this type are called
tabulata. The specimen presents another character (see Fig.

92 GEOLOGICAL BIOLOGY.

6) ; we notice the prismatic form of the corallites, their close
crowding together to form a massive colony, like a honey-
comb, and the septa are rudimentary, or reduced to mere
striae on the inside of the theca, and still further we observe
that the theca are perforated by minute holes, and that the
tubes are horizontally partitioned off by tabulae, making each
to consist of a series of superimposed chambers. These
several morphological features are characteristic of the family
Favositidae, and we say, therefore, that the family to which
the specimen belongs began in the Niagara, and 21 genera
are assigned to this family, all restricted to the Paleozoic time.
The specimen is also a particular kind of the Favositidae ; the
coral is massive, the corallites are closely approximated and
sharply polygonal, mostly six-sided, the pores are regular and
of definite circular form, the tabulae are regular, of nearly
equal distance apart throughout the length of each corallum,
and the septa are but rudimentary pseudosepta, and twelve
in number. This is a more restricted combination of mor-
phological characters and distinguishes the genus Favosites.
The genus is limited in range to the Paleozoic, and in the
genus there are 53 species found in American rocks. Each
of these species has some special mode of growth or size of
corallum, or other distinguishing morphological characters,
and each species is confined mainly to a single geological
epoch, or, at greatest, to a single period ; to the Niagara in
the case of F. niagarensis, or to the Hamilton, as F. dumosus,
Winchell.

Geological Eange and Taxonomic Ranks of the Characters. —
Thus, we may say of Favosites niagarensis, Hall, that its
specific characters (speaking only of morphological characters,
or the arrangement of matter in a particular mathematical
shape) are characteristic of the geological time when the
Niagara series of rocks were forming, that is, the lower part
of the Silurian system, or the Eosilurian period of time. Its
generic characters — viz., the massive polygonal tabulate coral-
lites, however, have a longer range ; they began in the Silurian
and range through the Devonian and Carboniferous eras.
Again, its family characters — viz., the perforation of the walls,
one of the characters of the Favositidae — range from a little

FOSSILS— THEIR NATURE AND INTERPRETATION. 93

earlier and appeared in the Ordovician era, but ceased with
the Paleozoic time, and its subordinal characters — viz., the six
primary septa — date back as far as the Cambrian era, and are
being repeated in the generation of species living at the pres-
ent time. Thus, in the case of an individual specimen of
Favosites niagarensis, we can point to one character and say,
This character continued to reappear in other individuals until
the close of the Niagara era, then it ceased ; of another, This
character continued to reappear until the close of the Paleo-
zoic time ; and of a third character, It is still appearing in
individual organisms now living in the ocean. The facts in
the case may be graphically expressed by the following table :

TABLE EXPRESSING THE GEOLOGICAL RANGE OF THE CHARACTERS

OF THE FOSSIL FAVOSITES NIAGARENSIS (HALL), ARRANGED

ACCORDING TO THEIR TAXONOM1C RANK.

Specific characters

Generic " (Favosites)

Family " (Favositidee)

Group " (Hexacoralla)

Subordinal " (Madreporaria)

Ordinal « (Zoantharia)

Class " (Anthozoa)

Branch " (Ooelenterata)

Time- values of the Characters of an Individual Differ according
to their Taxonomic Rank. — We learn from this analysis that any
particular fossil represents a particular living animal, whose
time of living was identical with that of the formation of
the rock in which it was buried ; also that the fossil ex-
hibits morphological characters of various taxonomic rank,
and these characters have a time-range quite of the same
order as their taxonomic rank. In any particular organism,
fossil or living, the characters of highest rank in classification
are historically the oldest, and the characters of lowest taxo-

c

o

s

D

Cr

T

J

K

Ty

QR

—

94 GEOLOGICAL BIOLOGY.

nomic rank, as the specific characters, are of most recent
origin and their geological range is of the shortest duration.
In studying fossils, therefore, and using them as time-indi-
cators, or studying the history of the organisms represented
by them, it is all-important to notice the taxonomic rank of
the morphologic characters under consideration, since it is
true that the less the taxonomic value of the character the
sharper and more diagnostic is its time-value.

Although the successive eras are distinguished by change
in the specific and in the generic types of organisms, and it
may be supposed that some of them at each era are directly
descended from those of different species of a previous era,
it is not so clear that the succession should present any
analogy to the succession of morphologic form exhibited by
the individual in its various stages of growth, as will be seen
by the following considerations.

Stages of Growth in Ontogenesis. — In the growth of the
individual there are certain stages called (i) infantine, or
larval, (2) adolescent, (3) adult, (4) senile, which may be
sharply distinguished by morphological characters, and dur-
ing the life of the individual by distinct physiological opera-
tions. These stages are found by Hyatt and others to be
so characteristic of the period of time in the growth as to be
precisely named ; Bather* has called them terms of auxology.
Hyatt, in a later article, f suggests the propriety of using
the term bathmology, first proposed by Cope, for this
classification of the stages of individual growth. The
technical names proposed by Hyatt are slightly modified
by Bather, and are as follows, viz., the infantine or larval
stage or form is called embryonic and brephic, the adolescent
stage is called neanic, the adult stage is ephebic, the old age
or senile stage of development is called gerontic, with a de-
clining, catabatic, and an hypostrophic or atavic substage.
Bather proposed the application of these terms to the tem-
poral stages in individual development by the addition of the
prefix morpho — thus morphephebic — to denote the characteristics

* ZooL Anzeigcr, Nov. 14 and 28, 1892, pp. 420, 424.
f Proc. Boston Soc. Nat. Hist., xxvi. p. 61, etc., 1893..

FOSSILS-THEIR NATURE AND INTERPRETATION. 95

of the adult ; and the prefix phyl — thus phylephebic — to denote
the characteristics of adulthood in racial evolution, assuming,
as these authors do, that races in evolution have their charac-
teristic stages corresponding to the stages of development of
the individual. There can be no doubt that in the growth of
the organism there is this general law of progressive change
of form and structure with its embryonic, adolescent, adult,
and senile stages, more or less distinctly marked. To this
process of progressive morphological change observed in the
growth of the individual the term ontogenesis has been ap-
plied.

No Successive Stages of Functional Activity seen in Phylo-
genesis.— A comparison of living forms with fossils arranged
in series in the order of their sequence in the rocks (i.e., chron-
ologically) has led to a belief that races, like individuals,
have their beginning, adolescence, maturity, and old age, and
the term phylogenesis was suggested by Haeckel to express
this idea. The fact must be emphasized, however, that in
individual development there is a change of function associ-
ated with the several stages of ontogenesis; while it is diffi-
cult if not impossible to imagine any corresponding change
of function in the successive representatives of a common race,
and while there are many analogies between the stages of
development of ontogenesis and the stages of evolution in
the history of organisms (pnylogenesis), great caution is neces-
sary not to force this theory of correspondence between the
ontogenetic stages of functional activity and the order of
differentiation of new characters expressed in the phyloge-
netic history of organisms.

Contrast between the Developmental Stages of the Individual
and the Succession of Species. — The two series of phenomena
present this marked contrast, that in the one (ontogenesis)
each particular phase of development is a repetition of phe-
nomena which have been repeated in the same way from the
beginning of organic life; in the other (phylogenesis) each
change is a step in advance of anything that has occurred
before ; the series is a single progressive series, with modifica-
tions and increment, but with no cycles of repetition. De-
velopment begins anew with each individual organism.

$ GEOLOGICAL BIOLOGY.

Evolution was already progressing as far back as we can find
fossils, and appears to be going on still. Organic develop-
ment repeats itself over and over and over again, producing
cycles of changes, each one of which constitutes the life-history
of an individual organism, each cycle with almost impercepti-
ble variations, the same from generation to generation in each
series ; but organic evolution, although it is by slow pro-
cesses, constitutes a continuous series ; there are no repetitions
in the series. Looked at from the point of view of our knowl-
edge, the series had a beginning, and the evolution has been
continuous since the beginning, and is not stopped to-day.

But the evolution has been an evolution entirely of form
and function, not of substance. The same substance, that is,
matter, has been used over and over again : the materials
have preserved the same chemical and physical properties,
have been temporarily built up to form new combinations,
have taken organic form, have performed their function, have
died and gone back to their simple condition again. As far
as can be ascertained, no change has taken place in the nature
of matter ; what it is to-day matter has been as far back in
time as science can penetrate.

Evolution an Organic Process, and not Applicable to Inorganic
Things. — Thus we reach the undeniable conclusion that or-
ganisms, which fossils represent, are something unique and
distinct from other things in nature. The physical constitu-
tion of matter presents no evolution. What it is, it was back
into the mists of eternity. Chemical properties of matter
offer no law of evolution. We interpret the chemistry of the
sun, or the most distant stars, by the same tests we use in
our working laboratories upon the things about us. The
crystalline properties of minerals offer no evolution ; the
angles of a quartz crystal and the system of its crystallization
in the Archaean granite are precisely those exhibited by quartz
crystallizing at the present day.

Fossils furnish the Direct Evidence of Evolution. — Fossils.first
exhibit to us true evolution; and this evolution, which we
recognize as an orderly sequence or progress of events, be-
comes the fundamental characteristic of organisms, and is an
essential peculiarity of organic activity. Fossils not only

FOSSILS— THEIR NATURE AND INTERPRETATION. 9/

represent organisms, but fossils alone record for us and reveal
to us the actual laws of organic evolution. But the paleon-
tologist has ever to bear in mind that he has only the records,
not the living organism, for study ; and he has to look to the
zoology and botany of living organisms for the interpreta-
tion of his records.

Living Organisms furnish Direct Evidence of Purposeful Devel-
opment.— The zoologist finds the organism to be essentially a
machine accomplishing a multitude of acts, which he calls
functions, because every act of the organism appears to be
purposeful, the end seems to be more essential than the
means, and the organism grows to be a complex structure,
with a variable number of parts, each constructed with adap-
tation to the function to be performed. This is what is
found upon analysis of the living individual ; the organism is
already active, performing its functions, and building or con-
structing parts for the fuller performance of those functions,
or for performance of other functions. As the individual
organism is seen in activity, the changes it undergoes, or
technically its development, is seen to be definitely pur-
poseful.

When it is compared with other organisms it is looking
forward to distinct functions to be performed in the future,
and when we look backward along the course of its develop-
ment we see it arising in the midst of a perfected individual
like itself, and it imitates in its development the very steps
taken by this earlier organism. Because of this imitation,
because of a repetition of what was before, we assume this
ancestral model to have determined the particular form, and
function too, of the newly arising individual. In all this
study we find the living organism to be incessantly changing.
If we make histological examinations we find every particle
changing, but relative integrity and solidarity of some of the
parts which perform definite functions is preserved. These
parts are called organs. These organs are the parts of the
machinery with which the individual works. The active,
living individual is thus between two forces. The ancestry
behind it determines its development, but the conditions into
which it comes determine it from before, and the product is

98 GEOLOGICAL BIOLOGY.

the resultant of these two forces, ancestry and environment,
working together.

The soft, active organs are the chief parts of study for the
zoologist; they best express the stages of ontogenetic devel-
opment, but the characters of the hard parts best record the
phylogenetic evolution. So long as there is plasticity in the
characters themselves there is possible adjustment, but when
we find a rigid resisting body formed, it expresses a perma-
nent step taken in the evolution and established.

Fossils and Geological Biology. — Geological biology treats of
the organism as a unit, with its relations to its ancestors, to
its race, to time, and to environment ; zoological biology
treats each organism as a complex bundle of organs with
their numerous functions adjusted together, but ever distin-
guished by their specific histological and anatomical peculiari-
ties. The zoologist studies organs and functions as they are
combined in the individual organism ; the paleontologist
studies varieties and species as they are combined to make up
faunas and races, and as adjusted to the varying conditions of
time and place. In studying a fossil he asks, not only and
not chiefly, what place has it in systematic classification ? but
how is it related to what has gone before, and what is its
ancestry? and how is the organism related to what follows,
or of what is it prophetic?

These questions lead us to seek such characters as will
indicate, first, genetic affinities and, second, effects of environ-
ment.

Hard Parts express both Relation to Environment and Relation
to Ancestry. — For these purposes the hard parts are of the
greatest value, and why? The hard parts are such as teeth;
organs of offence and defence, as horns, hoofs, spines, scales,
shells ; and skeletons, external and internal. They represent,
not the active vital part of the animal, but some part built
up between the living animal and nature ; hence they have
an outer and an inner surface, the outer suffers degradation
with use ; the inner expresses the form assumed by the ani-
mal in the natural function of animal growth. Fossils are
the result of growth, and hence express the final morpho-
logical result of the living individual. As hard parts they

FOSSILS— THEIR NATURE AND INTERPRETATION. 99

express the effects of struggle with environment more accu-
rately than do any others, for it is with the hard parts that
the animal has met environment, struggled with and resisted
it ; hence, fossils, so imperfect as evidence of the anatomical
structure of the organisms, are the best of evidence of the effect
of the interaction between the forces of ancestry, working
through the laws of generation tending to repeat the ancestral
characters, and the forces of the environment working through
the laws of struggle for existence in modifying those characters
by adjustment.

Kinds of Hard Parts of the Animal Kingdom preserved as Fos-
sils.— As we deal, then, with the hard parts only, a few words
will be said regarding the kind of hard parts which are found
in the several classes of the Animal Kingdom.

We glance over the Animal Kingdom and see that there
are large groups of animals now living, which, if they were to
die and every advantage were offered for their preservation in
their natural habitat, would leave no trace of their existence
a year after their death. It is important, therefore, to learn
at the outset to what extent the paleontological record will
be found silent because of impossibility of preservation of the
evidence.

Protozoa. — Among the lowest group of animals, the sub-
kingdom Protozoa, the Gregarinidae, found mainly within
other animals, would be absent because they form no hard
parts nor framework which could be preserved.

Among the Rhizopoda, differing from the former class in

FIG. 10.

FIG. ii.

FIG. 10. — Foraminifera. Globigerina bulloides d'Orb. Miocene. (S. and D.)
FIG. ii.— Radiolaria. Stichocapsa Grothi Rust. Jurassic. (S. and D.)

the possession of pseudopoda, and leading a more active and
independent life, the orders of Monera and Anmba, as far as

100

GEOLOGICAL BIOLOGY.

known, do not develop any structure which would be likely
to escape disintegration and resolution in the ordinary process
of fossilization. But the other two orders of the class, Fora-
minifera, Radiolaria, develop hard skeletons of lime or silica,
and great numbers of them are preserved in a fossil state.
The Infusoria (a higher class than the others, in the possession
of mouth and vibratile or contractile cilia) are not known to
exist in a fossil state, though now abundant under proper
conditions, and though most probably they lived in like con-
ditions back to earliest geologic time. Figures 10, 11.

Coelenterata. — Of the Ccelenterata the classes Spongia and

FIG. 12. — Spongia. Astylospongia prcemorsa Gf. sp. Silurian. (S. and D.) A, vertical sec-
tion ; B, lateral view ; C, silicious skeleton, greatly enlarged.

Anthozoa and the Hydroid Zoophytes (Hydrozoa) are repre-
sented. All of the orders of the Anthozoa have families
producing some hard parts, " corals," which are preserved in
the rocks, but in each order there are some families not devel-
oping calcareous skeletons, hence not preserved ; and in the
Hydrozoa (class) several orders and a few whole subclasses
(as the Lucernaridae, Siphonophora, etc.) are of such a nature
as to be wanting in any geologic record, and therefore in so
far the history of the Ccelenterata is necessarily imperfect.
However, Corals are among the most abundant fossils, and
Graptolites (related probably to the Hydroid Polyps, or

FOSSILS— THEIR NATURE AND INTERPRETATION. IOI

Sertularidae) are also abundant in a few zones in the Paleo-
zoic rocks. Figures 12, 13, 14.

FIG. 13.

FlG. 13. — Graptolite. Diplograptus palmeus Barr. Silurian. (S. and D.)

FlG. 14. — Coral. Parasmilia centralis Mant. sp. Cretaceous A, corallite, longitudinally
sectioned ; 2>, the same seen from above ; s and 1-5 = septa, c = columella.

Echinodermata were represented in fossil form, developing
some hard parts in each order, viz. : Crinoidea, Blastoidea,
Cystidea, Ophiuroidea, Asteroidea, Echinoidea, and even the
Holothurioidea probably recognized in the spiculae. The
Solecida (parasitic worms, whether grouped with the Echino-
dermata, or with annelids under Vermes) are all soft, and do
not come within the province of the paleontologist. Figures
15-19.

Vermes. — Among the Vermes (the leeches, earthworms,
and sea-worms) there are some which produce earthy cases
of mud, others have left their tracks where they bored
through the tenacious mud ; also teeth have been found, sup-
posed to belong to thrs group. (See Serpula, Spirorbis, etc.)
Still, these are rare fossils, and probably represent but very
imperfectly the worms living in ancient seas. Figure 20.

Arthropoda. — Of the Arthropoda, including all those ani-
mals composed of definite segments arranged longitudinally,
one behind the other, and the locomotor appendages of which
are jointed or articulated to the body, we have four great
classes: Crustacea, Arachnida, Myriapoda, Insecta. All of
these produce a more or less enduring, horny or calcareous crust
or case, within which the soft parts are contained, making the

102

GEOLOGICAL BIOLOGY.

a 1 "

*l __

<$W C5 "i* C^ ^^

>m ^

,ji 1 1

ScTuufeabtrgtr.ge*.

FIG. 16. FIG. 1 8.

FlG. 15. — Echinodermata, Crinoid. Taxocrtnus multibrachiatus Ly. and Cass. Carboniferous.
Above: calyx with stem. Below: the plates of the calyx dissected, st = stem, br — free arms, air
= anal interradial plates ; rh = right posterior, Ih = left posterior, vr = anterior right, ?'/ =
anterior left, vu = anterior medial radial plates ; irk = right posterior, ilk = left posterior,
irv = right anterior, ilv = left anterior interradial plates ; z/", infrabasalia ; pb = parabasalia ;
fj—^4 = radialia ; </I, d\\ — distichalia, first and second rank ; br = brachialia ; a\-a^ = anal
plates ; *>,-/> 8 = larger interradial plates ; t — smaller interradial plates ; ss, plane of sym-
metry. (After S. and D )

FlG. 16. — Blastoid. Pentatremitidea Eifeliensis F. Ro. sp. Devonian, b = basalia ; r =
radialia ; p = ambulacra. (After S. and D.)

FlG. 17. — Cystoid. Caryocrinus ornatus Say. Silurian. A, calyx with stem s \ br = arms ; I,
II, III, first, second, and third zones of plates of the dorsal capsule ; / = porous plates ; i =
place of attachment of arms ; a = anal opening £, view of the ventral dome, c = central
summit plates, i and a as above. C, inner surface of a plate of the second row, showing the
pores of the Hydrospires (/*) and their connecting canal (c). (After S. and D.)

FlG. 18. — Ophiuroid, A-H. Ophioceramis ferruginea Bohm. Jurassic. _ A, a complete
specimen, from under side ; br = arms. B — the disk from the inner side ; bl = bursal
shields ; / = ambulacral pores. C? mouth-skeleton from below; miu = mouth-angle; / =
papillae; me = angle-plates; ms = side shields; m = oral plates; b = second ventral plate. Z>,
disk from above; r = dorsal shield. E, a part of an arm from below; b = ventral shields;
s = lateral shields; / = ambulacral pores. F, a part of an arm from above; r and s as above;
st — spines G, the same, lateral view. //, cross-section of an arm. /, Geocoma planata
Qu. sp.; bs = bursal slit; bl = bursal shield. (Steinmann and Doederlein.)

FOSSILS— THEIR NATURE AND INTERPRETATION,
possibility of fossil remains. But, except in the case of

FIG. 19. FIG. 20.

FIG. 19. Echinoid. Botkriocidaris Pahleni Schm. Ordovician. A, side view ;«= anal

opening ; am = paired ambulacral plates with two double rows of pores and small spines ;

ia — single row of interambulacral plates. Z?, summit region with the anal opening (a). C,

under side, with the mouth opening.
FIG. 20. — Vermes, Annelida. Serpzila. A, S. (Spirorbis) omphalodes Gf. Devonian. B, C,

S. (Galenlaria) socialis Gf. Jurassic. C, cross-section of the tubes. Z>, Serpula gordialia

Schl. Cretaceous. E, S. (Rotularia) spirulaa Lara. Tertiary.

Crustacea, it will be observed
that the animals belonging to
these classes live mainly on land
and in the air, and when we bear
in mind that fossilization is a
process usually requiring water
for the preparation of the matrfx
(sand, mud, gravel, etc.), and for
the covering of the body with
the material when prepared, it is
evident that all land and aerial
animals, although possessing
parts capable of fossilization,
and living in abundance, run
very small chance of being
found in the deposits made
under water, in which fossils are
mainly preserved. Hence Crus-
tacea, being water animals, are
preserved as fossils in con-
siderable numbers, while the
other classes of Arthropoda,
that is, insects, spiders, and
Myriapods, although occasionally found, are

Trilobite,

Silurian.

FIG. 21. — Arthropod, Crustacean.
Calyinene Blumenbachi Bgt.
k = cephalic shield ; r = thorax ; s = pygi-
dum ; gl = glabella ; iua = cheeks ; ww'
= free part of the cheek ; n = facial su-
ture ; / = border ; a — eyes ; st = frontal
lobe; sf= lateral furrows; nf= neck-fur-
row; tf/'= occipital furrow ; nr = neck-lobe;
or = occipital ring ; rf = dorsal furrow ;
rf— marginal furrow; sp= axis;^/= plurae;
a, ax = axis; $', si = lateral lobes of the
pygidum; 1-13 = the 13 thoracic segments.

rare, and prob-

104

GEOLOGICAL BIOLOGY.

ably represent in only the most meagre way the forms of
these classes which lived in past ages. Figures 21, 22.

FIG. 22.— Arthropod. A, Pterygotus anglicus Ag. Devonian. Dorsal view. B, Pt. osiliensis
Schm. Silurian. Under side of head. k = cephalic shield ; r — thorax ; .$• = abdomen ; a =
eyes ; o — eyelets ; f\-f* = cephalic appendages ; 1-6 = thoracic segments ; 7-13 = abdom-
inal segments; t = terminal segment or telson ; e/> = epistoma ; kl = masticating plates of the
sixth pair of appendages ; m = metastoma ; z = median plates ; « = median suture.

Molluscoidea. — The Molluscoidea, including the Polyzoa
and the Brachiopoda, is a group of much interest to the Pal-
eontologist. The Brachiopods are well preserved, and are,
perhaps, from the point of view of the scientific paleontolo-
gist, the most important group of animals he is able to study.
Of their history, the record is more complete, the condition,
as a whole, more perfectly preserved, the missing links fewer,
than for any other group. They have been studied more
thoroughly, are of greater value as marking geological hori-
zons, probably/than any other. They develop a chitonous
'or calcareous bivalved shell, the external and internal form of

FOSSILS— THEIR NATURE AND INTERPRETATION. 10$

which, and the intimate structure of the shell substance, are
generally well preserved. Figures 23, 24.

FIG. 23.— Molluscoida, Bryozoa. A, B, Fertestella retiformis Schl. Permian. A, a funnel-shaped
stock from the outside. B, enlarged view showing the cell mouths (o) and the perforations (/)
between the cell-rows of the ccencecium. C, Archimedes wortheni Hall. Carboniferous.
The stock consists of a broad coencecium (£/), wound spirally about a central axis (a). Frag-
ments of the ccencecium separate from the axis present a structure similar to that of Fenes-
tella.

FIG. 24. — Molluscoida, Brachiopoda A, B, <?, Inarticulata. i^ingula. A, L, anatina Brug.>
living, pedicle valve from within, st = pedicle ; s, d, a, j', muscular impressions. B, L.
tenuissima Br. Triassic. C, L. Beani Phill. Jurassic. Z>, E,, Brachiopoda articulata.
Atrypa reticularis L. sp. Devonian. Z>, surface view of brachial valve. £, view of in-
terior, the brachial valve being in great part removed \f=- foramen for passage of the pedicle;,
cr = crura ; b — jugal processes or jugum ; sp = spires or spiral coils of the brachidium.

Mollusca. — Of the true Mollusks, all the four classes,
Lamellibranchiata, Gastropoda, Pteropoda, Cephalopoda,
construct, in most of their genera, calcareous or horny shells,
external or internal, which are preserved, more or less per-
fectly, in a fossil state. Gastropods and Lamellibranchiates
in the older rocks are very apt to be in the condition of im-
pressions and moulds, the substance of the shell being dis-
solved and carried away ; this is also the case with many

io6

GEOLOGICAL BIOLOGY.

families of the other two classes, so that very much is want
ing to a complete record of these classes. Figures 25, 26, 27

FIG.

FIG. 26.

FIG. 25. — Mollusca, Lamellibranch. Venus multilamella Lmk. Tertiary. A, a right valve,
outer surface ; lu = lunula. /?, the same interior. C. hinge of the left valve, m' = anterior,
n' — posterior, muscular impressions ; nib = pallial sinus ; / — ligamental pit ; mz = cardinal
teeth.

FIG. 26. — Gastropod. A, Paludina pachystoma Sdb. Tertiary Miocene. £, P. avellana Neura.
Plidcene.

FIG. 27. — Cephalopod. Ceratites nodosus d. Haan. Triassic. A, complete shell from the
side ; B, front view of the same ; inr = rim of the outer chamber ; ss, ss^, ss^ hs = saddles of
the sutures ; <?/, s^ sl^ hi = serrate lobes of the suture lines.

Vertebrata. — Of this branch there is scarcely an order that
does not develop hard parts of some kind, which might be
preserved in fossil condition under favorable circumstances.
Among the lowest orders (Lancelot, Hag-fish, Lampreys)
there is nothing likely to be preserved, except small teeth.
In the cartilaginous fishes teeth are the main parts of suf-
ficient hardness to resist decay and disintegration, while the

FOSSILS— THEIR NATURE AND INTERPRETATION. IO/

bones and scales of other fishes are hard and enduring if well
buried under water, but are easily destroyed if left exposed in
contact with the atmosphere for a long
time. So again, while many fish and
reptiles and a few mammals are in-
habitants of the ocean, birds and most
mammals and many reptiles are in-
habitants of land, and many fish and
reptiles are fresh-water species only.
Again, the remains of Vertebrates are
subject to the destructive agency of
lower animals and of themselves, so
that it is not to be supposed that under
the most favorable natural conditions

FIG. 28.— Vertebrate. Fish. Lepidotus elvensis Blv. Jurassic.
a = anal fin ; c — caudal fin, hemi-heterocercal ; d = dorsal
fin ; p — pectoral fin ; r> = ventral fin ; f— fulcra (on the
front edge of all the fins) ; k — gill-covers.

FIG. 29. — Vertebrate, Amphibian.
Brachiosaurus ainblystomus
Credn. A young form (B. gra-
cilis Credner). Triassic. co =
coracoid ; _/" = femur ; fi = fibu-
la ; h — humerus ; r = r adius ;
j = scapula; sr = sacral rib; / =
tibia; th.l = lateral and th.m —
medial thoracic plates; u = ulna.

FIG. 30. — Vertebrate, Reptile. Ichthyosaurus quadriscissus Quenst. Jurassic. Skeleton of a
young individual A = coprolite. (After Steinmann and Doederlein.)

anything more than the most meagre representation of the
vertebrate life of the world would be preserved in fossil con-
dition, and of those preserved, the more abundant would be
reptiles, fishes, and larger mammals, with a few birds. (Fig-
ures 28-32.)

Looking over the Animal Kingdom, in this general way,

io8

GEOLOGICAL BIOLOGY.

:T

Owen. Jurassic. Restored in the

position of the Berlin specimen, c = carpus ; cl = clavicula ? ; co — coracoid ; A = humerus«
= radius ; J = scapula ; u = ulna ; I-1V = 1-4 fingers. (After Steinmann and Doederlein.)*

FIG. 31. — Vertebrate. Saurura. Archceopteryx macrui
position of the Berlin specimen, c = carpus : cl = cla

FIG. 32. — Vertebrate, Mammal. Phenacodus Wortmanni Cope. Eocene.

FOSSILS— THEIR NATURE AND INTERPRETATION. 1 09

we find a few classes among the several subkingdoms, produc-
ing parts which could be preserved as fossils, but there are
reasons why even these are not present in abundance except
for a very few orders ; the rest may be represented by here
and there a specimen, but only rarely, and any conclusions
drawn from their study will be conjectural to the extreme.
In the study of the laws of organic history it becomes neces-
sary, therefore, to make judicious selection of those classes of
organisms whose records are sufficiently abundant and con-
tinuous to furnish the desired evidence.

Summary. — To summarize : When we study fossils in their
simple physical aspect, as mathematical forms in the rocks,
we find them presenting an orderly arrangement of sequence,
one after the other, in strict chronological order. When
classified by their likeness to each other into groups to form
natural species and genera, and when separated from each
other by their points of difference to form separate families,
orders, and classes, we find that there is the closest relation-
ship existing between the form they assume and the periods
of time when they lived. Taking a single suborder of the
Ccelenterata (the stony corals, or Madreporaria, with 448
known genera), of which fossil remains are found all the way
along, from the earliest fossil-bearing rocks to the sea-shores
of our modern ocean, we find all the genera relatively short-
lived, rarely exceeding the period of two systems in length of
duration, and the genera most nearly allied to each other in
form are always found in the systems chronologically nearer
to each other ; and uniting the similar genera into families,
the families presenting greater contrast are found farther sepa-
rated chronologically from each other than from the families
presenting less strong contrasts. When we carry our study
further and interpret these fossils as the remains of organisms,
and say that they represent living organisms, we come face to
face with the fundamental law of organisms, that is, the law
of change and variation. All organisms have a history. So
unchangeable are the physical properties of matter, so invari-
able are the laws of crystallization of minerals, and so con-
stant are the chemical properties of substances, that any
irregularity in any of them at once suggests the influence of

IIO GEOLOGICAL BIOLOGY.

organisms. This fact is apparent to every one, and it is no
new discovery in this age. Naturalists have for centuries
known !hat animals and plants grew, and have been seeking
for some mysterious living property by which to distinguish
living form and living matter: they have been examining
dead organisms, they have described hundreds of thousands
of different organisms, looking always for, and in most cases
only grasping, the dead products of life ; they have examined
the organic mechanism and observed its mode of action and
the results attained : but it is only recently that, under the
names development and evolution the fundamental character-
istic of all vital phenomena has become an object of serious
study and investigation. The morphological relations of
organisms have been thoroughly studied, but their time-
relations have only begun to be scientifically investigated.

CHAPTER VI.

GEOGRAPHICAL DISTRIBUTION— THE GENERAL RELA-
TION OF ORGANISMS TO THE CONDITIONS OF ENVI-
RONMENT.

IN the last chapter it was shown by an analysis of the
characters of the genera of Madreporaria — a group of organ-
isms well adapted to furnish this evidence (because of their
living under the same conditions required for the making of
the strata themselves, and producing hard parts, easily pre-
served from the earliest times onward) — that the form of an
organism has an intimate relationship to the geological period
during which it lived.

The natural conclusion from this observation is that the
order of sequence in the appearance of organisms is the ex-
pression of a natural law of their succession in time, or that
it is a law of nature for organisms to succeed each other in
this observed geological order.

We observed that the classification of organisms by their
morphological characters, as expressed in their arrangement
in the classes, orders, families, and genera of the zoologist,
shows that this relation of characters to time of appearance is
expressed in every detail of structure, and the more minute
our inspection the more distinctly is the truth of this princi-
ple brought to light.

A species or genus has not only a particular relationship
to other species or genera, but every genus has a particular
period in the time-scale when it lived, and a particular dura-
tion of geological time to which its living was limited, before
which it did not exist, and after which it failed to reappear.
This illustrates the general law that the particular morphologi-
cal characters assumed by an individual organism are immedi-
ately related to the ancestry which is behind it; but if we turn
our attention to the facts of geographical distribution, we

in

112 GEOLOGICAL BIOLOGY.

shall find that organisms present as close a relationship to the
conditions of the environment into which they are born.

The Importance of the Study of Geographical Distribution. —
Geographical Distribution is a subject which no one has studied
more thoroughly and with keener appreciation than Alfred R.
Wallace, and a quotation will, in a few words, express the
importance of the subject. He says: "So long as each
species of organism was supposed to have had an independent
origin, the place it occupied on the earth's surface, or the
epoch when it first appeared, had little significance. It was,
indeed, perceived that the organization and constitution of
each animal or plant must be adapted to the physical condi-
tions in which it was placed ; but this consideration only
accounted for a few of the broader features of distribution,
while the great body of the facts, their countless anomalies
and curious details, remained wholly inexplicable ; but the
theory of evolution and gradual development of organic forms
by descent and variation (some form of which is now univer-
sally accepted by men of science) completely changes the
aspect of the question, and invests the facts of distribution
with special importance." "The time when a group or a
species first appeared, the place of its origin, and the area it
now occupies upon the earth become essential portions of
the history of the universe. The course of study, initiated
and so largely developed by Darwin, has now shown us the
marvellous interdependence of every part of nature. Not
only is each organism necessarily related to and affected by
all things, living and dead, that surround it, but every detail
of form and structure, of color, food, and habits, must, it is
now held, have been developed in harmony with, and to a
great extent as a result of, the organic and inorganic environ-
ments. Distribution becomes, therefore, as essential a part
of the science of life as anatomy or physiology. It shows us,
as it were, the form and structure of the life of the world
considered as one vast organism, and it enables us to compre-
hend, however imperfectly, the processes of development and
variation during past ages which have resulted in the actual
state of things. It thus affords one of the best tests of the
truth of our theories of development [evolution] ; because the

GEOGRAPHICAL DISTRIBUTION. 113

countless facts presented by the distribution of living things, in
present and past time, must be explicable in accordance with
any true theory, or, at least, never directly contradict it." *

In studying the geographical distribution of organisms
the understanding of the nature of the conditions of environ-
ment can scarcely be overestimated.

The Natural Conditions of Environment — Nomenclature. —
There are various conditions of environment which modify the
growth and life of organisms. Among the chief of these are :
The (i) medium — air or water; (2) temperature — or climate
and limits of annual temperature ; (3) in water — the depth, the
purity, the salinity, the light, the motion ; (4) on land —
secondarily altitude as affecting climate and temperature ; (5)
the other organisms, because all animal life appears to require
other animal or plant organisms for its own food, hence (50)
struggle for existence; also (5^) the amount of organic food
determines the growth of higher organisms which require the
food. Medium and Habitat are the names applied to the
immediate conditions in which the organisms live. Province
is the name of the region occupied by a group of organisms
which are naturally adjusted to each other. Zone is the name
of the tract of sea-bed between boundaries of depth, variously
determined. Flora is the name applied to all the plants,
naturally associated and adjusted to the conditions of environ-
ment, of a particular province or geographical area. Fauna is
the name cf the group of animals so associated and adjusted.

Natural-history Provinces. — The primary classification of
the conditions of environment as affecting organisms is con-
sidered under the terms Terrestrial (land plants and animals),
and Marine (those living in the ocean). It is found that
the present life of the globe is divided into numerous floras
and faunas, the boundaries of which are not absolutely fixed,
cither in species or in conditions ; but the areas are distinct
in some of their features, and the association of organisms
is peculiar for each, although some of them may be com-
mon to neighboring areas. These provinces, both marine
and terrestrial, differ in their outlines for different kinds of

* Article " Distribution," (,th ed. Encyclo. Brit., vol. vn. p. 267.

114 GEOLOGICAL BIOLOGY.

organisms. To distinguish them they are called Natural-
history Provinces. We say, for instance, that the natural-
history province marking the distribution of flowering plants,
differs in its boundaries from the province marking the dis-
tribution of fresh-water Mollusca. The reason is apparent
when we note that the limiting cause of the distribution is
perhaps temperature and climate in one case, and community
of fresh-water channels in the other. The boundary of the
water-bed of a great river-system is the limiting cause of the
distribution of the Mollusca, the conditions of temperature
and rainfall that of the plants.

Normal Adaptation to Conditions of Environment. — We have
spoken of distribution as applied to organisms. This term
implies that each organism is normally adapted to a certain
set of conditions, which is called by the general name En-
vironment. Within limits the individual adjusts itself to*
slight change of the environment, but extreme change of the
conditions of environment restricts the possible living of the
particular organism, and for each particular organism the dis-
tribution is supposed to mark the particular extent of differ-
ing conditions in which it is normally adapted to live.

Specific Centre of Distribution and Varieties. — Theoretically,
each organism is supposed to be qualified to live under a
certain set of conditions, and to adapt itself to change of
those conditions to a greater or less extent. While geolo-
gists do not find a species to be determined rigidly by any
one criterion, general usage applies the name Species to'those
plants or animals which possess common morphological char-
acters, and are confined in their distribution to one natural-
history province (but taking this as a general definition, ex-
ceptions are recognized in the case of species distributed over
two or many provinces). Practically, too, each species ap-
pears to have a centre of distribution, at which point (or
specific centre) the combination of environing conditions are
the more favorable ; the species may be distributed from this
centre, but it is not so abundant outside, and is often seen to
present slight differences of form, size, color, or minor differ-
ences on the outskirts of the province of its distribution.
These differences from the typical form at the centre consti-

GEOGRAPHICAL DISTRIBUTION. 11$

tute what are called Varieties of the species. The conditions
of environment existing at a specific centre, or metropolis of
the species, as Forbes called it, constitute the normal habitat
for that species. The particular morphological and structural
characters which the species express are called its typical
specific characters. The modifications from these typical
characters which are seen in representatives of the species
on the borders of its specific distribution are its varietal
characters.

The Distinctness of the Flora and Fauna of Distinct Provinces.
— The species associated together in a natural-history prov-
ince are the flora and fauna of that province, and as generally
defined, not over one half of the species of two distinct prov-
inces are identical; or, to put it in the converse form, about
half the species of any province are distinct, or peculiar to
that province. Such a rule is purely arbitrary, and will vary
greatly as applied by different naturalists, but such a general
rule is applied in the distinguishment of the provinces of
marine species.

The Various Classifications of Natural-history Provinces. — In
the classification of provinces in Woodward's "Manual of
Mollusca" we find eighteen such marine provinces recognized,
and the land regions are defined under twenty-seven names.
Sclater (1857) defined six terrestrial regions, which were after-
wards adopted by Wallace (1876) * and subdivided into twenty-
four sub-regions. Fischer (1887) combined and extended the
former classification, and defined thirty regions distributed in
the following seven zones, viz. : Palearctic, African paleo-
tropical, Eastern paleotropical, Australian, Neantarctic, Neo-
tropical, Nearctic. Each of these regions is again subdivided
into sub-regions with their special faunas ; as, for example, the
region Circamediterranean is the second of the Palearctic
regions ; this is subdivided into the sub-regions (a) occidental
or Atlantic, (b) Meridional or Mediterranean (with the four
Faunas, Hispano-Barbaresque, Egypto-Syrienne, Hellado-
Anatolique, and Italo-Dalmate). (c) Centrale or Pontique,
(*/) Orientale or Caspique.f

* A. R. Wallace, " The Geographical Distribution of Animals," 1876.
f Paul Fischer, "Manual de Conchyliologie," Paris, 1887.

Ii6 GEOLOGICAL BIOLOGY.

Marine Organisms Particularly Important to the Paleontologist.
— Because of the fact that preservation of fossils is almost
entirely dependent upon the covering by water of the remains
preserved, the questions of distribution and environment of
chief interest to the paleontologists are those of marine and
fresh waters.

Haeckers Classification of the Marine Conditions of Life. —
Walther, in his " Bionomy of the Sea," presents a classifica-
tion of organisms according to their bionomic character, as
follows : The sum of the marine faunas and floras is called
Halobios, corresponding to them the fresh-water life is called
Limnobios, and the land organisms receive the name Geobios.
The Halobios, or marine organisms, are further classified
into (l) Benthos — those animals and plants living on the
sea-bottom, distinguished further as (a) sessile, ($) vagile,
(c) littoral, and (d) abyssal Benthos; (2) Nekton, or the life of
open sea, with strong powers of active locomotion ; and (3)
Plankton, the more or less passive life of open seas. Haeckel
(from whom Walther adopts the nomenclature) further sub-
divides the Plankton, or open-sea life, into the following five
groups: The neritic Plankton includes the swimming flora
and fauna of the coast regions of continents, archipelagos,
and islands; the oceanic Plankton includes the swimming
flora and fauna whose habitat is the open ocean ; the pelagic
Plankton inhabits the ocean surface and approximately 200
metres below ; the bathybic Plankton inhabits the waters from
the bottom for about 100 metres up, and between the latter
two lives the zonaric Plankton.*

Walther's Further Analysis of Conditions of Environment. —
Walther has amassed very interesting statistics to show the
particular influence upon distribution of the various condi-
tions of light, of temperature, of salinity, of tides and waves,
of currents and ocean circulation, and has classified the floras
and faunas of the seas in relation to these conditions.

The flora of the shores, littoral flora, are divided into that
of the (i) dune and sand-plain zone, (2) flora of coast rocks,
(3) of the mud zone, (4) the sand-plants flora. Four different

* Walther, " Einleitung in die Geologic als historische Wissenschaft, i.
Theil, Bionomie des Meeres," 1873, pages 16-22; also Haeckel, " Plankton-
studien," Jena, 1890, page 18, etc.

GEOGRAPHICAL DISTRIBUTION. 1 1/

zones of coast vegetation are recognized in the tropics by
Schimper: the Pescaprae formation, the Barringtonia forma-
tion, the Nipa formation, and the Mangrove formation.

The fauna of the coast is also determined in its composi-
tion by the conditions of the shore itself, and thus we find
different kinds of animals associated with the rock beach, the
bowlder beach, the pebble beach, the sand beach, and the
mud beach.

Under the sea surface downward a number of zones have
been distinguished, defined most easily by their depth, which
present strong contrast in their faunas. We owe it to Ed-
ward Forbes that we have a nomenclature for these zones of
depth. The divisions made by him are the littoral zone, the
laminarian, coralline, and the deep-sea zones; the latter, as the
result of deep-sea dredgings, has been divided into the zone
of deep-sea corals, or brachiopod zone, and the abyssal zone.

In a Report of investigations made upon the faunas of the
seas off the New England coast, Professors Verrill and Smith
found it to be a fact " that there are in the waters of this re-
gion three quite distinct assemblages of animal life, which are
dependent upon and limited by definite physical conditions
of the waters which they inhabit."* These are described
under the following divisions, viz. :

1. The fauna of bays and sounds;

2. The fauna of the estuaries and other brackish waters;

3. The fauna of the cold waters of the ocean shores and
outer banks and channels.

This classification of environments is not bathymetric, but
is chiefly on the basis of temperature and purity of the waters.

It is altogether probable that every kind of difference in
the environment, which could be described as beneficial or
otherwise to the vital functions of organisms, is also repre-
sented by greater or less adaptation of the organization, to
profit by the favorable conditions or to avoid the evil effects
of those which are unfavorable.

Relations of Organisms to Time and to Environment Equally
Significant. — When we consider alone the historical relations

* United States Fish Commission, "Report upon the Invertebrate Animals
of Vineyard Sound and Adjacent Waters,'1 p. 5, etc.

Il8 GEOLOGICAL BIOLOGY.

of the organisms, as expressed in their geological sequence,
the order of the phenomena appears like a mere unfolding of
successive phases of organic life upon the globe, each phase
preparing the way for the next ; and had we no preconcep-
tions, I think this evolution would seem to be the most
natural thing in the world.

Gradual modification with each step of generation would
be found in each case the sufficient explanation and cause of
that which followed.

But when it is observed that each living organism is
closely adjusted to a particular set of environmental condi-
tions, and that specific organic form and specific conditions
are closely co-ordinate factors, the question as to the influence
exerted by environment upon the organism becomes a prob-
lem of equal importance.

An Explanation required for Succession of Species as well as for
Adjustment of Species. — The study of the relations of organisms
to geological time and to geographical space first brings out
the simple fact that differentiation of organic form is actually
related to both. There is an adjustment of the organism to-
each of the phenomena, time-succession and place-extension.
If we turn from this simple statement of fact to seek for some
reason why organisms differ in form, and why one organism
has one form and another organism of another time and place
differs from it, then there appears back of geological succes-
sion and of geographical distribution an element of causation.
There are conditions in the succession and in the distribution
which we may suppose have been the cause, or at least the
occasion, of the changes of form exhibited by the organism.

Evolution and Adaptation both observed Facts. — We have
already remarked that the examination of a series of forms in
the rocks shows the modification and change in their form to
be co-ordinate with progress of time, and on following them
from the lowest rocks upward through the geological column
to the present, each series ends in recognized living organisms ;
hence we conclude that it is a characteristic of organisms to
pass through continuous change in time. This process of
changing morphological characters, expressed in the history
of organisms, is called Evolution.

GEOGRAPHICAL DISTRIBUTION. IIC>

Second, organisms now living are so distributed in relation
to the conditions of environment that we are led to recognize
this general law : that the morphological characters of organ-
isms are in some way associated with or related to the phys-
ical environment in which they live.

Ancestry and Environment as Causes of Evolution. — Thus, by
looking only at the superficial relation of organisms, i.e.
those which may be expressed in number and ratio, we find
a definite relationship existing between organic form (mor-
phology) and both geological time and physical conditions on
the earth's surface. We may express the relationship by the
proposition, that the morphological characters of any particu-
lar organism Jiave come to be what they are through the opera-
tion of two sets of conditions : first, the organic conditions-
which were antecedent to the appearance of the given organism;
and, secondly, the external physical conditions into which it was
born. The first set of conditions is expressed by the general
term Ancestry, and the second by the term Environment.

Differences of Opinion respecting Interpretations not Facts. —
So long as we confine our attention to the simple relationship
existing between organic structure and the passage of time or
the varying conditions of environment, we have touched only
the fundamental facts of the real problem before us.

The series of correlated phenomena are as they are, what-
ever be our interpretation of them. The reason for first care-
fully spreading out the facts themselves is in order to show
that they are not invented by any theory, that they exist
independently of any preconceived view, and that the differ-
ences in opinions regarding them are not matters of observa-
tion, but are matters of philosophy.

Introduction of Causation into the Discussion. — And here we
introduce a new element into the discussion. We assume
that cause and effect are involved in their relationship. We
assume that in the course of time the organisms which went
before must bear the relation of determining cause to those
that follow, and that in physical space or environment, the
conditions of geographical locality are a determining cause in
relation to the species adjusted to particular natural-history-
provinces.

120 GEOLOGICAL BIOLOGY.

Ancestry and Environment in Relation to the Beginning of Each
Individual. — From this point of view we recognize two classes
of phenomena which are all-important factors in determining
the particular form and structure of every organism, and the
fundamental difference between the two groups is found in
the relation they bear to the beginning of the development of
each individual. The one set of conditions have exerted their
effect when the first germ of the new individual arises, and
to them is applied the general name Ancestry. The other set
begin to influence the individual only after development has
begun, and to this set of conditions the general term Environ-
ment is applied. Evolution is the name given to the results,
in structure and function of organisms, which are traced to
Ancestry and Environment as determining causes.

It is from this philosophical point of view that the follow-
ing definitions become appropriate :

Definition of the Terms " Ancestry" and " Conditions of Environ-
ment."— Ancestry, as defined in the Century Dictionary, is
" the series of ancestors, or ancestral types, through which an
organized being may have come to be what it is in the process
of Evolution;" and in the same work the term conditions of
environment is defined as " the sum of the agencies and
influences which affect an organism from without ; the
totality of the extrinsic conditioning to which an organism is
subjected, as opposed to its own intrinsic forces, and there-
fore as modifying its inherent tendencies, and as a factor in
•determining the final result of organization. It is an expres-
sion much used in connection with modern theories of evolu-
tion in explaining that at a given moment a given organism
is the resultant of both intrinsic and extrinsic forces, the latter
being its conditions of environment and the former its in-
herited conditions."* Ancestry and Environment are, in
the abstract, names for these intrinsic and extrinsic factors of
evolution.

If we examine only the paleontological series, we might
•conclude that the course of evolution was determined entirely
by the first set of conditions, Ancestry ; and, on the other

* Century Dictionary, vol. I. pp. 201 and 958.

GEOGRAPHICAL DISTRIBUTION. 121

hand, if we look alone to the relations of organisms to envi-
ronment, this set of conditions appears sufficient to account
for the course of evolution, because in both cases we find
adjustment of morphological character to the conditions pre-
existing at the beginning of each individual life.

Two Factors Producing the Effects of Evolution. — Assuming
these definitions to be formulations of the truth in the case
to such a degree of accuracy that they may be adopted as
working hypotheses, the next step in our analysis is to ascer-
tain what part each of these factors plays in bringing about
differentiation of organic form and structure.

Three Views Possible. — There is practically but one of three
opinions to take in the matter: either (i) the differences ob-
served among organisms are accounted for entirely by ances-
try— that is, the potency of all organic differentiation and
evolution is found in the ancestry at any particular moment
of the process ; or (2) environment is the efficient factor in
bringing about all modification of organic structure ; or (3) the
actual course of evolution as it takes place is the resultant of
the co-operation and antagonistic action of both factors.

The extreme old school (of Cuvier, for instance) adopted
the first opinion, the extreme natural selection or Darwinian
school holds substantially the second view. It is believed by
the author that the truth will be found in the third position.

First Cause of some sort Essential to any complete Theory of
Evolution. — The discussion of evolution has for the past fifty
years chiefly centred about the theory of the origination of
species. Ancestry, in the general sense here used, includes
all the antecedent intrinsic conditions of an individual life.
When we analyze the theories to their ultimate essence the
great contrast between Creationism and Evolutionism does
not lie in the fact that the one acknowledges God to be the
first cause or ultimate ancestor of every living thing, while
the other, in magnifying the agency of the environment in
controlling the origin of species, denies all first cause : for, in
both cases, some pre-existing power or potency that is quite
godlike must be assumed as the necessary antecedent to the
phenomenal appearance of organisms in all their variety upon
the earth.

122 GEOLOGICAL BIOLOGY.

Edward Forbes on Origin of Species and Centres of Creation.
— When we ask how. did species arise, we find two dom-
inant opinions have 'existed regarding the nature of the
antecedent condition immediately preceding the individual
organism in each case. According to the first view, im-
mediate physical ancestry has explained only the repetition
and perpetuation of its own morphological characters, and the
origin of any particular combination of such morphological
characters was not accounted for, except through the agency
of a primitive first cause. The sequence of organisms in
paleontology was clearly recognized by naturalists at the be-
ginning of the century, but neither ancestry nor environment
was deemed competent to explain anything but what were
called varietal modifications of species. It was this idea that
was in the mind of Edward Forbes* when he described a
natural-history province to be "an area within which there
is evidence of the special manifestations of the creative power;
that is to say, within which there have been called into being
the original or protoplasts of animals or plants." And again
he says: " The diffusion of the individuals of the characteristic
species of a province is found to indicate that the manifesta-
tion of the creative energy has not been equal in all parts of
the area, but that in some portion of it, and that usually more
or less central, the genesis of new beings has been more in-
tensely exerted than elsewhere." This notion led to the use
of the terms centres of creation and specific centres, at which
the species was supposed to have originated, and from which
it was distributed, or migrated in the course of time.

Reality of Specific Centres Not Questioned; the Fact Variously
Interpreted. — It is a well-known fact, and one that Forbes
clearly understood, that each natural-history province is such
a specific centre for rarely more than one species of each
genus of its fauna; or, in other words, each well-defined
species is typically developed in some such specific centre and
distributed within such a natural-history province. The
specific centre may not be geographic. Geography, in gen-
eral, is the most commonly observed criterion of distribution

* Edward Forbes, " The Natural History of the European Seas," 1859.

GEOGRAPHICAL DISTRIBUTION. 123

of organisms, but in the case of insects the root and leaf of
the same tree present greater contrasts in conditions of en-
vironment than two trees of the same species a thousand miles
apart. Geographical distribution, and other terms associated
with it, have reference fundamentally to conditions of environ-
ment, whether the distribution is on geographical or other
lines.

Representative Species, Common Descent, and Migration of
Species. — Similar species of the genus in other provinces were
called representative species by Forbes. Another idea, in-
cluded in this hypothesis, was that all the individuals of a
species had a common descent. The idea of common descent
was associated with the definition of species, and when the
same species was recognized in two distinct provinces, the fact
was explained by the theory of diffusion, or migration of species;
and in defence of the theory of the specific centres Forbes held
that provinces, to be understood, must be traced back, like
species, to their history and origin in past time ; and again,
that " species have a definite existence, and a centralization in
geological time as well as in geographical space, and that no
species is repeated in time."

Darwin did not deny the Facts, but explained them differently
from Forbes. — Darwin, who gave a different interpretation of
the facts, recognized the truth of the proposition set forth by
Forbes. In his famous "Origin of Species" he says (in reply
to the question "whether species have been created at one or
more points of the earth's surface," and after some discussion
of the topic): "Hence, it seems to me, as it has to many of
the naturalists, that the view of each species having been pro-
duced in one area alone, and having subsequently migrated
from that area as far as its power of migration and subsistence
under past and present conditions permitted, is the most
probable."

Forbes' Explanation of the Origin of Species. — In Forbes' no-
tion of "specific centres" is included the idea that ancestry is
responsible for the "specific characters" of the individual.
41 Every true species presents, in its individuals," he says,
"certain features, specific characters, which distinguish it
from every other species; as if the Creator had set an ex-

124 GEOLOGICAL BIOLOGY.

elusive mark or seal on each living type." And in the dis-
tribution of fossil as well as living species was seen evidence
of " relationship of descent" and of "the derivation from an
original protoplast." But descent was supposed by him and
his school to be without modification; it was the transmission
without change of the ancestral characters to their offspring.
Whatever modification might appear was considered an irreg-
ularity of individual growth, the cause of which was looked
for in idiosyncrasies of the individual or in accidents of en-
vironment. Forbes was not ignorant of the paleontologic
succession of species. Ancestry determined the specific
characters, but it was supposed to determine their likeness,,
and not their differences. All the evolving of new forms was
traced to antecessory causes and conditions, but the immedi-
ate ancestors, it was believed, were capable of transmitting
only the characters which they received from their ancestors.
There is nothing wrong with "geographical distribution," or
"specific centres," or "specific characters," as used by the
older naturalists; the new light has come into the interpreta-
tion of descent and the nature of species.

The Meaning of Evolution by Descent. — It is important to dis-
tinguish between the names of things and their explanation.
The term evolution by descent is in this respect faulty, for it
means both more and less than is intended. More, in that
the most important factor brought forward in explanation of
evolution to-day, that of natural selection, is among the
extrinsic rather than the intrinsic forces, when the conditions
of environment are strictly discriminated; while descent, or
ancestry, can be applied only to those forces or conditions
which are intrinsic. It expresses less than is intended in that
it is not meant that descent alone determines the steps of
evolution.

Distinction between Evolution and Development. — Huxley's
definition, "evolution, or development, is, in fact, at present
employed in biology as a general name for the history of the
steps by which any living being has acquired the morpho-
logical and the physiological characters which distinguish it,"
is defective in that it includes a definition of both evolution
and development. Development of the individual organism,

GEOGRAPHICAL DISTRIBUTION. 12$

from the germ to the adult, is a very different thing from the
history of the steps by which the same individual acquired
the differences which distinguish it from other species of the
same genus, which is the particular meaning of evolution.
Evolution is the process of modification of specific characters,
and development is the process of formation of individual
characters. There are also conditions incident to these proc-
esses— conditions which are both outside of and exist before
each step of these processes. When these conditions are
essentially connected with the preparatory organic functions
by which the processes are carried on, they are intrinsic, and
they are defined under the general term ancestry ; when they
are accidental to the time or place when and where the pro-
cesses are acting, they are extrinsic, and are called the condi-
tions of environment.

Immutability or Mutability of Species. — The fundamental
difference between the old and new schools of naturalists is
found in their opinions regarding the origin of specific differ-
ences : the old school held the doctrine of the immutability of
species, the new holds the doctrine of the mutability of species.
The result of the change of view has not invalidated the
observations of the earlier naturalists, but it has produced a
complete revolution in the methods of interpretation of
natural history.

In this conception, defined by Forbes, we see that among
the contributions which ancestry brings to the actually known
individual there are what he called the " specific characters"
which distinguish it from every other species, and the posses-
sion of these "specific characters " was taken to support the
notion of derivation from the original protoplast.

Descent was recognized as without modification ; that is,
the law of descent was the perpetuation of the ancestral
"specific characters" in the offspring. There was in the
definition no consideration of the origin of such specific char-
acters. Whatever modifications occurred in the offspring
were defined as irregularities of growth, whose cause was
located in the idiosyncrasies of the individual, or in what is
above called environment, but they were not supposed to be
perpetuated.

126 GEOLOGICAL BIOLOGY.

This school, which Forbes represents, assigned all the
steps of progress observed in the history of organisms to
causes entirely antecedent to each individual's birth. The
explanation was confined to ancestry in its abstract sense of
*' antecessor " (as the Latin original has it): all cause of
changes of a specific rank was entirely antecedent to the
organic individual expressing them. The fundamental charac-
teristic of this view is found in the doctrine of the " immuta-
bility of species," as contrasted with the doctrine of " muta-
bility of species " of the new school.

Mutability of Species the Central Thought in the New Theory
of the Origin of Species. — Nothing that has occurred in the
present century has so stimulated investigation of the facts of
nature, and has so pervaded the whole realm of philosophical
thought, as that which has centred about this question as to
the nature and origin of the organic species. Darwin's
famous work ft The Origin of Species," first published in
November, 1859, struck the key-note of the present age of
the science. He clearly announced the opinion that species
are mutable, and as the whole science of natural history was
built on the idea of their immutability, a complete readjust-
ment of the science to the new conception has resulted.
The importance of a clear conception of the meaning of
species is thus apparent, and it will be discussed in detail in a
following chapter. The idea of immutability of species ob-
structed the way to the clear comprehension of the evolution of
organisms, very much as the catastrophe theory of the end of
the last century prevented geologists from reaching a clear
understanding of the agencies and methods by which the
earth reached its present condition. Uniformitarianism
played much the same role for Geology which evolutionism is
working for the science of Biology.

Two Extremes of Opinion Regarding the Mode of Origin of
Species by Evolution. — Among those to-day who adopt evolu-
tion as the explanation of the mode of origin of the different
forms of organisms, there are two extremes of opinion with
many intermediate compromises.

All will agree in recognizing ancestry and environment as
each taking some part in the evolution ; but the extreme

GEOGRAPHICAL DISTRIBUTION. I2/

school, on the one hand, holds that environment is the chief
factor determining the direction and extent of the modifica-
tions, which heredity tends to perpetuate, and that ancestry
plays only the part of holding and preserving, in its offspring,
what it gets from the agency of environment.

The other extreme is the opinion that ancestry is the more
efficient factor in bringing about the evolution ; that in what
is called variability there is working out, not a mere acci-
dental reflex of environment upon the plastic organism, but a
fundamental property or force of organisms, ever tending
from homogeneity to heterogeneity, and resulting in the
specialization of functions and the differentiation of organic
structure always ; the line of evolution followed out by any
particular race being influenced little by environment, — the
adjustments being active and not passive, — the successful
organisms seeking and adopting conditions favorable for their
existence if out of them, dying out if the conditions favor-
able are not within reach, or if crowded out of them.
Natural selection, to this school of opinion, plays rather an
eliminating role than one of causation, and explains rather
why there are gaps in the series of organisms than why the
characters assumed in the modified forms are what they are.
In this latter view the successive steps of modification of a
race are as much controlled by the ancestry as are the succes-
sive steps of development in the growth of the individual.

In the former view there is the replacement of the theory
of immutability of species by that of the mutability of species,
but the process of reproduction is still looked upon as immut-
able, reproducing the characters of the parents in the offspring
without change ; in the second view reproduction itself takes
a part in evolution and normally accomplishes modification of
form, either slowly or suddenly, but progressively, and evolu-
tion is an intrinsic law of, organism.

An Unknown Cause assumed to explain Origins by both Forbes
and Lamarck. — The naturalists of Forbes' school, with the
fundamental notion of immutability of species, had no other
way to explain the series of successive forms which they knew
from paleontological research than to call in the resources of
a first cause; but they were not ignorant of the series.

128 GEOLOGICAL BIOLOGY.

Lamarck, who looked upon species as mutable, still found his
ignorance impelling him to use the theory of spontaneous
generation to start his series. However much they may-
seem to be independent of a first cause, no scientific theory
even of evolution is complete without recognizing the potency
of the things as existing before their appearance.

Conclusions. — It will be apparent now that the discussion
of the relation of organisms to environment, or geographical
distribution, touches the fundamental problems of natural
history. Forbes was of the Linnaean school, who with
Cuvier and all that earlier school of naturalists held to the
conception of a species immutable; but his studies of distri-
bution were among the more important agencies in clearing
the way for the abandonment of that conception of species.
The explanation he gave of the origin of species was the
most rational one so long as the species was supposed to be
immutable. We often imagine that evolution, which has
been made the watchword of the new view, is a newly dis-
covered truth ; not so. The processes of evolution have beeii
elaborately investigated by the new school, but evolution of
organisms, in the abstract sense,' had been promulgated
almost from the beginning of philosophy, as already stated.

Darwin, in his li Origin of Species," frequently, and with
apparently no more hesitation than he had for the use of
species, spoke of Creation; he adopted, too, Forbes' term
"Centres of Creation." Haeckel, one of the most radical
defenders of the new views, entitled one of his most impor-
tant books " The History of Creation."* These illustrations
show that the attempt to explain the process and cause of
Evolution is quite distinct from the recognition of the facts
of Evolution, and we may conclude that mutability of organic
species and the evolution of organisms in geological time are
established facts, in the accomplishment of which both ancestry
and the conditions of environment have played a part.

* "Nattirliche SchopfunjiSgeschichte." Berlin, 1868.

CHAPTER VII.

GEOGRAPHICAL DISTRIBUTION : SPECIAL CONSIDERA-
TION: THE ADJUSTMENT OF ORGANISMS TO EN-
VIRONMENT.

R6sum6. — In the case of the Madreporarian corals it was
observed that as geological time progressed new genera actu-
ally were initiated, and the succession of genera and the rate
of their increase was seen to be definitely associated with suc-
cession of time. Likeness of structure and likeness of time,
dissimilarity of form and separation in time, slowness or
rapidity of initiation of new genera, and a particular geologi-
cal period of time for each family, order, or class, are inter-
preted to mean that there is a definite relationship existing
between differentiation of structure and passage of time.
This we assume to be a law of the order of events, and we
infer the general hypothesis that the form and structure of
organisms of one geological period are in some measure deter-
mined by the form and structure of the organisms of the
period immediately preceding.

This hypothesis involves two particular propositions :

(1) That each organism is genetically related to some pre-
existing ancestor whose form and structure were not exactly
like its own.

(2) That the process of organic reproduction is not a stereo-
type process of repeating in the offspring the exact characters of
the ancestry, but that the production of differences between the
parent and offspring is a normal factor in the reproductive
process, either continuously or occasionally in operation.

There is, however, another fact to be noted : the innu-
merable differences in the conditions of environment are more
or less distinctly expressed by differences in the kinds of
organisms associated with them. All kinds of animals are not

129

130 GEOLOGICAL BIOLOGY.

found in every place or condition, but in each particular kind
of environment particular kinds of animals are found, and
their living is more or less dependent upon those conditions.

Hence we infer another general hypothesis:

(3) That the conditions of environment do in some measure
determine the particular form and structure of each organism.

The Gastropoda Illustrate the Law of the Relationship between
Organisms and Environment. — In order to show more particu-
larly how the differences of form (expressed by different
species, genera, and families in scientific classification) are
related to differences in the conditions of environment, a
class of the Mollusca, the Gastropoda, may be examined in
detail. This group of organisms is convenient for the pur-
pose because of the full statistics already accumulated regard-
ing the geographical distribution of its species.

Meaning of the Classification of Organisms. — Without defining
the morphological characters indicated by the classification,
it is important to remember that zoological classifications are
fundamentally based upon morphological differences, that
organisms of two distinct classes present greater morphologi-
cal difference than those of a single class, that lesser diverg-
ence in form is expressed by division of the class into sub-
classes, and that the animals of the same order present
greater resemblance to each other than to those of different
orders. Families are again subdivisions of the orders, and
each family includes two or more genera, and the species of
each genus are alike in their general form, differing only in
some of the more minute details. Hence when we describe
the peculiarities of the distribution of genera, we are express-
ing the law of association between the generic form and the
conditions of environment indicated by the geographical dis-
tribution. Thus, the common sea- whelk, Buccinum unda-
tum (Fig. 33), represents the class Gastropoda as contrasted
with the Dentalium (Fig. 37), belonging to the class Scaphop-
oda, Hylaea, a Pteropoda or Chiton (Fig. 36), a representative
of the class Placophora. The Gastropoda, Scaphopoda, Ptero-
poda, and Placophora together constitute that division of
Mollusca called Glossophora, being alike in the possession of
a more or less distinct head-portion of the body, and of a

GE 0 GRA PHICA L D IS TRIE U TION.

well-developed tongue (radula), which is
with minute denticles set in rows (Fig. 34).

generally armed
The other types

FIG. 33.— A Gastropod, the common whelk, Buccinum undatunt, showing the spiral shell on
back of the animal, its large flattened foot, distinct head with two tentacles, at the base

the

base of

which are the eyes. The siphon si and the optrculum op are special parts not found in all
Gastropods.

FIG. 34. — Examples of the dentition of Gastropoda, single transverse rows of the denticles of the
lingual ribbon (radula), greatly magnified, of (A) Natica, (B) Nassa, (C) Pleurotoma, (Z>>
Scalar ia.

of Glossophora are adjusted to various conditions of environ-
ment, but for our purpose it will be better to confine our
attention at present to the single type of the class Gas-
tropoda.

Distinguishing Characters of the Class Gastropoda. — The com-
mon external characters of all Gastropods are these, viz. r
Head and sense organs well developed, the former often?
bearing tentacles; a ventral muscular foot and undivided,
mantle, which frequently secretes a plate-shaped, or spirally
twisted shell. The paleontologist knows Gastropods by their
calcareous, more or less spirally twisted, univalve shells.
These Gastropods, of which several tens of thousands of
species are described, are specifically adjusted to all kinds of
conditions of environment, and are distributed from the bot-
tom of the ocean to the tops of the mountains.

132

GEOLOGICAL BIOLOGY.

Zones of Environment in which Gastropods are Distributed. —
If we arrange their environmental conditions in tabular order
we have the following series, viz. :

1st. Abyss of the ocean, or an abysmal zone, extending
from 500 metres, or 250 fathoms, to the lowest known depths
of the ocean.

b

FIG. 35.— Schematic Mollusk. (After Lankester.) a, tentacle ; 3, head ; c, margin of mangle ; </,
margin of shell ; e, edge of body ; /, edge of shell depression ; g, shell ; %c. cerebral gan-
glion; gj>e, pedal ganglion; gpl, i leural ganglion ; h, osphradium ; /', ctenidium ; £, reproduc-
tive pore ; /, nephridial pore; 7«, anus; n and p, foot; r, coelom; j, pericardium ; t, testis ;
», nephridium ; z/, ventricle of heart ; z/, liver.

2d. Zone of Brachiopods, or of deep-sea corals (72-500
metres, 50-250 fathoms).

3d. Zone of Nullipores, or of Corallines (27-72 metres,
15-50 fathoms).

4th. Laminarian Zone (low tide to 27 metres, 1-15
fathoms).

5th. Littoral Zone (between low and high tides).

6th. Brackish water, sea-shores above tide, where fresh
and brackish waters are mixed, and where the surface may be
exposed to the air part of the time.

GEOGRAPHICAL DISTRIBUTION. 133

7th. Fresh water, as in rivers and lakes.

8th. Amphibious conditions, fresh water and land.

9th. Land, the surface of the land, or in the air.

These are zones of environment, which express a series of
varying conditions of light, of oxygen, of air, of moisture, of
degrees of temperature, of pressure of the medium, of depth,
of height.

Reasons for Selecting the Gastropods. — The Gastropoda are
selected because of the wide range of adaptation expressed in
their distribution, and because the statistics are particularly
full. The classification found in Zittel's Handbuch is adopted,
so far as nomenclature and inclusion of genera are concerned ;
but Gastropoda will be spoken of as of the rank of a class, the
more common usage of zoologists,* and the morphologically
specialized forms, the Chitons (Placophora, Fig. 36) and the
Dentalia (Scaphopoda, Fig. 37), will be omitted from the true
Gastropoda, as is done by Zittel, following Ihring and Lacaze
Duthiers : the Pteropoda will also be omitted.

Peculiarity of the Divisions of the Gastropods as to Range of
Adaptation. — Ranking Gastropoda as a class, with the restric-
tions above mentioned, it will include the following four
orders, viz. : Prosobranchia, Opisthobranchia, Pulmonata,
and Nucleobranchiata (or Heteropoda). The whole of the
Heteropoda are specialized in structure and restricted in dis-
tribution to the surface and upper parts of the ocean water,
and structurally they may be ranked with the monotocardian
Prosobranchs. Six living genera with about 50 species are
known, and a few fossil genera are referred to this order.
The Pulmonata (Fig. 38, 38^) are air-breathers, and (with the
exception of the Siphonaridae) are restricted in distribution to
land and fresh water. Six thousand (6000) living and 700

* Lankester's classification is (Encycl. Brit., art. " Mollusca," p. 633):
Phylum mollusca :

Branch A, Glossophora.

Class i. Gastropoda. Class 3. Cephalopoda.

Br. a. Isopleura. Br. a, Pteropoda.

Br. b. Anisopleura. Br. b. Siphonopoda.

Class 2. Scaphopoda.

Branch B, Lipocephala ( = Acephala, Cuvier).
Class i. Lamellibranchia (syn. Conchifera).

134

GEOLOGICAL BIOLOGY.

fossil species are described. The Opisthobranchia (Fig. 39)
are all sea-snails, and appear to be restricted in distribution to
the coastal waters, near the land, and near the line of contact
between salt and brackish water habitats; about 1200 species

39

FIGS. 36-42.— Gastropoda Illustrations of the chief types ; 36, Placophora, Chiton ruber ; 37,
Scaphopoda, Dentalium Indianorum \ 38, 38^, Pulmonata, Physa heterostropha ; 39,
Opisthobranchia (Nudibranchia) sEolis pilata ; 40, 41, 42, Prosobranchia,— 40, Cyclobran-
china, Achmaea testudinalis ; 41, Aspidobranchina, Haliotis sp. • 42, Ctenobranchina, Turn''
tella sp. (After Packard and McMurrich.)

are described, including fossil forms; the gills are behind the
heart. The remaining order, the Prosobranchia (Figs. 40, 41,
42), includes mainly marine species, which are adapted to a
great variety of marine conditions; there are known some
14,000 species. They are divided into three suborders, sepa-
rated primarily upon the differences in their breathing organs,
viz.: A, Cyclobranchina; B, Aspidobranchina; C, Cteno-
branchina, or better known as the Pectinibranchia of Cuvier.
In all the Prosobranchs the gills are in front of the heart,
that is, the branchial vein enters from the front. They are
dioecious (while the Opisthobranchs and Pulmonates are
hermaphrodite).

GEOGRAPHICAL DISTRIBUTION. 13$

The mode of existence of the Glossophora is compactly summarized as follows
by Zittel (translation from " Handbuch der Palaeontologie," vol. n. p. 161) : The
greater number of Glossophora are aquatic animals, and the majority marine.
The Pteropods, Placophors, Heteropods, Opisthobranchs, live exclusively in
the sea. The great order of Prosobranchs comprise also a majority of marine
forms. There are a certain number which live in brackish water near the dis-
charge of lakes (Potamides, Neritinas, Rissoas, Hydrobias), and others in.
fresh water (Paludinidae, Melaniidae, Valvatidae). The Pulmonate genera, fur-
nished with gills, are adapted to a terrestrial life (Cyclostomidae, Helicinidae).
The Pteropods and Heteropods are pelagic animals, free swimmers, inhabiting-
the open sea ; the great part of the other Glossophora are coast animals, crawl-
ing upon plants, rocks, and shore debris. Some Prosobranchs are amphibious
(Littorina, Truncatella, Patella, Nerita), and are able to live a long time dry,
without water ; they then retire within their shell, close the operculum, and
breathe the water which they have retained with them. The Ampullarians
have the advantage of two different kinds of respiratory organs, and can live
alike on land and in water. Some Prosobranchs bore in the sand and mud like
Lamellibranchs (Oliva, Mitra, Natica, Buccinum) ; others inhabit coral reefs,
or live as parasites in other animals (Entoconcha, Stylifer). The shells of fresh-
water Gastropods are generally covered with a greenish-olive or brown epidermis,
their apices are often broken or absorbed ; their shell is thin and horny (Lim-
naeus). Many Gastropods subsist on fresh flesh or carrion ; there are some
which perforate shells of other mollusks with their tongue, and devour them
through the little hole thus perforated (Natica, Murex, Buccinum); the majority
of Glossophora (almost all the Pulmonates, and the holostomate Prosobranchs)
live upon vegetable food.

Their geographical distribution is little known except for the littoral, fluvia-
tile, and terrestrial species ; it is known, however, that the Pteropods and
Heteropods, being pelagic, have a very extended distribution ; the Scaphopods
and the Placophors are equally found in all seas and all latitudes. There are
only a few pelagic forms among the Opisthobranchs and the Prosobranchs.
The geographical distribution of the marine Glossophora, besides the influence
of centres of origin, is determined greatly by the character of the bottom, the
form of the coast, the flux and reflux of tides, the currents, and the saltness and
depth of the water. The sandy shores are little favorable to Gastropods ; they
prefer rocky shores, where algae flourish. The shores much cut up furnish great
variety of conditions of habitat, and accordingly have a richer fauna than great
estuaries. The movements of the tide produce changes and bring in food, and
thus favor life. There are currents, also, which greatly affect geographical dis-
tribution. Most of the marine Glossophora die as soon as they are transported
into fresh water. There are, however, some which have the faculty of adapting*
themselves to change of medium. Such notably are certain species of the genera
Patella, Rissoa, Trochus, Purpura, Littorina, and Cerithium. Some fresh-water
species, conversely, are able to live in salt water (Limnaeus, Planorbis,
Melania, Melanopsis, Physa, Neritina). It is probable that all the actual ter-
restrial or fluviatile species are traceable to a common origin, and that they de-
scended from marine types of the geological epochs, modified by adaptation.

The temperature has a great influence upon the development of the Glos-
sophora : heat is favorable to them, and they are much more abundant in the
seas and lands of tropical regions than in temperate or polar regions. The
marine bathymetric zones, as the hypsometric zones on land, exercise their influ-

136 GEOLOGICAL BIOLOGY.

upon the Glossophora, as well as upon other animals. The study of their
conditions has a particular interest to the paleontologist, since he is thus able to
account for the conditions under which the fossils lived, and the mode of forma-
tion of marine sediments. One knows, in a general way, that the temperature
of the ocean goes on diminishing from the surface to the bottom, and that it
attains a temperature approximately constant of 4° to 5° Cent, at the depth of
500 feet; it descends scarcely to zero (32° F.) at great depths ; the conditions of
submarine existence are thus approximately constant in abysmal regions, while
they present the greatest range of variation in the shore regions of slight depth
in the tropics.

The bathymetric distribution of Mollusca was studied in 1830 by Andouin and
Milne Edwards ; and later, upon new data, by Sars in Norway (1835) and Ed.
Forbes in the JEgean Sea and in England. The most important results in this
•direction have been attained by the expeditions of the Porcupine (1869-70), of
the Challenger (1873-76), of the Gazelle (1874-76), of the Tuscarora (1874-76),
•of the Blake (1877-78), of the Voraigen (1876-78), of the Voraillem (1880).

The Zonal Distribution of the Ctenobranchina. — Restricting our
attention to the families of Ctenobranchina, and using for the
purpose the classification into families of F. Barnard,* which
are 44, we are able to see some evidence of the particular
connection between form and bathymetric distribution. Of
these families three have land species, and two of the fami-
lies are restricted to a land habitat (Cyclophoridae and Cyclo-
stomidae). There are five families of which the species are
all fresh-water species (Paludinidae, Ampullaridae, Bithyniidae,
Valvatidae. and Melaniidae). One family, Hydrobiidae, has
both littoral and brackish water species. The remaining
thirty-four families are all marine ; of them many of the lit-
toral species are able to endure exposure to the air and some
contamination of the water, but the normal habitat of all is
marine. Some of the families are limited in downward dis-
tribution : such are the families Truncatellidae, Hydrobiidae,
Janthinidae (a pelagic type), Cypraeidae, Solariidae, Purpuridae,
and Terebridae. Others reach downward to the abysmal depths,
as Littorinidae, Rissoidae, Cerithiidae, Naticidae, Scalaridae,
Pyramidellidae, Eulimidae, Muricidae, Pleurotomidae ; and it
is interesting to note that of these families, having a bathy-
metric distribution from the abysmal depth to the littoral
zone, several are also the most ancient in geological range ;
the Littorinidae, the Naticidae, and the Pyramidellidae are re-
ported from as early as the Silurian era. The second section,

Elements de Paleontologie," 1893.

GEOGRAPHICAL DISTRIBUTION.

Tcznioglossa (Zittel's classification), contains twenty-six fami-
lies ; of these, four families contain strictly fresh-water species,
a few genera of which are amphibious. One of the fami-
lies is made up of land species; the remainder are marine
forms. Species of, at least, three families have been taken
from the abysmal zone. If we consider only the genera
characteristic of the several zones, we find them distributed
among different families. Three of these are represented in
the ist, or abysmal zone; four in the 2d, or deep-sea Coral
zone; five in the 3d, or Nullipore zone; five in the 4th, or
Laminarian zone; five in the 5th, or littoral zone.

Genera of the Ctenobranchina characteristic of the Several
Bathymetric Zones. — The genera which have already been
found to characterize the several zones have been tabulated
(from lists derived from various sources) by Fischer.*

It will be noticed that some genera are restricted to single
zones, and others characterize the faunas of more than one
bathymetric zone. Examination of these lists shows the fol-
lowing genera of Ctenobranchia to characterize the faunas of
the respective zones.

(1) The Littoral Zone. — From high water to a depth of
12 metres, species of the genera Littorina, Hydrobia, Assi-
minea, Rissoa, Truncatella, Cerithium, Natica, Pyramidella,
Nassa, Purpura, Murex, Conus.

(2) The Laminarian Zone. — From low tide to 15 fathoms;
a zone characterized by species mostly phytophagous, of the
following genera, viz. : Phasianella, Xenophora, Triforis,
Rissoa, Aclis, Daphnella, Lacuna, Terebellum, Pterocera,.
Marginella, Mitra, Nassa, Phos, Drillia, Pleurotoma.

(3) The Nullipore Zone. — From 15 to 20 fathoms; the
zone of calcareous algae. The characteristic species are
mainly carnivorous, and of the following genera, viz. : Bela,
Buccinum, Cassis, Cassidaria, Chenopus, Eulima, Fossarus,
Fusus, Nassa, Natica, Pleurotoma, Trichotropis, Tritonium,
Trophon, Velutina.

(4) The BracJdopod Zone, or that of deep-sea corals, ex-
tending from a depth of 50 to 100 fathoms, has for its Cteno-

* "Manuel de Conchyliologie," Paris, 1887.

138 GEOLOGICAL BIOLOGY.

branch fauna species of the genera Bela, Eglesia, Fossarus,
Mangelia, Murex, Odostomia, Pleurotoma, Rissoa, Triforis,
and Turritella.

(5) The Abysmal Zone. — 500 metres, or 100 fathoms, or
more in depth, down to the profound depths, supports species
of the genera Aclis, Acirsa, Cerithium, Chenopus, Defranchi,
Eulima, Fusus, Hela, Natica, Odostomia, Pleurotoma,
Rissoa, Taranis, and Trophon.

Evidence of Adjustment of the Morphological Character to the
Environment. — An examination in the like manner of the dis-
tribution of species shows an adaptation of each species to
much more restricted bathymetric conditions, and to restricted
geographical areas or provinces. This fact might, however,
be accounted for by migration and sorting out of species from
choice, or the selection of environmental conditions ; but in
the case before us, where not only genera, but whole families,
— families whose representatives are found in all parts of the
globe, — are restricted to special conditions of environment, it
seems impossible to account for the fact except by the sup-
position that the morphological characters of the organisms
are adjusted to the environment.

When we examine animals whose structure is more
strongly contrasted, as in the case of the fish swimming in
water, the beast walking on land, and the bird flying in the
air, we are not impressed so much by the morphological
adjustment as by the physiological necessity of the restriction
to a particular environment ; but in the case of the Gastropods,
where the differences in form are relatively of small physio-
logical significance, the finding of a close correlation existing
between the specific, generic, and even family form, and the
particular conditions of environment seen in the zones of the
ocean, and climatal differences of land, impresses one vividly
with the immediate connection between differences of form and
differences of environment.

Law of the Adjustment of Organisms to Conditions of Environ-
ment.— We learn from these statistics that the morphologi-
cal differences, which are the basis of the classification of the
"various species of the ctenobranch Gastropods into genera
and families, are intimately connected with the differences in

GEOGRAPHICAL DISTRIBUTION. 139

temperature, depth, pressure, medium, and, in general, con-
ditions of environment in which they are distributed. And
at the same time we learn (a) that this is the case for a small
group of organisms whose general structure is alike, all secret-
ing a spiral shell, all having substantially the same organs,
arranged in much the same manner, and (b] that the range of
the differences of environment concerned — viz., in tempera-
ture, in depth, from the abysses to the tops of mountains, in
mediums, from high-pressure salt water to rarefied air — is
almost as complete as it would be possible to reach in habit-
able regions of the globe. The differences in form and
structure of the organisms as units are, therefore, not at
all in proportion to the differences of conditions of environ-
ment. Organisms very much alike, in the same genus even,
are found living under conditions of environment as strongly
contrasted, almost, as can be found ; and organisms of extreme
difference in structure are associated together in the same
conditions of environment. The conclusion we draw is, that
condition of environment is a fundamental cause in determin-
ing differences of form, but that whatever the structure or
organization of an organism may be, there have been, and are
constantly going on, adjustments to changed habitats, and that
the morphological changes resulting in these adjustments to
environment have been mainly of low order, i.e., varietal or
specific, and rarely are of higher than generic importance.

This is in strong constrast to the law observed regarding
relation of differences of form to time ; amount of time-sepa-
ration being co-ordinate with degree of difference in the
whole structure, and not merely in specific and generic
characters.

Summary. — In what has been said above the relations of
form to general conditions of environment have been dis-
cussed. Geographical distribution, in the particular use of
the term, is concerned with the association of like forms (the
same species or varieties) in areas presenting like conditions
of environment, and the distinguishing of different areas by
the different faunas and floras inhabiting them. It is sup-
posed that the adjustment, by various processes, of the species
to their changed environment may explain their differences

140 GEOLOGICAL BIOLOGY.

of form. What we have been illustrating is the fact that the
species of a genus, or the genera of a family, are found adapted
to different kinds of environment, and that the adaptation is
expressed by modification of the form of the organism. In
geographical distribution proper the fact is emphasized that
likeness of characters of form is associated with continuity of
like environmental conditions; viz., that the same variety is
restricted to a particular geographical area or province.

Geographical distribution emphasizes the fact that en-
vironment, by the law of adaptation, has the effect of confin-
ing the descendants of common parents within boundaries,
and thus tends to the continuance of like characters. Bathym-
etric distribution emphasizes the fact of adaptation itself,
by showing that the morphological differences distinguishing
the several species of a common genus, or the several genera
of a common order, are directly associated with differences in
the environment. The two groups of facts together point to
a most important biological law : that divergence of morpho-
logical characters is in some way associated with changing of
environmental conditions.

Distribution implies migration, and when we observe that
migration is accompanied with modification and adjustment
to new environment, we discover this second of the funda-
mental laws of evolution.

Relation between Zonal Adaptation and Geographical Range.
— An analysis of the classification of the Mollusca shows that
the Gastropod structure is adapted to all kinds of environ-
ment, because we find genera of Gastropoda in each of the
several zones expressing the full range of environmental dif-
ferences on the earth, from the abysses of the sea to the top
of the dry land.

Three of the orders of Gastropoda are somewhat special-
ized in adaptation to environment : the order Heteropoda
are pelagic forms; the Opisthobranchia are all marine, liv-
ing in the zones from Littoral down to the Nullipore zone.
The Pulmonata are restricted in adjustment to the high
littoral only of the marine zones, and to brackish, fresh-water,
and land conditions above the tide-level. The order Proso-
branchia has genera in every zone distinguished in our list.

GEOGRAPHICAL DISTRIBUTION.

This order is distinguished by the following characters, viz. :
dioecious, branchiate, shell-bearing, gills in front of heart;
from the latter character the name of the group is derived.
The Cyclobranchina (Fig. 40) and the Aspidobranchina (Fig.
41), two suborders, are all marine forms, but under the sub-
order Ctenobranchina (Figs. 33 and 42) — a division in which
all are so far specialized as to possess "a large cervical gill of
pectinate form on the left side, with small olfactory organ
(so-called rudimentary gill) ; a spiral shell is very generally
present ; the male possesses a penis on the right side ; most
are carnivorous, and possess a protrusible proboscis " (Claus
and Sedgwick) — there are genera adapted to each of the dif-
ferent kinds of environment, from the abysmal to dry-land
zones. Some of the genera of this suborder are restricted in
distribution, and constitute subdivisions of higher than family
value. The Ptenoglossa are all pelagic. The Rachioglossa,
the Toxiglossa, and the Rhipidoglossa are all marine ; but the
genera included under the T&nioglossa are adapted to differing
zones of environment from one extreme to the other. This
group is still further specialized, and in each transverse row
of the elongated radula of the tongue-like rasping organ of
the mouth, there are usually seven plates, and two small jaws
are usually found at the mouth-entrance.

There are two divisions of the Taenioglossa, the Siphono-
stomata, in which the opening of the shell is canaliculated for
the protrusion of a proboscis-like extension of the mouth ; in
the other, the Holostomata, the opening is entire. But when
we examine the genera grouped together by possession of
such likeness of structure, still we find in the former group,
of which most of the families contain marine species, that the
Ampullaridse are restricted to fresh-water habitat. In the
holostomatous division the Cyclostomidae, Cyclophoridae, and
Truncatellidae are air-breathers and live on land. The Palu-
dinidae, the Valvatidae, the Melaniidae are all fresh-water
species ; while the Littorinidae are marine forms, but have species
in the deepest part of the ocean, and others living between
tides ; and many other of the families of the latter group are
distributed through several zones. The forms of this divi-
sion, Taenioglossa, which are constructed to breathe air, and

142 GEOLOGICAL BIOLOGY.

thus are restricted to a habitat above sea-level, are included in
the five families Cyclophoridae, Cyclostomidae, Aciculidae,
Truncatellidae, and Assimineidae.

Of the eighteen genera of the first family, twelve are re-
stricted to Southern and Eastern Asia and neighboring islands ;
one genus (Pomatias) is distributed over North Africa and
South Europe; another (Craspedopoma) over the Canary,
Madeira, and Azores Islands. Another genus (Megalomo-
stoma) is found in the Antilles and Guatemala, and Apero-
stoma in Central and South America and Mexico.

The genera of Cyclostomidae have a similar distribution,
mainly in the lands bordering the Indian Ocean, and a couple
of genera (Choanopoma and Cistula) in the corresponding
lands bordering on the Gulf of Mexico.

The other three families, Aciculidae, Truncatellidae, and
Assimineidae, are all found within the same areas.

Families whose Genera have a very wide Range of Adaptation,
and Restricted Adjustment only among the Species. — If we pursue
the analysis still further, we find that there are some families,
like Cerithiidae, in which for some of its genera there is still
a very wide adaptation to conditions of environment ; species
of the genus Cerithium are living now between tide, and have
also been dredged from the abysmal zone. In such families
the zonal adaptation can be found only among the species.

Great Difference in the Closeness of Adjustment of the Charac-
ters of different Taxonomic Rank. — It is hence evident that
there is great difference in the extent to which organisms are
adjusted to restricted conditions of environment. In some
organisms their class characters are strictly adjusted to a par-
ticular group of environmental conditions, as is the case with
the insect whose mature structure with tracheal breathing
restricts it to a habitat in which air is accessible ; but even
among insects there are cases of adaptation to life in water.
In other cases one order is adapted to one mode of life and
another to a different condition of environment — as among
the reptiles there are aquatic Saurians, the Enaliosauria, and
the true lizards, Lacertilia, adapted to live on land. In such
cases as we have been considering, though there are in each
group some cases of restriction of adjustment to particular

GEOGRAPHICAL DISTRIBUTION. 143

conditions of environment, within the same group, be it
order, suborder, family, or genus, there are also those having
the same structure which are not so closely adjusted to the
environmental conditions.

Species Generally Closely Adjusted to Particular Conditions. —
This is the case until we reach the species. Species do appear
to be closely adjusted to some particular set of physical con-
ditions. Each one is so constructed that one environment is
at least most favorable, and to remove it from such condition
is either impossible without killing it, or leads to some adjust-
ment of its habits, and, it may be, structure and form to
.adapt it to the changes. The adaptation can only be varietal
for a single individual ; hence it is only among the specific
characters that we find the evidence of immediate change of
form to adapt the organism to changed conditions.

Fresh-water Families ; Restriction in their Distribution. — The
following families are made up of fresh-water species : Palu-
dinidae, Ampullaridae,Valvatidae, Melaniidae, and Hydrobiidae;
the latter two families containing a few brackish-water species.
Such species are by their specialized structure restricted,
therefore, to continental or island habitat.

The Paludinidae and the Valvatidae are restricted to the
northern hemisphere, are mainly in temperate zones, and
are not known south of the equator.

The Ampullaridae are from Central and South America,
Eastern Africa, Madagascar, S. Asia, Malaysia, the Philippines,
Australia, and vicinity.

The Melaniidse are chiefly intertropical species, being
most abundant in India, Indo-China, Malaysia, the Philip-
pines, Oceanica, Africa, Central America, South America,
running from Central America up into Mexico, and from
North Africa to Spain and Asia Minor.

The Hydrobiidae, which, according to Fischer, have been
distributed under eighty genera, are scattered over almost all
the lands between the temperate zones of the northern and
southern hemispheres.

Two Closely Allied Families, Separated in their Distribution. —
The Strombidae and Chenopodidae illustrate this law. The
shells of both these families are heavy, and more or less

144

GEOLOGICAL BIOLOGY.

specialized in their form, developing elongations, spines, and
processes giving them peculiar shapes. It is probable, there-
fore, that in their life they are not capable of much change of
local habitation.

The Strombus is confined to warm seas, — the Pacific, the
Indian, and the mid-Atlantic, including the Caribbean seas
and Mexican Gulf. The other recent genera of Strombidae
are from the Indian and Pacific warm seas. The genus
Chenopus of the second family is a North Atlantic form, and
is not associated with the Strombidae in habitat.

But representatives of both families are as old as the
Jurassic, and there are also several genera in each family from
Cretaceous rocks.

It is evident from this set of facts that the distinction be-
tween the two family types of structure was initiated in the
Mesozoic, and that there was adjustment to particular condi-
tions of environment very early — an adjustment which change
of time did not modify.

The following table will graphically illustrate this fact :

TABLE OF THE GEOLOGICAL RANGE OF THE FAMILIES
STROMBID^E AND CHENOPODID^.

.2

D
H-i

Cretaceous.

b

t

<u
H

Recent.

Strombidae:
Strombus (warm seas, Pac., Ind., Med., and Ant.)

ft

#
*

*

*

*

*

*

Rostellaria (Ind O Red Sea, and China)

•*

*

*

#•

*

Terebellum (Ind O)

*

•

Chenopodidae:

*

*

•

*

*

*

*

#•

*

#

*

The Relation of Antiquity to Distribution. — The distribution
of the genera in the family of Cerithiidae illustrates another

GEOGRA PHICA L D IS TRIB U TION.

145

law: viz., old genera are widely distributed, while younger
genera are more closely restricted in distribution.

Thus Cerithium is a genus of which species are known
from the Triassic, Jurassic, Cretaceous, Tertiary, Quaternary
and Recent Periods. It is known from all seas, warm and
temperate ; and a species of the genus has been dredged from
the abysmal zone, and other species are known up to the lit-
toral zone of the ocean. Fastigiella, on the other hand, a
genus known no farther back than the Tertiary, is confined
to the Antilles, as present knowledge goes.

Rissoa, of the family Rissoidae, has a similar history ; it is
known from the Jurassic up, and it is distributed in all seas.
In the accompanying table these facts are graphically repre-
sented.

TABLE OF THE GEOLOGICAL RANGE OF THE FAMILIES
CERITHIID.E AND RISSOID^E.

Triassic.

u
1

2

3
•— >

Cretaceous.

Tertiary.

J

1

K

Cerithiidae:
Triforis (Antil., Ind. and Pac. O.)

*

ft

Fastigiella (Antilles) . . . .

#

#

Cerithium (all seas warm and temp ). . . .

*

*

*

*

ft

Bittium (all seas)

*

•55-

*

•*

#•

Potamides (Ind. O., Afric. coasts, and Cal.).
Diastoma

•A"
*

*

? Sandbergeria. . .

*

#

*

*

•X-

ft

*

Ceritella

•K

*•

*•

*•

*

Rissoidae:

*

*

*

ft

Scaliola (Japan, New Caled., and Red Sea). .
Rissoina (warm and temp, seas, Antil., Med.,
and Pac.)

*

*

ft

ft
#

ft

Paryphostoma

*

Distribution in Relation to Temperature of the Waters. — Two
families maybe selected to illustrate this law. In the Lamel-

146 GEOLOGICAL BIOLOGY.

lariidae the genus Lamellaria is distributed in all three oceans
Atlantic, Pacific, and Indian. The three genera Velutina,
Marsenina, Oneidiopsis are confined to boreal seas, and the
fifth genus, Caledoniella, is from New Caledonia alone.

In another family, the Cypraeidae, all the genera are ad-
justed to warm and temperate seas; the principal genus,
Cyprsea, of which more than a hundred and fifty species
have been described, is confined entirely to warm seas ; the
majority of the species are from the Indian Ocean and the
Australian and Polynesian oceans. This genus also dates
from as early as the Middle Mesozoic, species having been
found in the Jurassic, Cretaceous, and Tertiary rocks. Other
genera of the family live in the Mediterranean waters, and ex-
tend across to the shores of the West India islands and Cen-
tral America, and are also seen on the west coast of America.

To select another family, outside the immediate suborder
we are now considering, in the Buccinidae we find as much of
an adaptation to cold waters as in the last case there was to
warm waters.

The genera Buccinum and Siphonalia are distributed in
both boreal and austral seas ; Chrysodomus has a circumpolar
distribution ; Liomesus is only found in arctic and boreal
seas. Other genera of the same family are distributed in the
intermediate seas, both Atlantic and Pacific; and several
genera which are associated by their structure in the same
family, as Phos Hindsia and Dipsaceus, are restricted to the
warmer seas about the Philippines, Indian, China, and Car-
ibbean shores, or the corresponding warmer western coasts of
America.

This family dates back to as early as the Cretaceous
era.

Tabulation of the Facts. — The following table expresses in
graphic form a summary for all Gastropods, of the facts re-
garding the actual present adjustment of the form and struc-
ture of these organisms (as expressed in the different classes,
orders, and families into which they are classified), and the
various conditions of environment (ranging from the abysses
of the ocean to the extremities of leaves of trees in the open,
air) in which they find their normal life habitat.

GEOGRA PHICA L DIS TRIE UTION.

147

TABLE EXPRESSING THE RELATIONS BETWEEN THE DIFFER.

ENCES IN STRUCTURE OF THE GASTROPODA AND

DIFFERENT CONDITIONS OF ENVIRONMENT.

Abysmal zone.

Brachiopod zone.

Nullipore zone.

Laminarian zone.

Littoral zone.

1
PQ

Fresh Water.

Amphibious.

j

*

M

*

arim

*

*

*

Abe
si

*

ve o
irfac
— » —

:ean

s.

1. * * Aerial

Class: i. Scaphopoda

*
( P

(P

*

el a
*
el a

*

*

*

*

g i

c )

*

Class Gastrofoda.

V

*

o

*

ft
*

*

*

#

ft

*
#

3. Opisthobranchia

Order Prosobranchia.

*
( P

*
el a

*
g i

ft

c)

ft

Suborder Cteno bran china.
Section: i. Ptenoglossa

2. Taenioglossa

*

ft

*

Ma

*

*

•

*

ft

rin

ft

*

#
*

e&

ft
*

ft
*

Fr

*

ft

esh

•K

w

*

*

ft
*
*

ate

*

*

#

Section Tanioglossa.

Family Cyclostomidse ... •... ..

Assimmeidse .

Paludinida^

Melaniidse

NOTE. — The stars opposite the name of each group indicate the kind of
environment to which the genera of the group are adapted.

Summary of Results. — This analysis of the distribution of
the various types of Gastropods may be summarized as follows :

The organic structure of Gastropods is such that it is
capable of adjustment to all the conditions of environment
found inhabited by living things on the face of the earth, or
in the waters under the earth.

148 GEOLOGICAL BIOLOGY.

There are examples in the class Gastropoda of orders, all
the members of which are restricted to a narrow and particu-
lar set of environmental conditions, as the Heteropods
(pelagic), and the Pulmonata (above tide-level).

There are other cases in which the structural adaptation is
of subordinal rank, as the Ptenoglossa (pelagic) among the
Ctenobranchia ; and still others in which members of the sub-
order are found under all kinds of environmental conditions;
but certain families are restricted in distribution, as among
trie Taenioglossa the family Paludinidae are all fresh-water
species, the Strombidae all marine, the Cyclostomidae all are
air-breathing and land forms.

Again, among the members of a family there are genera
which are restricted in their distribution to particular condi-
tions of environment, and other genera distributed over a
wider range of conditions, as in the Buccinidae Buccinum is
distributed in cold waters, and thus about the northern and
southern poles; and Phos is restricted to warm seas, and
thus near the equatorial zone.

And, to proceed one step further, particular species, of a
genus which is known to be distributed in all oceans, are gen-
erally restricted to living in a narrow range of environmental
conditions, to a particular limit of depth, to a particular zone
of temperature, and often to a particular geographical position
along one side of a continent or along the shores of a particu-
lar sea or gulf or island.

While, however, there is this great variation in the close-
ness of adaptation of structure to conditions of environment,
it is a general law that the higher the taxonomic rank of a
group of animals the greater is the range of environmental
differences to which its members are adjusted ; viz., the mem-
bers of a family, as a rule, are distributed more widely and
under more diverse conditions of environment than the mem-
bers of some particular genus of the family, or than a par-
ticular species of the genus.

CHAPTER VIII.

WHAT IS A SPECIES?— VARIOUS DEFINITIONS AND
OPINIONS.

What are Species? — Their Numbers and Importance. — In the
previous chapter reference is made to the great importance of
the idea of species to the study of natural history, and in the
following chapter an attempt will be made to answer the
question, " What are species? "

Bronn, in 1849, published a list of all the then known
fossil species.* The list comprised 2050 names of plants,
24,300 names of animals. When Zittel wrote his Paleontol-
ogy f he quoted Giinther's estimate of 320,000 species of liv-
ing animals, and 25,000 fossil animals, already described.
Of this 350,000 species of animal organisms, now known to
science, what is it in each case which the naturalist observes,
and names and enumerates as a species?

Ernst Heinrich Haeckel, in his " History of Creation," in-
sists upon the importance of the idea of species, as follows :
" Even now all the important fundamental questions as to the
history of creation turn finally upon the decision of the very
remote and unimportant question, ' What really are kinds or
species?' The idea of organic species may be termed the
central point of the whole question of creation, the disputed
centre, about the different conceptions of which Darwinists
and anti-Darwinists fight." £

Linne held that there are as many different species as there

* H. G. Bronn, "Index Palaeontologicus," etc. 3 vols. Stuttgart, 1848-49.

f K. A. Zittel, " Handbuch der Palaeontologie," vol. i. Munchen, 1876.

\ E. H. Haeckel, "The History of Creation; or, The development of the
earth and its inhabitants by the action of natural causes; a popular exposition
of the doctrine of evolution in general, and of that of Darwin, Goethe, and
Lamarck in particular; the translation revised by E. R. Lankester," 2 vols.
New York, 1883. Vol. I. p. 42.

149

ISO GEOLOGICAL BIOLOGY.

were different forms created in the beginning by the infinite
Being, and his binary nomenclature, in which each species is
given a specific and a generic name, is the foundation of mod-
ern Natural History.

Definitions of Species. — TOURNEFORT (1656-1708) defined
genus of plants to be " the assemblage of plants which resem-
ble each other in structure," and species as " the collection of
plants which are distinguished by some particular characters."

LlNNE (1707-1778) said that we count as species what has
been created of diverse form at its origin, and later Linne
considered that all the species of a genus were originally a.
single species.

BUFFON (1707-1788) described species as a " continuous
succession of similar individuals which reproduce themselves,
and the characteristic of the species is continuous fecundity"

DE CANDOLLE (1778-1841), the celebrated botanist (as
translated by Wallace in his book on " Darwinism ")* defined
the term thus: "A species is a collection of all the individu-
als which resemble each other more than they resemble any-
thing else, which can by mutual fecundation produce fertile
individuals, and which reproduce themselves by generation in
such a manner that we may from analogy suppose them all to
have sprung from one single individual."

CUVIER (1769-1832) gave what is probably the standard
definition of this school: "ISesptce est la collection de tous les~
corps organises ne's les unes des autres, ou de parents communs
et de ceux qui leur ressemblent autant quils se ressemblent entre
eux" In 1821 the first clause of the definition was changed to
" comprend les individus qui descendent les unes des autres"
This definition may be regarded as the foundation principle of
the school of naturalists of which Cuvier was, probably, the
most distinguished teacher.

ZlTTEL. — In his treatise on Paleontologie, Zittelf says of
species: The single species was considered, by the great
classification naturalists, Linne and Cuvier, as having a real
existence and fixed invariable value ; this opinion was almost

* A. R. Wallace, "Darwinism; an exposition of the theory of natural se-
lection, with some of its applications." London, 1889.

f K. A. Zittel, " Handbuch der Palaeontologie," vol. i. pp. 45, 46.

WHAT IS A SPECIES? !$!

universally admitted by all naturalists until Darwin came
to show that this category was also variable, changing,
and without fixity. The partisans of the first theory ac-
corded to the species a sum of particular immutable char-
acters ; it had always been such as we see it, (species tot
sunt diverse quot diver see forma sunt creatce). . . . The parti-
sans of the theory of Transmutation believe that species
have appeared slowly, the one after the other, and by suc-
cessive transformations. ... In order to limit living species,
the better criterion is furnished by their direct descent.
According to Cuvier, one should refer to the same species all
the individuals which were born, the one from the other, or
of common parents, and which resemble each other as much
as they resemble their parents; the individuals of separate
species are incapable of fertile union, or produce generally
only infertile progeny. In paleontology it is impossible to
control real consanguinity by physiological observation, and
consequently we are deprived of this criterion in the study
of fossil species. . . . One ought to recognize, moreover, that
the value of this criterion is not more absolute in the deter-
mination of living botanical or zoological species, as numerous
species are capable of reproduction without sexual union, (as
hermaphrodites, Jhe products of scissiparity, budding, alternate
generation, parthenogenesis), and there are other species,
recognized as good species, the crossing of which produces
fertile hybrids. ... If, then, the delineation of species is diffi-
cult in botany and zoology, it is evident that it will be more
so in paleontology. The paleontologist is limited to a knowl-
edge of the exterior forms of fossils, and these, moreover,
often incomplete, the better characters having been frequently
destroyed by fossilization. ... In general, there are referred
in paleontology to the same species all the individuals, or all
the fragments, ivhich present certain common characters, and
form a circumscribed group, independent of geological range or
geographical distribution ; they can, nevertheless, be associated
with neighboring groups by a small number of intermediate
forms.

The Theory of Mutability of Species and Evolution. — BONNET
(1720-1793) advanced the idea that diversity of climate, nour-

I52 GEOLOGICAL BIOLOGY.

ishment, etc., might produce new species, and the term evo-
lution, in its general sense, appears to have been first proposed
by him.

LAMARCK (1744-1829) definitely adopted this view, and
under the name of Mutability of Species. The term Develop-
ment was also used by him to express the formation of new
species from pre-existing species by gradual modification, and
the theory was elaborately expounded by him.*

The restricted use of the word Evolution (as adopted in
this treatise) meaning the gradual and progressive change in
the form of species, as distinct from the development of the
individual organism from the embryo upward, to which
Lamarck also likened it, was first adopted, it is believed, by
ETIENNE GEOFFROY ST. HILAIRE in 1825, in a report of his
travels in Egypt ; and the idea was finally elaborated in a book
published in 1831, entitled " Memoire sur le degre d'influence
du monde ambient pour modifier les formes animales." He
maintained the principles of mutability of species, common
descent of individual species from common primary forms, and
the unity of their organization, or unity of plan of structure.

LAMARCK was prominent for the promulgation of the
theory of the mutability of species, and there was warm dis-
cussion between the Lamarckian and Cuvieiiian schools long
before Darwin produced the " Origin of Species." But before
either of these great naturalists the philosophical notion of
mutation of organic forms had been definitely announced.
In the Ionian school ANAXIMANDER (6 1 1-547 B.C.) expressed
the view as a philosophical conception. In describing the
origin of things he gave utterance to the theory that out of
the vague indeterminate first principle by successive trans-
mutation man and animals have sprung.

Philosophical Importance of the Transmutation Theory of the
lonians. — Thus it is seen that as early as the beginning of
Greek philosophy the Ionian school of physicists (Thales and
Anaximander) recognized the principle of change in nature.
Without the idea of change cause has little meaning, and
from a philosophical point of view modern science traces back

* Introduction to " Histoire des animaux sans Vertebres," etc., published in
1815-1820.

WHAT IS A SPECIES? 153

its origin to this old school of philosophy, which recognized
the difference between the all and the parts, and found the
parts necessarily the changed forms of the all. The notion
of change of form, or Metamorphism, led to the seeking an
explanation of it; and the whole idea of evolution, or the un-
folding of things from that which they wrere not, grew up as
men thought on this subject.

Antiquity of the Notion of Evolution. — As Schurman
pointed out in a chapter on Evolution and Darwinism in his
recent book on the " Ethics of Darwinism": " Like most of
the fundamental conceptions of our knowledge, and our
science, the essential elements of the theory [of evolution]
are as old as human reflection ; and among the Greeks we
find these five constituent elements of the modern evolution
hypothesis: The belief in the immeasurable antiquity of man,
the conception of a progressive movement in the life of
nature, the notion of a survival of the fittest, and the two-
fold assumption that any thing, or any animal, may become
another, since all things are at bottom the same." *

Reality of Species Logically Antecedent to the Notion of Specific
Mutability. — But that particular form of the conception which
is formulated in the term mutability of species was first clearly
expressed in the latter part of the last century, and for its
expression it was essential that first there should be a formu-
lated idea of the reality of species. The idea of organic species
had to be conceived of as a fundamental entity at the found-
ation of the science of organisms, before any explanation of its
origin, or of the laws governing its existence, could arise.

The Idea of Species as Immutable. — The school of Linne
and Cuvier developed the idea of organic species, and in giv-
ing expression to the idea, which was abstract in itself, it
became necessary to find concrete delimitation of the species.
This idea of species is as essential to the biologist as the ideas
of atom, of molecule, of force, of energy, are to the physicist
and chemist ; and in the order of development of ideas, it was
natural that the primary definition of species should include
the idea of stability, and it was fully scientific too ; for, as

* J. G. Schurman, "Ethics of Darwinism," pp. 43, 48.

154 GEOLOGICAL BIOLOGY.

we have seen, the species, so far as superficial and even very
careful observation goes to-day, when expressed by so acute
an observer as Huxley, is fundamentally a group of like indi-
viduals, alike for space, alike for time duration. " A species
in the strictly morphological sense, is simply an assemblage
of individuals which agree with one another, and differ from
the rest of the living world, in the sum of their morphologi-
cal characters." *

A Mutable Species necessarily Temporary. — The idea of
" mutability," which was added to the conception of the
reality of species by the modern school of naturalists, is
intimately associated with the idea that the morphological
form of organisms, which constitutes their specific characters,
is temporary, and thus is distinguished from the characters of
atoms which are conceived of as continuing the same through-
out all time. The theory that the species is immutable was
associated with the conception of a primary principle under-
lying each form which was supposed to exist from the begin-
ning with persistent integrity.

The Question of Mutability of Species entirely Distinct from
that of the Origin of Species. — This discussion of species is also
a thoroughly legitimate process for the scientific investigator,
and the two views alike call for an explanation of their origin.
The Lamarckian school was not less free from the unscientific
cutting short of investigation by referring this origin to an
unknown cause. Cuvier and his school argued, We know the
species, but the first cause is a sufficient cause of its origin ;
here it is, and we do not know how it came to be. Lamarck
alike believed in scientific ignorance as to its origin when he
followed Aristotle in calling in spontaneous generation as the
explanation of its existence. According to Lamarck, Life
is purely a physical phenomenon. All the phenomena of life
depend on mechanical, physical, and chemical causes, which
are inherent in the nature of matter itself. The simplest
animals, and the simplest plants, which stand at the lowest
point in the scale of organization have originated, and do
originate, by spontaneous generation. In the first beginning

* T. H. Huxley, "The Crayfish," etc., p. 29.

WHAT IS A SPECIES? 155

only the very simplest and lowest animals came into exist-
ence ; those of a more complex organization only at a later
period.

The Fundamental Tenet of the Mutability School. — Thus we
find that the fundamental difference between the hypothesis
of the "immutability of species" of Cuvier, and that of the
" mutability of species" of Geoffrey St. Hilaire, Lamarck,
and Darwin, consists in the assumption by the more modern
school that the specific morphological characters of organisms
are temporary ; are constantly undergoing slight modification
from generation to generation; and, finally, that separate
species are not such from the beginning, but take their place in
an orderly sequence of phenomena ; that which constitutes the
specific character for each case having an explanation in what
preceded it, and bearing the relation of cause, or taking a part in
determining what shall follow.

The removal of "special creation" from the one theory
and " spontaneous generation " from the other was the natural
result of the progress of ideas, — an opening of the laws of
organic evolution to scientific investigation. These two
hypotheses were the natural recourses of ignorance, and the
present form of our philosophy is no less obliged to find an
unobservable origin for the things of whose existence we
have observable evidence.

State of Opinions when Darwin began his Investigation of the
Origin of Species. — This brings us to the stage in the history
of opinions when Darwin began his investigations. The
mutability of species had been announced and strongly sup-
ported by able advocates. The general principle of evolu-
tion had been formulated centuries before, but was rather in
the stage of speculative opinion than applied hypothesis ; the
facts supporting and illustrating it were not greatly accumu-
lated. Linn£, Cuvier, and their schools had already defined a
great number of species of plants and animals, had classified
them, and had erected an elaborate systematic botany and a
systematic zoology on the theory of immutability of species.
The new theory seemed to shake the foundation of the science
of Natural History. If there is no fixity to the idea of
species, the query arose, what can we talk about ? What is

156 GEOLOGICAL BIOLOGY.

there left for us to investigate ? But in fact, while the muta-
bility of species was received and advocated, the idea of
species was still retained, as evidenced by the title of Darwin's
famous book, "The Origin of Species."

New Conception of the Nature of Species. — The change was a
philosophical one ; no longer was the species considered to
be a permanent entity with definite boundaries, but in the
definition of organic species its time-relations and its geograph-
ical distribution were elements added to those of its morphol-
ogy and physiology. This was a great advance. The organism
came to be recognized as not a mere concrete being independ-
ent and standing by itself, constituted at the beginning what
it is and remaining so during its existence, but as a very de-
pendent part of a greater organism, nature itself, and related
intimately to its surroundings or environment, to the organ-
isms which preceded it or its ancestry, and to those which are
to follow or its descendants, as a sensitive, slowly changing
reflex of all that has been and is. In the new conception
there is the dim outlining of the idea (an old idea, but one
which is day by day growing more distinct and of fuller com-
prehension) that nature itself is a greater organism in which
the species is but one of the organs.

Remarkable Revolution of Thought started by Darwin's " Origin
of Species." — Darwinism, although not pure evolutionism, but
only one phase of it, has done more than anything else to
bring about these changed views of nature. Darwin took up
the general theory of evolution, and attempted to give an
account of the method of its working. The title of his work
clearly sets forth the essential scope of his theory: " On the
Origin of Species by Means of Natural Selection, or the Preser-
vation of Favored Races in the Struggle for Life." This defini-
tion of the origin of species implies two fundamental propo-
sitions, viz. : (i) That the species naturally varies in its
characters, for the natural selection is selection among char-
acters that differ; this is the idea of "mutation"; and (2)
that the reason why one character rather than another is pre-
served is its better adaptation to conditions of environment ;
this is the idea of 4< natural selection."

Darwin brought out prominently the fact, that what we

WHAT IS A SPECIES? 157

call species, i.e., the descendants of common parents, vary
among themselves, and that the variability is substantially
universal. This was elaborated by study of the variation of
plants and animals, and particularly of pigeons under domesti-
cation. The selection which man makes in his stock-breeding
suggested to Darwin the idea that the very conditions of
environment would act in the course of ages as selecting
agencies, favoring the growth and transmission of certain
peculiarities of structure or habit, and working against other
varietal characters, thus eventually perpetuating the favorable
varieties, and causing ill-adapted characters to become lost.
Undoubtedly his observations, when a boy, of the results of
stock-breeding among Leicester sheep and the ideas of Mr.
Bakewell, with whom he was acquainted, impressed them-
selves upon his memory and were the foundation of the
theory, the elaboration of which made him famous.

The Evolution Theory of Biology and the TTniformitarian
Theory of Geology. — Darwin's " Origin of Species " brought the
world to a vivid appreciation of the universal mutability of
all organic things, and the theory which bound together the
mutability of organisms was evolutionism. It is interesting,
from a philosophical point of view, to note that about fifty
years before, a like step of progress was reached through the
uniformitarian theory of Hutton, which set forth the principle,
that during all geological time, there has been no essential
change in the character of geological events ; but uniformity
of law and conservation of force are perfectly consistent with
the mutability in the results and the incessant evolution of
present life out of the dying past.

Evolution and Development Contrasted. — In its general sense
I find no better definition of evolution than that given by
Huxley: " Evolution or development '," he says, " is, in fact, at
present employed in biology as a general name for the history of
the steps by which any living being has acquired the morphologi-
cal and the physiological characters which distinguish it"
Evolution, as has been already noted, in this sense confuses
two processes which may co-operate in the result, but which
may be distinguished in their exhibition in actual facts of the
history. They are technically separated under the two cate-

158 GEOLOGICAL BIOLOGY.

gories of Ontogeny and Phylogeny. Ontogeny, or Ontogenesis,
is the technical term for the " history of the individual devel-
opment of the organized being." Phylogeny is applied to the
history of the genealogical development. Phylogeny, as
Haeckel used it, is associated with the theory * that the steps
of phylogenesis, or of ancestral development, may be deduced
from the observed history of ontogenesis or the development
of the individual. In order to free the term from any theory
of accounting for the history, it is proposed to restrict the use
of the term evolution to that part of the history of organisms
which is seen upon comparing the organisms of one geological
epoch with those most closely similar in the preceding geo-
logical epochs, and to restrict the use of the term develop-
ment to the history of those changes which are observed on
comparing the successive stages of growth of the individual
organism with each other, or the history of a single cycle of
organic growth.

Evolution the History of the Steps by which Variation is Ac-
quired, not Transmitted. — It is evident from this analysis that
in the case of any particular organism the steps by which it
acquires the characters which were possessed by its parents
are steps in the development of the individual ; but the steps
by which it acquires any characters not possessed by its ances-
tors are steps of evolution. The latter characters in every
case are the varietal characters.

It is the acquirement of variation, not its transmission, that
constitutes whatever there is of evolution in the history of
organisms. The terms thus restricted furnish us with names
which can be used independently of any theory. The facts,
or series of facts, may be scientifically observed, recorded, and
defined, and an explanation sought for them.

A Definition of Darwinism. — For the meaning of Darwinism
we may adopt the excellent definition of the Century Diction-
ary. " That which is specially and properly Darwinian, in

* The Recapitulation theory. See, for a clear statement of the principal
features of this theory, the President's address to the Biological Section of the
British Association, delivered at Leeds, September 1890, by Arthur Milnes Mar-
shall, entitled "The Recapitulation Theory," and republished in "Biological
Lectures and Addresses," 1894, pp. 289-363.

WHAT IS A SPECIES? 159

the general theory of Evolution, relates to the manner, or
methods, or means by which living organisms are developed,
or evolved, from one another; namely, the inherent suscepti-
bility and tendency to variation according to conditions of
environment ; the preservation and perfection of organs best
suited to the needs of the individual in its struggle for exist-
ence ; the perpetuation of the more favorably organized
beings, and the destruction of those less gifted to survive ;
the operation of natural selection, in which sexual selection is
an important factor; and the general proposition that at any
given time any given organism represents the result of the
foregoing factors, acting in opposition to the hereditary ten-
dency to adhere to the type, or * breed true ' '

The Lamarckian Theory of Evolution. — " The portion of the
theory of Development [Evolution] which maintains the com-
mon descent of all species of animals and plants from the
simplest common original forms might, therefore, in honor of
its eminent founder, and with full justice, be called Lamarck-
ian ; on the other hand, the theory of Selection, or breeding,
might be justly called Darwinism, being that portion of the
theory of Development [Evolution] which shows us in what
way, and why, the different species of organisms have de-
veloped from those simplest primary forms." *

Phylogenetic Evolution. — We may quote again from the
Century Dictionary the definition of Phylogenetic Evolution :
"It is the name for that form of the doctrine of Evolution
which insists upon the direct derivation of all forms of life
from other antecedent forms, in no other way than as, in
ontogeny, offspring are derived from parents, and conse-
quently grades all actual affinities according to propinquity,
or remoteness of genetic succession. It presumes, as a rule,
such derivation or descent, with modification, is from the
more simple to the more complex forms, from low to high in
organization, and from the more generalized to the more
specialized in structure and function ; but it also recognizes
retrograde development, degeneration or degradation."

The law of Evolution is put in a terse form by Huxley, who

* Haeckel, "Hist, of Creation," etc., vol. i. p. 150.

160 GEOLOGICAL BIOLOGY.

expands the Latin phrase of Harvey " omne vivum ex ovo"
into " omnum vivum ex vivo" and carries the evolution idea
still further in the phrase " omnis cellules cellula."

The Fact of Evolution Established Beyond Controversy; the
Eeal Nature of Evolution to be Learned only by a Study of the
History of Organisms. — The followers of Cuvier, with their
"immutability of species," recognized the principle of " de-
velopment " in the sense above defined, but they believed that
the metamorphoses, which are called evolution, are the results
of independent originating force, or they discarded the belief
altogether. The more modern school, represented by the
idea of the " mutability of species," fully accepts both devel-
opment and evolution as established facts in the order of
nature. This principle of evolution is so far-reaching in its
application, and so dominates the speculations of our times,
that typical illustrations of it as exhibited in the history of
organisms are worthy of special study in order that these
applications to other things may be correctly made, for only
by understanding precisely what evolution is in nature can
one apply the term correctly in discussing the philosophical
application of it to other things.

What is an Individual ? — When we push the analysis of
organic nature farther, we meet the question, What is the in-
dividual ? A very superficial consideration of the problem
shows us that the organic individual is not merely the sum of
the matter constituting the body of the individual at any par-
ticular time. The matter of the individual is not made in the
course of the growth, but it is only organized. The matter
in the case is the food, which before was not a part of the
individual. So that it is true to say that an organic indi-
vidual develops, but the matter it uses is not in any sense
characteristic of the individual, nor is the particular structure
of the cells or tissues, for this is common to other individuals,
but each individual differs from others in the mode and pur-
pose of its activities, and in the results of such activities as
expressed in its morphological characters.

In other words, the organic individual is what it is in each
case, not by virtue of the chemical or physical materials of
which it is composed, but by virtue of the form, structure, and

WHAT IS A SPECIES? l6l

activity of the whole as constructed. Thus the likeness in
form and function, which leads to the classification of organ-
isms as of the same species, does not arise by virtue of like-
ness of the matter involved in its construction, but by virtue
of likeness of the agency by which the particular construction
is brought about. To put this proposition in concrete form, a
particular cat has the form and function it possesses, not by
virtue of any qualities inherent in the bones or muscles or tis-
sues of which it is composed, or in the cells or in the ultimate
chemical elements of which it is composed, but its individual
characteristics are altogether determined by the fact that it
developed from a cat which was its mother.

Descent is the explanation of the particular characteristics
of each individual. In dealing with such characteristics, we
are dealing with the phenomena of life which are continuous, so
far as our experience tells, and depend for their expression
not alone upon the immediate surroundings of the individuals,
but upon pre-existing living organic individuals, its ancestors.

CHAPTER IX.

WHAT IS AN ORGANISM ?— THE CHARACTERISTICS OF
THE INDIVIDUAL AND ITS MODE OF DEVELOPMENT.

Mutability of Organisms a Foundation Principle of all Evolu-
tion.— In an analysis of the meaning of evolution, it is essen-
tial to remember, at the outset, that the evolution takes place
only in respect of mutable things. The species is said to be
mutable, but it is the organic species as contrasted with
everything else. The mutability, therefore, is respecting
organisms only. I have shown how the organic " species,"
which one school of naturalists calls ''mutable," is in one
sense a mere abstract idea but in another it stands for an
aggregate of real existing individual organisms. Such an
earnest advocate of mutability of species as Oskar Schmidt
says, " The retention of species is, moreover, scientifically
justifiable and necessary, if only the determining impulses be
taken into account and the definition reduced to harmony
with reality;" and the definition he gives is, "While we re-
gard species as absolutely mutable, and only relatively stable,
we will term it, with Haeckel, ' the sum of all cycles of repro-
duction which, under similar conditions of existence, exJiibit
similar forms? "*

Morphological Similarity the Characteristic of Species. — The
essential notion in species is similarity of form. The fact
recorded in the term species is the occurrence in nature of
numerous organisms of almost identical form and structure —
individuals which seem, in general, to live and grow sepa-
rately, but are also organically associated together. In order
to explain this community of form among the individuals of
the same species, we must examine into the laws by which

* "The Doctrine of Descent and Darwinism," p. 103, New York, 1878.

162

WHAT IS AN ORGANISM? 163

the individual attains its form, and to this end we must
analyze the characteristics of an organism.

The Definition of an Organism. — Organism may be defined
in two ways: we may point to a concrete example and say,
" That cat is an organism" and then takeaway all those char-
acteristics which are peculiar to the particular example, as its
hair, its limbs, its eyes, its teeth, in fact, all its special organs
and parts, and come down to a fully abstract definition of an
organism, of which the cat is a concrete example; or we
may take the philosophical definition, and with Kant define
the organism to be ' * that whose every part is at once the means
and end of all the rest" For our purposes it is better to
combine the two methods, and say, An organism is a living
being whose every part is at once the means and end of all the
rest ; for it should be insisted that, whatever its full meaning
may be, living is an essential quality of any organism that
either develops or evolves, and the idea of organism includes
the necessary relationship of the parts to each other and to the
whole.

Living and Performance of Physiological Functions are Essen-
tial Parts of the Definition of an Organism. -- " Under one
aspect," says Huxley, " the result of the search after the
rationale of animal structure thus set apart is Teleology, or
the doctrine of adaptation to purpose ; under another aspect
it is Physiology." *

Inversely, then, a dead animal is not an organism. It is
only a mass of organic matter which some organism has con-
structed. So much are we engaged in handling dead animals
and plants that we are apt to overlook this important distinc-
tion. Too often the modern naturalist conceives of the
organism as only an aggregate of matter having a definite
form and structure of parts, as a house might be defined as a
building made of mortar and bricks.

A Zoological Specimen in the Museum as much a Vestige of
Organism as a Fossil. — The animals we see in the zoological
museums and dissect in the laboratories are as much remains
or vestiges of organisms as are fossils ; growth and structure

* Thomas Henry Huxley, " An Introduction to the study of Zoology illus-
trated by the Crayfish," p. 47, New York, 1884.

164 GEOLOGICAL BIOLOGY.

are in intimate association in the organism, and the instant
the organism ceases those changes incident to growth there re-
mains the inert result of the' living, that is, the dead, animal.

Living Implies Change, and Change is Incessant in a Living
Organism. — Living implies change in the organism, and inces-
sant change. This change is what makes growth possible.
The organism at any particular stage is only the morphologi-
cal result of the previous growth, and what we recognize as
the adult form of the individual is as truly mutable as the
species itself. The individual organism, if exactly denned,
is not precisely the same for any two days or moments of its
existence, but one of its fundamental characteristics is that
it grows, i.e., it has development. Almost the same might
be said of any of the parts or organs : so long as they are
acting they are undergoing waste, and repair, and incessant
change ; as soon as this process ceases they cease their
organic function, decay, and return to their material ele-
ments. In the organ, in the individual, in the species, or
in the whole organic kingdom, the morphological form and
the physiological function are of a temporary nature, and
thus essentially differ from the physical or chemical proper-
ties of matter.

An Organism is an Aggregate of Cells. — An analysis of a
plant or animal demonstrates it to be composed of " cells."
Each individual organism is morphologically an aggregate of
cells ; these cells are not all alike, nor are they combined in
the same manner. Another proposition may be accepted
without further examination : every animal or plant begins its
" existence as a simple cell, fundamentally identical with the
less modified cells which are found in the tissues of the adult."

The Organic Cell the Morphological Unit. — The simplest form
of the cell, or, as Huxley calls it, a " morphological unit,"
may be conceived of as a mere mass of protoplasm devoid of
cell-wall and nucleus. He sets forth as fundamental proposi-
tions that, I. " For the whole living world the morphologi-
cal unit, the primary and fundamental form of life, is merely
an individual mass of protoplasm, in which no further struc-
ture is discernible; 2. That independent living forms may
present but little advance on this structure; and 3. That all

WHAT IS AN ORGANISM? 1 65

the higher forms of life are aggregates of such morphological
units, or cells, variously modified."*

The primitive form of the organic individual is the simple
cell of microscopic size, globular in shape and with no distin-
guishable differences in the structure of its contained proto-
plasm. If higher powers of microscope could be brought to
bear, it is not improbable that, like the nebulae of the macro-
cosm, this amorphous unit of the organic microcosm might be
resolved into complexity; but, as we know them, cells are
found almost universally to possess three elements of structure :
(i) the protoplasmic substance of the cell, (2) a cell-wall or
marginal sheath, and (3) a nucleus within.

The Three Ways by which Cell-modification is Accomplished.
— There are three ways by which diversity of form is attained
by the cell :

(1) By movement of the cell itself, exhibited in change of
shape of its exterior form, or of the cell-wall. This is seen
in the Amoeba (Fig. 5.1), which, by drawing in one part and
extending another, assumes various forms, temporarily, but
remains in the simple cell state.

(2) The second method of attaining diversity of form is
by cell- division, which is the common method by which
growth is effected. Reproduction of a new cell is accom-
plished by such division of the original cell, separation of one
part from the other, and completion of its outlines by each
part until division into two distinct cells takes place. The
Protozoa are characterized by this mode of development, and
by the necessary failure to attain complexity of structure of
the individual, which reaches no higher stage of diversity than
the unicellular stage.

(3) The third method of attaining diversity of form is by
cell-multiplication within the individual.

Metazoa Characterized by Histogenesis, or the Formation of
Tissues. — All the animals of the classes higher than Protozoa
are ranked together under the general name Metazoa, and are
distinguished from them by this differentiation of the sub-
stance of the body into cells. This, which is the second

* Huxley, Ency. Brit., gth ed., vol. in. p. 682.

1 66 GEOLOGICAL BIOLOGY.

method of organic development, is called histogeny, or histo-
genesis — the origination or development of tissues; and the
terms cryptogeny and cryptogenesis may be used to distinguish
from it the first method of organic development, which ends
in the reproduction of cellular units, and is confined to simple
enlargement of the cell, as in the Protozoa.

Histogenesis, Cryptogenesis, and Phylogenesis. — In histogencsis
the organic unit is enlarged by the division of the initial cell
into many separate cells forming a compound organism known
as the metazoal individual. In cryptogenesis the organic unit
is a simple cell. As histogenesis begins with cryptogenesis,
and is an enlargement of the scope of organic growth, so we
may conceive of phylogenesis as an enlargement of histogenesis,
in which the unit is the organic species, and the progress is in
terms of specific forms, new species arising by evolution of
the old and modification and expansion of the ancestral types
in their descendants. The growth is growth of the race, and
the specialization is in functions of the individuals, first seen
in the production of sex; this specialization is further de-
veloped in the co-ordination and co-operation of the members
of a family, and is still more highly elaborated in the com-
munity or the race.

Analogy between the Cell and Organism and the Molecules,
Elements, and Minerals of Inorganic Matter. — The results of these
several modes of growth of the organism are analogous to the
categories used in chemical nomenclature. There are physical
units which are called molecules, which may be compared to
cells, the organic units. The chemical element is a molecule,
or mass of molecules, exhibiting uniform properties, or chem-
ical reactions. A mineral is a combination, or it may be a
simple element, exhibiting definite and uniform chemical
composition, and physical characters of weight, hardness,
crystalline structure, etc. As the molecule is resolvable into
imagined atomic constituents, so the organic cell is resolvable
into its protoplasm, and according to the theories of some into
innumerable pangenes or ids, each having its personal char-
acteristics.

The Individuality of the Organism. — On the other hand, as
any particular mineral exists only temporarily and under

WHAT IS AN ORGANISM? l6/

special conditions, so the organic species may be looked upon
as a temporary thing made up of a certain number of actual
individuals, living at a particular time and under particular
circumstances, the individuality perpetuating itself by the
process of generation. But here the analogies cease, as is
explained elsewhere ; the incessant changing of the organic
form and function of the living organism distinguishes it
fundamentally from matter in any other condition.

Growth and Reproduction of the Protozoa and of the Metazoa,
Contrasted. — As will be seen from the above remarks, the
function of reproduction in the Metazoa is a specialization of
the simpler function of growth of the unicellular Protozoa.
Growth in the Protozoa seems to be limited by what may be
called the capacity of the organic cell, and reproduction then
consists merely in producing new cells, or in the multiplica-
tion of unicellular organisms.

Generation the Fundamental Function of an Organism. — In
the Metazoa the growth capacity is enlarged, and in these
higher animals reproduction or generation is no longer the
function of the whole organism, but is specialized off as a part
of its activity ; and in the structure of the organism special
parts, tissues, or organs are set apart or differentiated for
the execution of this special function. The remaining activi-
ties are spent in the development of the individual. Indi-
vidual development, and all auxiliary activities, have to da
with actually existing conditions of life, but generation looks
forward in its very essence to conditions that have not yet
appeared. Generation is, therefore, at the foundation of all
organic life and history, and in the process of generation organs
are constructed before they act, and independently of the exter-
nal environmeut to which they must be adjusted when they act.

Summary of the Steps of Progress in Organic Development. —
To summarize the steps of progress in the organic develop-
ment, we find, first, simple growth ; the cell increases by
absorbing matter from outside, accumulating it, and thereby
augmenting, both as to physical size and to the amount of its
organic force, whatever that may include. This process goes
on until the cell reaches the limit of its individual capacity,
until growth ceases.

l68 GEOLOGICAL BIOLOGY.

Secondly. Some sort of division or fission sets in which
begins with the cell-nucleus; if fission becomes complete, it
is unicellular reproduction and the organism is protozoal.
This process, repeated over and over again, is what may be
called cell-genesis, or cryptogenesis. This is unspecialized
growth, and the cell, when considered as carrying on inde-
pendent existence, may be called an undiflerentiated organism.

Thirdly. When the fission of the developing cell is incom-
plete within the walls of the cell, the process goes on until
there is repeated cell-division, or segmentation, and the
dependent cells are more or less specialized and combine to
form tissues.

Fourthly. The tissues develop into separate organs, capa-
ble of carrying on special functions, and we have a metazoal
animal, in which the several parts act for the interest of the
whole body. The product is a complex organism with organs
made of specialized cells performing special functions.

Growth, strictly speaking, is thus a function of the cell,
which culminates in cell-multiplication by fission, or partial
fission, augmenting the mass and force of the individual.

Development is that kind of growth which takes place in a
multicellular organism when, by generation, a nucleated cell
is set apart, protected, nourished, and by division and differ-
entiation is elaborated into a complex organism, without
regard to the growth of the parent — even at its expense, and
when fully constructed is set free to begin independent life
for itself.

Evolution is that kind of growth which is expressed in the
specialization of functions and differentiation of organic struc-
ture in some members of a species, enabling them to exceed
the capacity of their ancestors, and to adapt themselves to
conditions beyond or other than those to which the parent
form was adapted. The evolution is exhibited in a series of
forms, succeeding one another, in which varietal, and ultimately
specific, differences distinguish the later from the earlier mem-
bers of the series. Such a series is called a race, and the repre-
sentatives of a race which are alike are called a species.

Embryology. — The development of the individual is par-
ticularly discussed under the name of Embryology, and the

WHAT IS AN ORGANISM? 169

student is referred to special treatises on this subject for in-
formation regarding the details of the process, but a few-
general statements may be of use in forming a correct notion
of the nature of organisms in general.

The typical cell is composed of a mass of protoplasm with a
more or less distinct cell-wall, and, generally, a nucleus, very
minute in size and escaping resolution into its elements, but
giving evidence of performing some very important functions
in the cell when examined with the highest powers of the
microscope.

FIG. 43. — Agamogenesis by fission. c-g= the several steps in the process of generation from the
parent form a to the production of four separate individuals g.

The Functions of a Metazoal Organism; Generation. — In the
Metazoa there are three groups of functions, viz., sustenta-
tion, generation, and correlation. Generation is the name of
the function by which organic individuals are produced, or, as
is commonly said, reproduced.

Agamogenesis. — There are three (or, including alternate
generation, four) modes of generation. Agamogenesis, of two

o a a

FIG. 44. — Agamogenesis by budding. Generation in which the parent individual retains its in-
tegrity, sending off a young but relatively immature offspring (/) as an external bud.

kinds, by fission (i) and by budding (2). This mode may be
represented diagrammatically by the following series :

I. In this series the simple parent individual (a) by sub-
division into sub-equal parts becomes four separate individu-
als (g), each capable of independent existence (Fig. 43).

II. The second mode of agamogenesis may be represented
by the above diagram (Fig. 44).

Here a modified fission takes place, the original individual
retaining its integrity and sending off a bud, which, after
partial development, is separated, completely or partially,,

GEOLOGICAL BIOLOGY.

from the parent ; this is called budding, from its similarity to
the mode of budding in vegetable growth.

III. This budding process may proceed within the parent
individual when separation takes place by an act of expulsion,

o d 0

FIG. 45. — Agamogenesis by internal budding, in which the young germ is formed within the body
cavity of the parent, and when complete is suddenly expelled as a free individual.

suddenly instead of gradually, which gives a third type of
agamogenesis, as may be illustrated by the diagram Fig. 45.

In this case the offspring comes forth immature in develop-
ment, but complete in organization. All three of these modes

o o

FIG. 46. — Monoecious gampgenesis. Sex differentiation, represented by the symbols A male, -f-
female, taking place within the parent individual (a), the several steps consisting of union of
the two elements (£), development of the germ (c), its discharge (d e) and becoming a free
individual, the parent retaining its integrity (f).

of generation are called agamogenesis, because there is gen-
eration without sex differentiation.

Gamogenesis. — Gamogenesis is that mode of generation in
-which distinction of sex is accomplished in the individuals be-

©' O'' c*5' rt ft n cl-fl

w V±y v±y v±y v±y \±J v+y v+y

FIG. 47. — Dioecious gamogenesis. In this mode of generation sex differentiation has taken place
before the individual is complete, and co-operation of two distinct individuals is essential to
each act of generation (a a'). Separate organic elements are developed in the sex individuals
(bb') : the spermule is extruded from the male individual (c i 2), is brought into contact with
the ovule (3 £•'), the two elements unite (d\ segmentation and development of the ovum (e f)
take place, the ovum is developed as a dependent individual until it is capable of independent
existence, when it is extruded and set free (g i and 2) either as a male or as a female individual

Or 2).

fore generation begins. Gamogenesis may (IV) be moncecious,
in which case the sex differentiation has proceeded only so far
as to differentiate the organs within the body of the individual
organism, each individual developing both of the sexual ele-
ments. This mode of genesis may be represented by Fig. 46.

WHAT IS AN ORGANISM? l?l

In this case the generation is sexual, but hermaphrodite,
and the product of generation is set free after being developed
sufficiently to carry on independently the functions of life.

The other mode of gamogenesis is dioecious (V), in which
the differentiation of sex has proceeded so far as to affect
individual life, and to require the co-operation of two differ-
ent individuals for the accomplishment of the function. This
is the more frequent mode of generation in the animal king-
dom. It may be represented diagrammatically by Fig. 47.

The Several Stages of Development in the Higher Organisms. — In
this series there are several stages of development which it is

'FiG. 48.— Segmentation of the ovum. A, B, <7, various stages of segmentation. Z>, blastula.

(After McMurrich.)

important to note. There is, first, the stage of sex differentia-
tion in the individual, the one being called male, the other
female. This appears early in the life of the individual, but
in its earliest stages there appears no discernible difference of
form in the organs of the two sexes.

Second. This distinction is carried on independently in
the growth and development of each kind of individual ; organs
-are specialized and differently formed, and finally result in the
production of specialized cells, called in the one case
Spermule (Spermatozoaii], and Ovule (Ovum) in the other.

Third. The conjunction of the spermule and ovule, formed

1/2

GEOLOGICAL BIOLOGY.

within the organism of separate individuals, is the next essen-
tial step in the process, and the ovule, thus fertilized (as the
result of this conjunction is called) proceeds under proper
conditions to further develop, and when sufficiently developed
for independent life is thrust out of the parental organism,
is separated, and becomes a separate individual, as repre-
sented by the stages, d-g, Fig. 47. The distinction of sex is
again represented in the new-born individual which is born
already differentiated (2 g, Fig. 47) in this respect, and as it
matures develops the organization of either a male or a female
individual, and only as thus differentiated is the continuation
of the process of reproduction possible.

With the third stage of cell-development, above described,
begin the processes of cellular differentiation, or histogenesis,
within the walls of the cellular organism. The segmentation
of the contents of the interior of the ovule is the first step in
this process, and results in the formation of innumerable

spherules. The cell in this con-
dition is called a blastula, or
morula (according to the extent
and mode of its segmentation).
The blastula results when the
segmentation affects only a part
of the cell-contents, and a hol-
low ball-like cell is formed ; in
the morula the whole cell-
contents are segmented, or, at
least, the unaffected part is
relatively very small, and the

FIG. 49-Gastrula stage of the ovum. (After result is a Solid ball of Cellules
McMurrich.) (pig< ^

Fourth. The next stage of development is the formation
into a gastrula, in which specialization of the secondary
spherules or cellules take place, and an outer and an inner
layer are formed. The typical gastrula is formed by the
dimpling in of the hollow sphere of the blastosphere to form
a two-layered cell (Fig. 49).

The Primitive Tissues, Endoderm, Ectoderm, and Mesoderm. —
The Ectoderm and the Endoderm are the primitive undifferen-

WHAT IS AN ORGANISM? 1 73

tiated tissues from which develop, as growth proceeds, the
special organs. There is also formed very early in the de-
velopment of most of the higher animals, the Metazoa, an
intermediate layer called the Mesoderm.

These several stages of histogenic development distinguish
the Metazoa from the Protozoa, and the distinction might be
stated by describing the Protozoan as a cellular animal, the
Metazoan, as a tissue-bearing animal.

The Special Organs Arising from Primitive Tissue Layers. —
This is not the place to go into further details regarding the
mode of development of organisms, but, as illustrative of the
degree of specialization of function already outlined in the
distinction of the tissues of the gastrula into Ectoderm, Endo-
derm and Mesoderm, the following summary of the organs
which develope in the Vertebrate from each of the primitive
tissue layers is given.

1. From the Ectoderm arise the epidermis, the nervous
system, and the infoldings at each end of the intestinal cavity.

2. From the Endoderm arise the mesenteron and its ex-
tensions, the lung, liver, etc., and the notochord (later, the
backbone).

3. From the Mesoderm arise dermis, muscles, connective
tissue, bony skeleton, and probably the reproductive organs.

The Embryo Stage, characterized by Dependence and Passivity,
is not subject to Individual Struggle for Existence.— Fifth. The
stages of development, enumerated under the preceding
section, take •» place either within the cavity of the parent
body or within a food-holding case provided by the parent ;
in other words, the organism is not free, building up its
growth by its own energies, but it is still attached and de-
pendent upon the vital conditions and resources of the parent.
It is called a germ, and the embryo stage of development.

In the development of each metazoal animal there is this
dependent stage of development, the embryo stage, of greater
or less length, in which the young organism is not an inde-
pendent individual, and therefore is not subject to the action
of struggle for existence.

The most important fact to note regarding this stage, is,
that it is the stage in which all the differentiation of tissues

1/4 GEOLOGICAL BIOLOGY.

(up to the formation of the completed organs — those, at least,
that are essential to independent activity) is carried on with
relative passivity of the embryo itself; and the determination
of all this development is traceable directly to the parent, and
not to the environment of the developing organism. How-
ever much the length and extent of this embryo stage may
differ in different kinds of animals, it is clear that there is
such an embryo stage of development in all metazoa.

The Stage from the Free Existence of the Individual to the
Maturing of its Functions. — Sixth. The next step in the de-
velopment is the setting free of the organism from its
embryo stage ; its birth marks the beginning of the infantine
stage in the higher Vertebrates. The higher the differentia-
tion and the more complex and specialized the organization,
so much the longer is the dependent or preparatory stage
extended.

In the higher animals, for instance, some of the systems
of organs are not completed at birth, particularly the genera-
tive system ; these gradually mature, and the stage from
birth to the perfection of this system of organs is the infantine,
larval, or juvenescent stage. Full maturity is reached only
when the whole organism is fully developed and capable of
independent life and the execution of all the functions of life.

The Cell an Organism. — From what has already been said
the essential elements of the organism may be learned. Re-
curring to Kant's definition of an organism as "That whose
every part is at once the means and end of the whole," we
observe that one of the first marks by which we recognize the
simplest cell to be an organism is its division into parts, with
what we assume to be different functions, because they do
play different parts in the history of the cell.

Differentiation of the Cell a Mark of its Organic Nature. —
The simplest differentiation of parts which we are able to
observe is that expressed by the cell-wall. This is a differentia-
tion of the superficies as a protective shelter for the interior.
If, in contrast, we break open a crystal there is no essential dif-
ference between the outer and inner parts. A further special-
ization of parts and function is seen in the nucleus as a differ-
entiated part of the cell. All cells do not appear to be pos-

WHAT IS AN ORGANISM?

sessed of special cell-walls, but the lack of them may be due
to the imperfection of our vision, or to imperfectly formed
cells ; although the cells whose existence appears intrinsically
dependent upon their own activity possess the nucleus, it
is not fully evident what the function is which the nucleus
plays. It is sufficient for the present purpose to note that it
is a specialization, by the activity of the whole cell, of a part
of itself for the execution of some function essential to the
existence of the cell as a whole. Morphologically it is a dif-
ferentiation of form and structure; physiologically it is a
specialization, a division of labor or function, within the cell.
When the cell acts in generation the same principle is at work ;
that is, a partition of material substance, or of morphological
characters, with a retention of common interests. So long as
the segmentation of the yolk goes on there is the differentia-
tion of parts, but each part is essentially a part of the whole,
and the segmentation is but an increasing of parts with the
growth of the individual. As the segmented parts arrange
themselves into orderly series, and, like soldiers dividing into
platoons and companies, they march off to construct them-
selves into organs and tissues, the same principle of organic
growth is expressing itself in the organism — the enlargement
of the function of the whole by the increase of the number of
active parts.

Differentiation and Specialization the Marks of an Organism.
— Differentiation and specialization are intrinsic marks of an
organism. They are essentially processes of increment of parts
and functions by division, and not by addition. The activity of
the organism ever tends to increase heterogeneity, or dissimi-
larity of kind of its parts. The activity of non-organism
tends to the decrease of heterogeneity. In gravitation this is
illustrated wherever the law of gravitation expresses itself in
action ; two things tend to approach more nearly to a state of
uniformity regarding the law of gravitation, and so the final
end of activity of the law of gravity would be a perfectly
homogeneous mass, in which the attraction in every direction
would be uniform. So chemical action is a process by which
the heterogeneity of chemical composition is reduced ; the acid
and the salt unite to form a more stable compound, each of

176 GEOLOGICAL BIOLOGY.

the heterogeneous chemicals uniting to form a homogeneous
compound. The final result of chemical action is the com-
pound with homogeneous properties throughout, theoretically
and historically composed of sundry elements, but effectively
simple, uniform and homogeneous. So, too, in crystallization
the tendency is, in the heterogeneous solution, for the like
things to associate according to regular arrangement of parti-
cles; from heterogeneity of arrangement the law is toward
simplicity and regularity of form.

The Attainment of Heterogeneity. — When these two modes of
activity come into conflict the organism expresses its vitality,
we say, by overcoming the disintegrating chemical and physi-
cal forces about it. The intrinsic tendency of organism is, then,
to attain heterogeneity, or dissimilarity of kind, dissimilarity
of form, morphologically, and dissimilarity of function,
physiologically. This we see in the development ot the
cell, in the construction of tissues and of organs, in the
growth of the individual, or technically, in all the stages of
ontogenesis.

Grand Results of Ontogenesis, or Development of the Individual-
— This is not the place to discuss the details of ontogenesis, —
in the departments of Histology, Physiology, Zoology, and
Botany these details are fully elaborated ; but it is important
to note what are the general results involved, or the history of
the stages by which the individual attains its distinguishing
characters. The first analysis of the organism shows us that
the two primary characteristics of organism are form and
growth, and, in describing any individual organism, to be com-
plete, our description must include an account of both the
morphological and the physiological characters. From the
earliest life of the cell this development is a process of divi-
sion— division of substance or differentiation, division of
action or function, i.e., specialization. The great complexity
of the higher organism is accomplished, not by addition and
aggregation of new particles from outside, but it is a work of
the cells from within, taking in crude physical matter, assimi-
lating and reconstructing it, and then, by subdivision de-
veloping the general structure. In the higher organism the
result of this elaboration is seen in a great elaboration of

WHAT IS AN ORGANISM? 1 77

structure and differentiation of parts, called organs, and of
the specialization of the functions of these organs

Classification of the Functions of a Vertebrate. — Analysis of a
highly specialized organism, such as a vertebrate, presents us
with three groups of functions, viz. ; Sustentation, Genera-
tion, and Correlation.

L Sustentation, or Assimilation, is seen in the various
processes of what we are accustomed to call growth, the tak-
ing in and digesting of food, and the building up of tissue.

In assimilation two kinds of results are attained. The
morphologic effects are technically called metabolic changes;
these may be divided into changes of two kinds : Constructive,
or Anabolism ; and Destructive, or Katabolism. The destruc-
tive process, or katabolism, results in two special functions:
Secretion, which is the preparation of products necessary to
the anabolism, or to the constructive work of the organism ;
and Excretion, or throwing off of useless products of the
activities of the organism.

II. Generation, or Reproduction — vegetative multiplica-
tion.

III. Correlation, exhibited in higher organisms in two
ways ; as (a) Contractility — seen in muscular action, and
(b) Irritability — as seen in responses to any exciting cause
or sensation.

In the following table is shown the relation of the systems
of organs and special tissues to these groups of functions :

I. SUSTENTATION.

(Itt) Nutritive ...... Alimentary system : mouth, stomach, intestines, etc.

(Ib) Circulative ...... Circulatory system : heart, veins, etc.

(Excretory organs : kidneys.
nO Purificative J Respiratory organs : lungs, etc.

j Secretory organs : liver, salivary gland, pancreas,

[ etc.

i Generative organs : ovaries, etc.
Auxiliary organs : egg-capsules, uterus, mammae,
etc.
111. CORRELATION.

f Muscles.

/TTTa* /- »• Skeletal parts : exo- and endo-skeleton, for fixation

1 1 support, protection, and offence, as teeth, clawsi
(_ bones, shell, coral, etc.

Irritability \ Nerve-ganglia : nerves and brain.
itaoiuty - organs . eye> eafj etc

1/ GEOLOGICAL BIOLOGY.

Such are the steps of the growth and development of the
individual by which it passes from a condition of homogene-
ous protoplasm to the elaborate organization of the highest
animal.

Are the Laws of Ontogenesis the Same as those of Phylogenesis ?
— If we are right in stating that this increasing of the heter-
ogeneity is an essential and fundamental law of organism,
does it follow that it is also an essential and fundamental
law in the processes of phylogenesis, or evolution of species?

The Meaning of Function. — Before answering this question it
is necessary to consider that the use of the term function, as
applied to an organ or part of an organism, is quite analogous
to the use of the term property as applied to a chemical or
physical substance. The mineral loses its crystalline proper-
ties when it is melted and the morphological arrangement of
its particles is destroyed, although it is the same matter as
before, and for the reason that the crystalline properties con-
sist in the morphological arrangement of the molecules, not in
their chemical composition : so the animal has lost its proper
organic function when the physiological processes cease to
operate, although the morphological form and constitution of
the organic structure still remain. As the crystalline proper-
ties are the peculiar marks of the mineral, so the physiological
functions are the peculiar marks of the organism, and, teleo-
logically, the structure of the organism is built up for the
purpose of these functions. The question thus arises: In case
there are hindrances to the accomplishment of the functions
of any organism as it develops, is it not according to analogy
in the other fields of nature to expect the organ to adjust
itself to the hindrances to the extent of the capacity of the
organism to vary its form?

A mineral in crystallizing arranges its particles so that, left
free to express its characteristics, a particular crystalline form
will appear. If a physical obstruction appears in its way, this
form will be imperfect, but the law of crystallization is ex-
pressed as far as possible ; the whole process of crystallization
does not cease because of the hindrance to its perfect action.

If we consider function to exist prior to, and to be the
raison d* etre of organization, it is to be expected that func-

WHAT IS AN ORGANISM? 1/9

tional activity of growth and development will go on normally
at the expense of change of morphological form.

Normal Growth. — This explanation assumes that there is a
normal growth, and the determining of what is normal to
each individual is found in the ancestry; i.e., at the outset of
embryonic growth the normal function of the development of
the individual is already determined. This includes the attain-
ment of the morphological and the physiological characters of
the class, order, family, genus, and species to which the
organism belongs. The egg at the first appearance of the
embryo is determined not only to be a vertebrate, but a bird,
of the order Rasores, of the suborder Gallinae, of the family
Phasianidae, of the genus Gallus, and of one of the many
varieties of the species Gallus domesticus. Such is the normal
development for that particular embryo. The laws of the
development in its every step may be studied, and have been
very fully traced in this particular case, and the knowledge of
the law is based upon the observed order of these steps in the
development ; the inference which we naturally draw is that
every new development of a similar egg will be the same.

Natural Selection, as an explanation of the changes which
transpire in phylogenesis, assumes that the slight adjustments
of the morphological characters, which take place in the onto-
genesis of the individual, are added to the determining factors
of development for the next generation ; that adjustments
which are very slight in each case, by accumulation from
generation to generation, bring about the differences which
distinguish the various species, genera, families and orders of
the classes of the animal kingdom. And this is what is
meant by " descent with modification." Instead of the idea
of descent along a uniform line, in which the offspring differs
in only unimportant and strictly variable characters from any
of its ancestors, the school of Darwin holds that the slight
variations observed (between the offspring and parent, or
among the offspring of a common parentage) do not tend to
become less in succeeding generations, but that the variations
have unequal values in relation to the advantage of the in-
dividual ; and in the struggle of individuals for life, those
individuals possessing the slightest advantage over their fel-

ISO GEOLOGICAL BIOLOGY.

lows will, in the long-run, survive them in the race, and they
will increase and prevail while the others will drop out and be
lost.

Definition of Ontogeny and Phylogeny. — In the analysis of
Huxley's definition of evolution (or development) the two-
fold division of the history is adopted, which is expressed in
part by the terms Ontogenesis and Phylogenesis, introduced
by Haeckel. Haeckel briefly defined these terms, as follows:
Ontogeny, or Ontogenesis: The history of the development
of the individual (including Embryology and Metamorphol-
°gy) '• Phylogeny, or Phylogenesis : The paleontological history
of the development of the ancestors of a living form. It is
proposed to restrict the term development to the meaning
expressed by Ontogenesis, and to restrict the use of evolu-
tion to Phylogenesis. In Ontogeny we find the individual
organism beginning with a great majority of its lines of
development or steps of metamorphism already determined
for it. Take, as an example, the crayfish, which Huxley
has so interestingly dissected and described,* and of it we
can say at the first stage of the embryo that in case it lives
at all, whatever the conditions of environment may be, it
will develop all the characters of the branch, class, order,
family, genus, and species to which it belongs. Its name,
Astacus fltiviatilis, applies to it in all stages of its develop-
ment from the embryo up (Fig. 50).

The Main Features of Development Predetermined before they
Begin. — We can predict before any trace of the characters
appear (with as great a degree of certainty as we can predict
the result of combining a given acid with a grain of chemical
salt) what the path of development will be which the embryo
will take if it continues to grow. It will surely develop a
jointed body, with the articulated limbs and chitinous crust of
the Arthropoda. It will surely develop a breathing apparatus
of gills situated on the maxilliped and legs of the Crustacea.
The appendages of the cephalothorax will certainly be an-
tennae, and the specialized biting mouth parts of the sub-class
Neocaridae, not the simple legs of the more ancient sub-class

*"The Crayfish, an Introduction to the Study of Zoology" (Appletons,
1880).

WHAT IS AN ORGANISM? l8l

Paleocaridae ; it will have the twenty segments, the special-
ized carapace, the pair of mandibles, the two pairs of locked
maxillae, and other characters of the order Decapoda, and all
the peculiarities of the family Potamobiidae will be strictly
carried out. These concern the whole of the morphology, but
in some characters of still less importance the certainty is
not so great. This individual will develop on the first somite
or ring of the abdomen small appendages, — certainly if it be
a male, and exceptionally if it be a female, — whereas, if its

FIG. 50. — Astacus fluviatilis. Side view of a male specimen (nat. size), bg, branchiostegite; eg,
cervical groove ; r, rostrum; /, telson; i, eye-stalk ; 2, antennule ; 3, antenna; g, external
maxillipede ; 10, forceps; 14 last ambulatory leg; 17, third abdominal appendage ; xv, the
first and xx the last abdominal somite. (After Huxley.)

ancestors had been the closely allied Parastacidae, no append-
ages would be developed. Again, in all the details of struc-
ture of parts it will be a true Astacus, and not a Cambarus, a
closely allied genus; and finally, if it were taken to California,
and placed under identical conditions with the native Astacus
nigrescens, it would still differ in all its specific characters from
that species — characters which consist mainly in differences of
form and proportion of the parts, which are in number,
structure, and function the same for the two species.

Slight Possible Effect of Environment. — Environment might
produce slight modification in some of its very insignificant
characters, but otherwise rts total anatomy and physiology

182 GEOLOGICAL BIOLOGY.

were predetermined when it began its development. So it is
with all organisms. It was this fact, of the perfect repetition
of all the essential characters of the ancestors in the new
individual, that seemed, in the minds of the early naturalists,
so absolutely to fortify the belief in the immutability of
species. The slight modifications in unimportant details
appeared as mere accidental imperfections of the individual.
But it was in these slight variations that Darwin found the
secret of evolution.

CHAPTER X.

WHAT IS THE ORIGIN OF SPECIES?— THE PROBLEM AND
ITS EXPLANATION.

WE have seen that there are organic individuals; that they
all, however complex their organization, begin as simple cells,
and pass through, in each case, definite stages of development,
assuming by degrees greater and greater differentiation of the
cell. The chief stages of this development are the cellular
segmentation, the formation of tissues in embryonic growth,
and the attainment of maturity by steps of modification which
are in almost every observable particular the exact repetition
of steps of modification which their immediate parents passed
through in attaining their maturity.

Variation and Mutability Essential Presumptions in the Discus-
sion of Origin of Species. — The differences which the individual
presents, when closely compared with its parents, are called
variations, or varietal characters. The characters which each
individual possesses in common with its parents are classified
according to their importance and permanence, and arranged
in order from lesser to greater, as specific, generic, family,
ordinal, class, or branch characters.

It is a generally accepted belief that the assumption by the
individual of all of the characters which it bears in common
with its immediate ancestors is sufficiently accounted for by
what are called the natural laws of reproduction ; that the
slight departure from exact repetition is an insignificant and
indeterminate accident of all organisms, or that it is an expres-
sion of the imperfection with which the process of reproduc-
tion acts.

The theory of zoologists of the first half of the century
was that the species were immutable ; that variations were
not cumulative, but were always simply variations, the spe-

183

1 84 GEOLOGICAL BIOLOGY.

cies continuing so long as the race continued to reproduce in
its original integrity. With this theory there was no way to
account for species, except by assuming that the difference be-
tween two species is intrinsic, and is not to be accounted for
by the natural laws of reproduction.

The problem of the origin of species came to be a ques-
tion for scientific investigation and speculation at the time
when the idea of fixity of those characters was replaced by
the theory that variability belonged to the specific as well as
to the so-called varietal characters. In other words, in dis-
cussing the origin of species we assume that reproduction is
not a process of exact, but of inexact repetition of characters,
or of imperfect reproduction of ancestral characters in the
offspring. i

Variability an Inherent Characteristic of all Organisms. — Vari-
ability is thus assumed to be an inherent characteristic of all
organisms, and origin of species has primarily to consider how
comparative permanency of characters, and of different sets of
characters in different lines of descent, is brought about.

The Origin of Form, not of Matter. — The origin of organic
matter takes us back to the earliest stage of the universe, and
.as to a choice between a spontaneous origin from inorganic
matter, or an eternal existence of the two kinds of matter,
theories may differ, and for our purposes it is useless to in-
quire. Our search is for the origin of forms expressed by
organisms, and since our studies of paleontology present us
with an orderly procession of changing forms, it is quite le-
gitimate for us to seek among fossil forms for a scientific ex-
planation of the origin of the separate forms, which we call
species.

Definition of Species whose Origin is Sought. — The definition
of species, quoted from Huxley, will suffice for the present
stage of this study of science: "The species regarded as
the sum of the morphological characters in question, and
nothing else, does not exist in nature ; but it is an abstrac-
tion, obtained by separating the structural characters in which
the actual existences, the individual crayfishes, agree, from
those in which they differ, and neglecting the latter."

But again: " Species, in the strictly morphological sense,

WHAT IS THE ORIGIN OF SPECIES? 18$

is simply an assemblage of individuals which agree with one
another, and differ from the rest of the living world in the
sum of their morphological characters;" and further, "in
the physiological sense, a species means, firstly, a group of
animals the members of which are capable of completely fer-
tile union with one another, but not with the members of any
other group; and, secondly, it means all the descendants of
a primitive ancestor, or ancestors, supposed to have originated
otherwise than by ordinary generation."*

Meaning of "Origin of Species." — What, then, do we really
mean -when we ask, What is the origin of species? It is
not the sum of morphological characters, which Huxley says
does not exist, but the morphological characters themselves,
which concern us. It is not the assemblage of individuals
which agree or differ one from another, or the group of ani-
mals which have certain capabilities and have certain com-
mon ancestors, whose origin we are seeking ; it is the origin of
those differences and agreements in morphological characters
which are the marks of the morphological species, and of the
capabilities and disabilities which constitute the characteristics
of the physiological species, that is meant by the phrase * ' ori-
gin of species.'"

Development of Individual Characters Known and Observed. —
The naturalist is familiar with the development of the indi-
vidual ; he knows very well that the adult differs by well-
marked morphological characters from the infant, and more so
from the embryo; and he further knows that the stages of
development are brought about by successive minute changes
of form. The difference existing between the gamecock,
with its complex physical organization and high qualities
of courage, skill, and determination, shown while fighting its
fellow to the death, and the motionless and apparently homo-
geneous yolk suspended in its bed of albumen, are differ
ences brought about by the processes of ontogenesis in a very
short space of time.

The Law of Development. — The origin of the individual or-
ganism with all its complexity, both morphological and phys-

* Loc. cit., pp. 242, 291, 296.

1 86 GEOLOGICAL BIOLOGY.

iological, is not explained by simply calling it development.
Development is the history of the steps by which these char-
acters are attained. It is the term by which we express the
law of this history ; and so long as the idea of the immutabil-
ity of species prevailed there was supposed to be a particular
law of development for each species. This law of develop-
ment was alike for all the descendants of a common ancestor.
By the expression law of development is meant a regularity
in the order of changes or in the sequence of steps by which
the results seen in the mature individual are attained. Every
descendant of a common parentage was thought of as
passing through the same stages of growth in reaching its
maturity.

No Analogy between the Origin and Development of an Immuta-
ble Species. — The origin of the species from this point of view
was explained, necessarily too, in some other way than by
natural development; the reverent were satisfied with consid-
ering it a special act of the Creator; others preferred to ex-
plain it by the fortuitous concourse of atoms. Neither found
any explanation in the natural laws of either generation or
development. That there are different species and that new
species have arisen were accepted facts; but the idea that
different species could be explained by any laws noted in the
development of the individual was not maintained. It was
believed that the characters were specifically distinct for each
species, and that this difference was in itself original.

Inorganic Properties and Organic Characters Compared. — The
case was somewhat analogous to our idea of two kinds of
mineral substances, as gold and iron ; as to seeking an ex-
planation for their origin, we do not attempt it : we either say
they were created so in the beginning, or they appeared
spontaneously concurrent with cooling of the solar system.
Their differences we conceive of as their intrinsic properties.
So with the idea of immutability of species logically there
was associated the other idea, that the characters both mor-
phological and physiological are essential properties or
qualities of the species, and it was no more to be expected
that one would ask why do birds have feathers and dogs have
hair, than why is gold yellow and iron gray. The sufficient

WHAT IS THE ORIGIN OF SPECIES? l8/

answer in each case was, they are the natural properties of the
species.

The Idea of Mutability at the Foundation of the Discussion of the
Origin of Species. — Thus we see that the attachment of the
idea of mutability to organic species naturally led to the
inquiry as to the origin of the properties or distinguishing
marks of different species; and still further, it led to the dis-
sociation of the characters from the species, causing them to
be considered separately. The difference in point of view is
a radical one, and the great amount of dispute and contro-
versy which has resulted may be traced in great measure to
the radical difference of meaning which the two schools
attached to the word species. To a naturalist of the older
ideas it was as absurd to speak of the origin of species as to
speak of the origin of gold ; both of these were supposed to
occur in the world naturally, and that was enough.

What is Mutable? — When we speak of mutability, then, we
ask, " What is it that is mutable? " Physiologically, the muta-
ble element about species is the steps of the development ; that
is, there is not a perfect fixity of the law of development of
the offspring when it starts upon its individual career as an
embryo. Morphologically, the mutable characters of the
species are among the most unimportant of the characters it
assumes ; for each individual of the species they are called its
varietal characters.

A Concrete Example ; Its Characters Symbolically Represented.
— In order to fully answer the question what is mutable,
and therefore what is it that is evolved in the course of the evo-
lution of a new species, we are obliged to consider a concrete
case. We must take an actual individual specimen of a
particular species, and ask, What is it about this specimen
organism which is mutable and has arisen by the evolutional,
as distinct from the developmental, processes of the individual
growth ?

Such an example, whatever it be, has numerous characters
which are recognized by the systematic zoologist, and are
defined by him under separate heads arranged in the order of
rank, the whole constituting the taxonomic definition of the
particular species. To express the relation of these characters

188 GEOLOGICAL BIOLOGY.

to each other and to the individual it is not necessary to
describe them, but symbols may be chosen to stand for them,
and by examining the symbols we may arrive directly at the
meaning of the expression ' ' origination of characters and
species."

If we then express the morphological and physiological
characters by symbols, using the letters B for the characters
of the branch, C for those of the class, O for those of the
order, F for those of the family, G for those of the genus,
S for those of the species, V for the varietal characters, and
the numerals I, 2, 3, 4, etc., for the different types of each
category, we may combine these symbols in such a way as to
express the sum of the characters of a particular individual
organism.

Spirifer striatus Martin, var. S. Logani Hall, taken as the Ex-
ample.— The example chosen for examination is a well-known
fossil, specifically recognized in each of the continents in
limestones of Eocarboniferous age, Spirifer striatus Martin.
The variety which is found in the Keokuk limestones of the
Mississippi valley is called Spirifer Logani Hall in the " Iowa
Geological Report " (vol. I. pt. 2, PL XXL, p. 647). In order
to fully define this specimen and assign its place in the classi-
fication of organisms we must refer it to the branch Mollus-
coidea (B 6) of the Animal Kingdom* to the class Brachiopoda
Dumeril (C 2) and subclass Arthropomata Owen, to the
order Telotremata • (O 4) of Beecher,f to the suborder Heli-
copegmata Waagen, ^ family Spirifer ides King § (F4), sub-
family Trigonotretaria Schuchert, J genus Spirifer Sowerby^f
(G 10), species striatus Martin,** and variety (so-called

*Claus and Sedgwick, " Elementary Text-book of Zoology," translated and
edited by Adam Sedgwick, with the assistance of F. G. Heathcote, part n. p. 71.
London and New York, 1884.

f Am. Jour. Sci., sen in. vol. XLI. p. 355.

\Palczontologia Indica, ser. xiii., " Salt Range Fossils, "by William Waagen,
Pt. i., " Productus-limestone fossils," iv. p. 447. Calcutta, 1883.

§Thos. Davidson, "British Fossil Brachiopoda," vol. I. p. 51. PaUonto-
graph. Soc.t London, 1853, etc.

\Am. Geol., vol. n. p. 156.

^[ Schuchert's list, Am. Geot.t vol. II. p. 156.

** See Davidson's " Brachiopoda," vol. n Pt. V. p. 19, PI. II. and III.

WHAT IS THE ORIGIN OF SPECIES? 189

species) Logani Hall.* Taking Miller's "American Paleozoic
Fossils, "f we count this as species 115, and variety 2.

Thus, to express it symbolically, we should have the
formula for the characters developed in this single particular
species ='B 6+ C2 + O4+F4 + Gio + Sii5+V2;
and all of this is implied in the common scientific name for the
species, Spirifer Logani Hall.

This expresses the morphological characters of the species
arranged in the order of their respective ranks.

New Species Conceived of as Arising by a Process of Variable
Characters becoming Permanent. — Thus it is seen that there
are various degrees of mutability of the characters expressed
by any particular specific individual. The accounting for the
repetition of the characters already known in the ancestors
is by the natural laws of generation. In the example before
us the characters represented by the symbols (B 6), (C 2), etc.,
to (S 115) are supposed to be relatively fixed characters so
far as transmission by generation is concerned, but the
characters represented by (V 2) are distinctly mutable in
generation, the descendants expressing them with varying
degrees of modification from their ancestors. These varietal
characters in the course of successive generations either (a)
drop out by degrees, (b) do not reappear at all, or (c) con-
tinue to reappear in the offspring In case they continue to
appear in the offspring, then they become added to the more
permanent specific characters, and when so added, in place of
(S 1 15 -f- V 2) we have (S 1 16), or a new species, all the other
characters remaining the same. Species (S 116) may be sup-
posed to show further variation, and (S n6-|-V3) and
(S ii6-|-V4) appear, and assume the same relations of
repetition by generation, forming species (S 117), (S 118),
etc.; but after a time the species (S 116), (S 117), (S 118),
(S 119) become dominant. (S 115), (S 114) drop out, and we
have a new genus (G 1 1), composed of the newly arisen
species (S 1 16), (S 117), (S 1 18), and (S 1 19), the constancy of
what was once a specific character becoming more fixed, and

* " Geol. Survey of Iowa," vol. i. pt. 2, " Paleontology," by James Hall, p.
647, Plate XXI.

t3dEd., p. 374

GEOLOGICAL BIOLOGY.

the characters reaching a greater prominence and constituting
the marks of another higher group, and then they constitute
distinct generic characters.

This theory of the origin of species accounts for the
morphological appearance of the new species by supposing
that the future specific characters were first in the state of
simple varietal modifications of the parental forms, and be-
came fixed and permanent in the course of regular develop-
ment in the whole or a part of the members of the descended
race. Those members of the race permanently developing
the new characters constitute the new species.

The varietal character may be algebraically expressed as
either a plus or minus quantity; i.e., the variety may differ
from the typical species by the addition of some slight char-
acter, or by the absence of some character, possessed by the
normal species.

Characters of any Particular Specimen Differ Greatly in
Antiquity. — In regard to the antiquity of the characters the
following facts are known, as expressed in the following table :

TABLE REPRESENTING THE VARYING ANTIQUITY AND DIFFERENT GEO-
LOGICAL RANGE OF THE CHARACTERS OF AN EXAMPLE OF THE
SPECIES SP1RIFER LOG AN I HALL.

„,>,! i Taxonomic rank
Symbol. of characters.

c

0

S

D

Cr

T

J

K

Ty

Q.R

B 6 Molluscoidea

€2 Brachiopoda

O 4 Telotremata

G 10 Spirifer

S 115 S striatus

— —

Va V. Logani

The varietal characters, expressed by the name Spirifer
Logani Hall, appeared geologically for the first time in the
Keokuk limestone in Middle Eocarboniferous time in North
America. The specific characters, represented by the specific

WHAT IS THE ORIGIN OF SPECIES? 19!

name ,S. striatus Martin, are found in all parts of the world
and are characteristic of limestone rocks of the Eocarboniferous
period.* The generic characters of the specimen named Spir-
ifer began to appear in Eosilurian time and continued to appear
till the close of Paleozoic time. The family characters, Spiri-
feridae, do not date back earlier than the genus, and they con-
tinued to appear till the Jurassic era. The ordinal characters,
Telotremata, began in the Eoordovician period, and species
developing the ordinal characters are living at the present
time. The class characters, Brachiopoda, appeared among
the earliest Cambrian fossils, and are represented by numerous
species and genera in the seas of the present time, and the
same may be said of the branch characters, because we have
reached the beginning of our record. Thus it is seen that the
form of organism, called Spirifer Logani, although it has been
extinct for millions of years, developed certain characters,
described as ordinal and class characters, which are still being
repeated in organisms now living; and although the species
is characteristic of the Carboniferous era, and did not appear
earlier or later, it developed characters (genera and family)
which began as early as the beginning of the Silurian, and
others which began in the Ordovician, and still others that
began as far back as our record goes.

The Majority of the Characters of a so-called New Species have
Appeared Before. —When we say, then, that at a particular time
in geological history a new species arose, we do not mean that
the new species differs in toto from its ancestors, but that a form
has arisen which, agreeing with them in the great majority of its
structural characters, yet differs from them by certain so-
called specific characters, their specific rank being indicated
by the fact that they are transmitted to their offspring with-
out modification. The fact of their constancy is all that dis-
tinguishes these characters from varietal characters; and the
generic characters are like specific characters in this particular
of being transmitted without observable modification from
generation to generation.

Theoretically, however, it is assumed that this perma-

* See the time-scale on page 54.

GEOLOGICAL BIOLOGY.

nency is only relative ; that, somehow, the higher characters
become modified as well as the lower. Thus it is supposed
that by such gradual modification, taking place in the course
of genealogical descent, successive individuals arise which
differ specifically from their ancestors, later others which at-
tain generic difference, and after a great many generations
the family characters are changed ; and still later they differ
ordinally, and, theoretically, even such radical differences of
structure as distinguish one class from another may be thus
attained.

Fixed Characters those which are Transmitted Unchanged in
Natural Descent. — In ordinary natural development, or onto-
genesis, there is a law of constancy regarding all the charac-
ters expressed by the symbols B 6, C 2, etc., to S 115. These
may be then called the fixed characters of the species at any
particular time, and be indicated by the letter M. But, as
we have explained, in the course of time among the de-
scendants may appear a new genus, G 1 1 ; the point of geo-
logical time, or the stage in the history, marking such an
event is when the new species assumes dominance in indi-
viduals, and the old forms drop out, and leave a gap in the
series. The species M may be considered as expanding at
this point to include new generic characters, or we may con-
sider the new genus as arising as an offset from the old
forms. It will be seen that all the individuals possessing the
characters M form a common race, and that divergence of
race proceeds from varietal, through specific, generic, family,
etc., characters, and in the order here given; and that the
series, branch, class, order, etc., are expressive of the natural
order in rank of importance of the characters, in their an-
tiquity, and in their fixity.

Rank of Characters, the Precision of their Reproduction, and
their Antiquity. — If we arrange the characters in the inverse
order, thus: V, S, G, F, etc., we have expressed the char-
acters in the order of their increasing importance, increasing
fixity, and constancy of their repetition by generation.
There is thus seen to be a law of relation existing between
the certainty and accuracy of repetition in reproduction, and
the number of times the reproduction cycle has been re-

WHAT IS THE ORIGIN OF SPECIES? 193

peated. This leads us to the further analysis of this process
— the plasticity or the permanency of the characters.

Plasticity of Characters. — In the characters recognized as
plastic in the development of the individual there is possible
adjustment to changed conditions. So long then as any char-
acter is in a plastic, undeterminate condition, it is evidently
not essential. All varietal characters may be regarded as in
such a condition. The theory of Darwin explains that these
tentative characters will necessarily prove of advantage or of
disadvantage ; it may be extremely slight, but, in a close con-
test, sufficient to give the possessor greater or less chance of
success in the struggle for life ; and the perpetuation of such
characters will be brought about by the living of the possessor
of the favorable variation to perpetuate its kind, and the
death of the others.

Origin of Species from the Physiological Point of View. — At
this point we need to consider the origin from the physiologi-
cal standpoint. The name for the process of assuming mor-
phological and physiological characters by the individual is
development, as has already been explained. Reproduction is
that process by which one set of individuals initiates the cycle
of development for a new individual. The principle deter-
mining the repetition of like characters in the parent and off-
spring is called Heredity or Inheritance. Variability is the
principle expressed in the tendency of all vigorous organisms
to exceed the mere repetition of ancestral characters by diver-
gences. Darwin's theory of the origin of species was pro-
posed to account for the existence of different species by a
physiological process.

Darwin's Theory of the Origin of Species. — The full title of
Darwin's work is, " Theory of the Origin of Species by Means
of Natural Selection, or the Preservation of Favored Races in
the Struggle for Life," and its chief points are the following:

1. Variability Darwin found to be a natural law in the
development of all plants and animals.

2. Artificial Selection. — Darwin observed that men, by
selecting, under domestication, plants or animals which
already possess particular varietal characters, can, by breeding
them together, and by preventing their mixing with other

194 GEOLOGICAL BIOLOGY.

varieties, perpetuate the varieties, or can cause a race to
grow up in which the varietal characters shall become relatively
permanent. Numerous facts of this kind are familiar, as our
common breeds of horses, cows, pigs, domestic pigeons,
flowers, fruits, etc. As illustrative of the extreme modifica-
tion possible, the greyhound and the pug-dog may be cited.

3. Darwin further observed that varieties occur under
natural conditions ; that there are doubtful species, or forms
which are intermediate between the typical species.

4. He found by an analysis of the plants of twelve coun-
tries, and the coleopterous insects of two districts (and this
result was confirmed by later study), that the larger genera
present the greater number of varieties and are the more widely
distributed.

5. The natural increase of organisms by generation is
vastly in excess of the actual number reaching maturity ; in-
crease is by geometric ratio, but the increase of adults is, at
best, only a very slight arithmetical ratio. Linne showed that
from a single plant producing only two seeds, if all the seed-
lings were to live, in twenty years there would arise a million
plants. Darwin estimated that from a single pair of elephants
breeding at the age of thirty years, and continuing breeding
until ninety years old, producing three pairs of young in the in-
terval, at the end of the fifth century there would arise fifteen
million elephants alive at one time descended from the first
pair.

6. There are innumerable checks to increase, as nature of
climate and of food, but particularly mutual checks, as strug-
gling of individuals for the same food, or for the same set of
favorable conditions. This is the general law of struggle for
existence.

7. Natural Selection. — Darwin then argued that the con-
ditions of environment ; the abundance of food, or lack of it ;
the favoring climate, or the opposite ; the accessibility of
food, or difficulties in the way of obtaining it ; would all work
together and separately, as either favorable or unfavorable
conditions for each individual, according as he was more
poorly or better adapted to live under them than his fellow ;
that each of the characters of a varietal nature must have

WHAT IS THE ORIGIN OF SPECIES? 19$

some slight value in favor of the possessor, or against him, in
the struggle : the result would be the extinction of those less
well adapted and the preservation of the more favored — i.e.,
a survival of the fittest. This is the law of natural selection.

8. Darwin further added the principle of sexual selection ;
that is, that variations in habit, or even in color, are adapted
to cause a selection in pairing, which will lead to a further
perpetuation of certain characters and the isolation of varie-
ties into breeds, and thus the formation of species proper, or
larger groups of individuals, repeating by reproduction the
originally varietal characters of the few.

9. Darwin noticed that divergence of characters is pro-
duced in animals and plants under domestication, gradually
and as the result of continued artificial selection ; hence he
inferred that the selection acting in nature will also tend to
perpetuate more and more markedly the strongly contrasted
varieties, the intermediate ones blending with the stronger
types ; thus, he believed, the differences, or gaps marking
species from species, are formed.

There were other laws of variation which he noticed.
That use tends to develop, disuse to suppress characters, had
already been emphasized by Lamarck. Habit or custom
favors certain characters. Correlation of parts in growth
tends to cause variation in other parts, as adjustments to
changed organic conditions, and many others ; and the facts
of distribution of organisms were found in line with this theory
of origin of species, and paleontological succession is in har-
mony with it. In his sixth revised edition of " Origin of
Species," published in 1888, Darwin says definitely: " I be-
lieve that animals are descended from at most only four or
five progenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the be-
lief that all animals and plants are descended from some one
prototype."* And in the closing passage of the book he
sums up the essential points of his idea of the origin of
species, speaking of the laws by which all animals and plants
have been produced, thus: " These laws, taken in the largest

* Vol. ii. p. 299.

196 GEOLOGICAL BIOLOGY.

sense, being growth with reproduction ; inheritance, which is
almost implied by reproduction ; variability from the indirect
and direct action of the conditions of life, and from use and
disuse; a ratio of increase so high as to lead to a struggle
for life, and, as a consequence, to natural selection, entailing
divergence of characters, and the extinction of less improved
forms. There is a grandeur in this view of life, with its sev-
eral powers, having been originally breathed by the Creator
into a few forms, or into one , and that, whilst this planet has
been cycling on, according to the fixed laws of gravity, from
so simple a beginning, endless forms most beautiful and most
wonderful have been and are being evolved."*

Do Characters become of Higher Rank as they are Transmitted ?
— The natural and general inference from the Darwinian ex-
planation of the origin of species is that variations, by selec-
tion and invariable transmission, become, in the course of
generations, fixed and permanent characteristics in the off-
spring, which removes them from the rank of variations to that
of specific characters ; by degrees in the course of more genera-
tions these same characters are supposed to become of higher
rank and constitute the generic characters of their descend-
ants; and in the same way further fixation and repeated in-
heritance might make them to become still more important,
and thus to attain ordinal and finally class rank in classification.
The paleontologist may with good reason ask if this be the
fact. Are early genera made up of species whose distinguish-
ing specific characters constitute the distinguishing marks of
genera of later times ? There are those who question the
truth of this proposition as a matter of fact.

Evolution of Genera and Acceleration and Retardation. — The
opinion was expressed by E. D. Copef that the evolution of
generic characters has proceeded in a different manner from
the evolution of specific characters ; that the evolution of
generic and of specific characters has not been part passu, but
independently of each other. He further distinguished two
special laws of evolution — the law of acceleration and retar-
dation, and the law of natural selection.

* Pages 305, 306.

f "Origin of the Fittest: Essays on Evolution," p. 43. New York, 1887.

WHAT IS THE ORIGIN OF SPECIES? 1 97

The essential idea set forth by Cope may be found in the
following quotation from the chapter "On the Origin of
Genera" :

" There are, it appears to us, two laws of means and modes
of development [evolution]: I. The law of acceleration and
retardation. II. The law of natural selection. It is my pur-
pose to show that these propositions are distinct, and not one
a part of the other: in brief, that, while natural selection
operates by the * preservation of the fittest,' retardation and
acceleration act without any reference to ' fitness ' at all ; that
instead of being controlled by fitness, it is the controller
of fitness. Perhaps all the characteristics supposed to mark
generalized groups from genera up (excepting, perhaps, fami-
lies) have been evolved under the first mode, combined with
some intervention of the second, and that specific characters
or species have been evolved by a combination of a lesser
degree of the first with a greater degree of the second mode."

Growth-force or Bathmism. — The defenders of this view are
called by Wallace, in criticising them, the American school
of Evolutionists.* There is assumed to be a special develop-
mental force, called growth-force or " bathmism," which is
exhibited in variation itself, and becomes effective, as phylo-
gerietic evolution, through retardation and acceleration, in the
same way as the force which is expressed in natural selection
operates through the death of the unfit and "the survival of
the fittest toward the evolution of species.

The Origin of Species Still an Open Question. — Many other
theories have been advanced to explain the origin of species:
the examples above cited are sufficient to explain the nature
of the problem ; but it is aside from the purpose of this
treatise to go into detail in the discussion of theories.

It will be observed from the statements already made
that the two great factors in evolution and the origin of spe-
cies are species and mutations. Species with the repetition
of characters and the adjustment to environment are facts
which every naturalist is more deeply aware of the fuller his
knowledge of organisms becomes. Mutation, or the acquire-

* Wallace, "Darwinism," p. 420. This American school is in other places
called the Neolamarckian school.

198 GEOLOGICAL BIOLOGY.

ment of variation, is also a conspicuous fact in nature. To
explain the origin of species involves the accounting for the
becoming fixed or permanent of variable elements of organi-
zation, as well as the accounting for the previous variability
of the characters now fixed.

Darwin's theory and those like it are chiefly engaged in
accounting for the acquirement of permanency of originally
variable elements. The Lamarckians and Neolamarckians are
chiefly interested in accounting for the variability. While
natural selection is effective when the differences themselves
are already on hand, it assumes variability to be a fact without
explaining it. It is necessary to account for variation itself,
and those who assume, that any structural modification which
an organism may acquire during its lifetime may be trans
mitted to its offspring, necessarily emphasize the effects of use
and disuse, the retarding or accelerating of growth, and, in
general, all the factors of variation tending toward variation
of the individual during its life.

It is in the field of observation rather than in speculation
that the solution of these questions is to be found. So soon
as we admit the possibility that the transmission of characters
from one generation to another may not be absolutely con-
stant, we throw back the whole discussion into the field of the
actual laws of progress in generation If the organisms have
varying degrees of the growth-force, if they can in the least
degree choose for themselves the course of development of
their organization, the whole problem of evolution may be
accounted for by the operation of this force — a force which
then becomes the most important factor in the case. But
before we can reach a final theory of the origin of species
we need to know what the facts are. Hence it is that the
whole subject of variation, both in living forms and as ex-
pressed in the historical series, is of vital importance. Not
only is variation an intrinsic law of organic generation, but as
has been shown with overwhelming force, the discontinuity
which we observe separating the character of one species from
those of species next to it in likeness is not a result of natural
selection, " nor has it its origin in environment," " nor in any
phenomenon of adaptation, but it is in the intrinsic nature of

WHAT IS THE ORIGIN OF SPECIES? 199

organisms themselves, manifested in the original discontinuity
of variation." *

It is certain that more light is required upon these funda-
mental factors of evolution before the final word can be said
upon the origin of species. That which distinguishes the
species, in contrast to the variety, is the constancy of transmis-
sion of its specific characters, but it is evident that constancy
here is not absolute constancy — at least it is not known to
be absolute.

In variation, the nature, causes, degrees, and rate of
variation are the subjects of investigation which now promise
to give the true explanation of not only the nature but the
origin of species.

* " Materials for the Study of Variation, treated with especial regard to-
Discontinuity in the origin of species," by William Bateson, London, 1894, p.
567.

CHAPTER XL

THE PRINCIPLES OF NATURAL HISTORY CLASSIFICA-
TIONS.

ILLUSTRATED BY A STUDY OF THE CLASSIFICATION OF THE ANIMAL

KINGDOM.

FOR a clear understanding of the meaning ot the origin of
species it is essential to consider the nature of the nomenclature
of the classification of organisms We have already consid-
ered what species are and what the organic individual is, and
how development is an appropriate term for the growth and
perfection of the individual, and how evolution pertains to the
progressive modifications of the successive species of a genus.

Classifications in Natural History. — Classifications and sys-
tems of classification in natural history are but methods of
expressing, briefly, almost symbolically, the knowledge we
already possess of the characters of organisms and their rela-
tions to each other. A single word, the name of a class or
order, or even the specific name of a species, stands for all the
morphological and physiological characters peculiar to that
species, order, or class. Hence such terms are highly tech-
nical : and though it may not be possible to learn the full
meaning of any of them in a brief course of lectures, it will
be possible to describe the right manner of using them, so
that the knowledge of the details will be arranged in an or-
derly manner under the proper heads as it is gradually ac-
quired.

Species and Genus of Aristotle. — As the facts of biological
science have accumulated it has been found necessary to dis-
tribute them in some systematic manner, and for this purpose
a number of arbitrary divisions having definite names has
been gradually evolved. The use and meaning of these names
will be most easily explained by a brief examination of their
development from the terms Species and Genus of Aristotelian

200

CLASSIFICATIONS IN NATURAL HISTORY. 2OI

logic. Species, the translation of the Greek term ezdos, meant,
when applied to organisms, those having a number of like and
peculiar characters. Genus, the translation of the Greek
yevos, in logic was that which can be predicated of things
differing in species, and as a biological term it was applied to
a group which included several different species.

Scaliger's Terms. — Scaliger expanded the Aristotelian no-
menclature : by him Individual was used to indicate a single
organism (plant or animal), distinguished by having a separate
body, and having a separate and independent activity. Species
was used in the Aristotelian sense, but Genus was found of
three degrees of importance: the Genus pro ximum, the Genus
medium, and the Genus summum.

The Terms of Linn6. — Linne (1735-1766) classified organisms
(both plants and animals), retained the name Genus for the
Genus proximu m of Scaliger, and proposed the term Or do for
Genus medium and Classis for Genus summum.

Cuvier's Perfection of the Nomenclature and the Present Usage*
— These names were later adopted by Cuvier, about the be-
ginning of the present century, and he added the term Em-
branchment, or Branch; and thus was established the nomen-
ture still in use in Biology, which in English is as follows :
Individual, Species, Genus, Order, Class, and Branch (or Sub-
kingdom, or Phylum, or Type). To illustrate the meaning
of these divisions the following examples may be given: A
black and a bay horse would be called two individuals of the
same species. The horse and the ass are two species of the
same genus (Equus). A horse, an ass, and an elephant all
belong to one order (Pachydermatd). The horse, ass, ele-
phant, and lion are of the same class (Mammalia}. All these
would be united in the same branch with the alligator (the
branch Vertebrata). Further subdivision has been very com-
monly made of the order into suborders or families, viz., the
family of Elephantidce, including the elephants and the mas-
todon, and the family of Equidce, including the horse and the
Hipparion.

The Classification of Cuvier. — Linne recognized six classes
in .the Animal Kingdom {Mammalia, Aves, AmpJiibia, Pisces,
Insecta, Vermes). Cuvier made great progress in the distinc-

202 GEOLOGICAL BIOLOGY.

tion of the lower animals. He recognized four branches (Ani-
malia Vertebrata, Animalia Mollusca, Animalia Articulata,
Animalia Radiata). The first four classes of Linne's sys-
tem were united to form the first branch of Cuvier. The
most prominent character uniting them was the possession of
an internal skeleton, bound together by a segmented vertebral
column. The second branch of Cuvier, called Mollusca, in-
cluded six classes (Cephalopoda, Pteropoda, Gastropoda, AcepJi-
ala, Brachiopoda, Cirrhopodd), and the conspicuous charac-
ters of the Mollusca were the possession of a soft, bag-like
body, enclosed more or less completely by a hard exterior
shell composed of one, two, or more parts. Cuvier called the
third branch Articulata, including in it four classes (Annelida,
Crustacea, Arachnida, Insect a]. The chief character in this
branch was the segmented external skeleton, composed of
joints with lateral articulated appendages. The fourth branch
was Radiata, and included five classes (Echinoderms, Intestinal
Worms, Acalephcz, Polypi, Infusoria]. The prominent char-
acter was the radiate structure, typically exhibited in the Star-
fish or Sea-urchin, but ignorance of internal structure led to
the association of many unlike forms. Since Cuvier's time
great advance has been made in the knowledge of the struc-
tural anatomy of animals, especially in the smaller and lower
organisms, and many other classifications have been proposed,
but the majority of Cuvier's classes have remained. Animals
referred to some of the classes by Cuvier, and some newly-
•discovered animals, have been made the types of other classes,
and stricter definitions of the classes already established have
been made.

Uniformity of Usage of Specific and Generic Names. — The
branches have been considerably remodelled, especially by
later zoologists, according as one or other organ or system
of organs has been taken as of chief importance in distin-
guishing the groups. Of the later classifications those of
Leuckhart, Huxley, Claus, Gegenbaur, and Lankester have
expressed new points of view in the arrangement of the or-
ganisms, but in all the confusion of systems a common usage
has grown up in the application of specific and generic names
to animals and plants, and these have constituted the standards.

CLASSIFICATIONS IN NATURAL HISTORY. 2O$

At the present time hardly two standard authors of text-books
of Zoology or Paleontology will be found to apply the no-
menclature of classification in the same way throughout ; that
is, they will not distribute the genera in the same manner, or
will give different value, or will apply different names to
orders, families, and classes.

Selection of a Standard Classification. — It becomes necessary
to use some standard in the matter of classification, and Zit-
tel's " Manual of Paleontology" may be selected as the stan-
dard in the present case. Editions of Zittel are published in
both German and French, but at the present time (1895) no
English edition has appeared.*

Differences of Opinion regarding the Rank of the Characters —
The difference in usage of the nomenclature of classification
is determined by differences of opinion as to the taxonomic
value or rank of characters expressed by the organisms
rather than by any difference in recognizing the characters as
matters of fact. Classifications, therefore, although differing
in the hands of different authors, may be used with precision
when considered as descriptive of the combination of char-
acters expressed in actual organisms.

There are several standard classifications of more or less
common use among paleontologists, three of which may be
here referred to: Claus and Sedgwick's, as given in " Ele-
mentary Text-book of Zoology," 1884; Zittel's classification in
' ' Handbuch der Palaeontologie, " vol. I. , 1 876-1 880 ; Nicholson
and Lydekker, " Manual of Paleontology," 3d Ed., 1889.

Claus and Sedgwick's Definitions of the Nine Branches of the
Animal Kingdom. — Brief definitions of the nine branches, as
given by Claus and Sedgwick, are as follows, viz. :

" Protozoa. — Of small size, with differentiations within the
sarcode, without cellular organs, with predominating asexual
reproduction.

" Ccelenterata. — Radiate animals segmented in terms of
2, 4, or 6; mesoderm of connective tissue, often gelatinous;

* A briefer text-book in German has appeared : " Grundziige der Palgeon-
tologie (Palaeozoologie)," von Karl A. von Zittel, pp. i-viii, 1-971. and 2048
figures; Munich, 1895. An English translation of this work, with some revision
-by American paleontologists, is in preparation.

2O4 GEOLOGICAL BIOLOGY.

and a central body cavity common to digestion and circula-
tion (gastro-vascular space).

" Echinodermata. — Radiating animals, for the most part
of pentamerous arrangement ; with calcareous dermal skele-
ton, often bearing spines ; with separate alimentary and vas-
cular systems ; and with nervous system and ambulacral feet.

11 Vermes. — Bilateral animals with unsegmented or uni-
formly (homonomous) segmented body, without jointed ap-
pendages (limbs), with paired excretory canals sometimes
called water-vascular system.

" Arthropoda. — Bilateral animals with heteronomously-
segmented bodies and jointed appendages, with brain and
ventral chain of ganglia.

" Molluscoidea. — Bilateral, unsegmented animals with cili-
ated circlet of tentacles or spirally rolled buccal arms ; either
polyp-like and provided with a hard shell-case, or mussel-like
with a bivalve shell, the valves being anterior and posterior;
with one or more ganglia connected together by a perioeso-
phageal ring.

" Mollusca. — Bilateral animals with soft, unsegmented
body, without a skeleton serving for purposes of locomotion ;
usually enclosed in a single or bivalve shell, which is ex-
creted by a fold of the skin (mantle) ; with brain, pedal-gan-
glion, and mantle-ganglion.

" Tunicata. — Bilateral unsegmented animals with sac-
shaped or barrel-shaped bodies, and a large mantle cavity per-
forated by two openings ; simple nervous ganglion, heart, and
gills.

" Vertebrata. — Bilateral animals with an internal cartilagi-
nous or osseous segmented skeleton (vertebral column) which
gives off dorsal processes (the neutral arches) to surround a
cavity for the reception of the spinal cord and brain ; and
ventral processes (the ribs) which bound a cavity for the re-
ception of the vegetative organs ; never with more than two
pairs of limbs."

Zittel adopts the older Claus classification, in which the
fifth branch, Mollusca, includes Molluscoidea, Mollusca, and
Tunicata — divisions which are given higher rank in the newer
classification.

CLASSIFICATIONS IN NATURAL HISTORY.

Nicholson separates the Sponges from Coelenterata under
the branch name Porifera ; includes the Vermes and the Ar-
thropoda of Claus in one branch, the Annulosa, making of
them three sub-branches: I. Solecida, II. Anarthropoda (these
two sub-branches together constitute the branch Vermes of
Claus), and III. Arthropoda, which includes the same classes
as assigned to that division by Claus.

The Classes of Importance in Paleontology and their Known
Range in Geological Time — Those classes which are of impor-
tance to the student of the history of organisms are the fol-
lowing: the names are used uniformly so far as to include the
same organisms, but their theoretical relations to each other
are not stated alike by different authors. (See next page.)

Species and Genera of Chief Use in Tracing the History of Or-
ganisms.— When we come to the actual study of the historical
relations of organisms it is specific and generic characters with
which we chiefly deal, and the grouping of them into families,
orders, classes, and branches is the result of the study rather
than a matter of direct observation.

We agree with Zittel * that the systems of classification in
biology are only the expression of our actual knowledge of
the reciprocal relations of the organisms : they depend di-
rectly upon the present state of our knowledge, and are sub-
ject therefore to more or less profound modifications.

The higher categories are built up of generalizations de-
rived from comparison of the detailed structure of the indi-
viduals. All our systematic categories are artificial abstrac-
tions which rest upon the greater or less resemblance of
form in the individuals. The historical relations between the
characters marking these larger categories are not matters of
observation, but only of speculation. The history is to be,
observed in series of successive species, and the study of
classifications becomes of importance in restricting our at-
tention to the field within which all the evidence to be had
must be found. The actual evidence of the history, which
the paleontologist may se.e and examine, is presented in the
specific and varietal characters of the fossil remains preserved
in the rocks.

* See " Handbuch der Palseontologie," vol. I. p. 39, etc.

206

GEOLOGICAL BIOLOGY.

THE CLASSES OF THE ANIMAL KINGDOM AND THEIR GEOLOGICAL RANGE
GROUPED IN BRANCHES ACCORDING TO CLAUS AND SEDGWICK.

i. Protozoa

2. Coelenterata

3. Echinodermata

4. Verities

5. Molluscoidea

6. Mollusca

7. Tunicata

8. Arthropoda

9. Vertebrata

Monera . . . .

Rhizopoda . . .

Infusoria . . . .

Spongia . . . .

Anthozoa . .

Hydromedusa . .

Ctenophora . . .

Crinoidea . . .

Asteroidea . . .

Echinoidea . . .

Holothurioidea
f Platyhelminthes .

Nemathelminthes

Gephyrea . . .

Rotifera . . . .

Annelida . . .
( Bryozoa . . . .
' Brachiopoda . .
f Lamellibranchiata
J Gastropoda . . .
(. Cephalopoda . .

Tunicata ....
T Crustacea . . .

Arachnoidea . .

Myriapoda . . .

Insecta . . . .

Pisces

Amphibia . . .

Reptilia . . . .

Aves

Mammalia . . .

B

Cr

K

Q.R

CLASSIFICATIONS IN NATURAL HISTORY. 2O/

Species of the Paleontologist. — We have already considered
the philosophical notion of species, but the real species which
we deal with in Paleontology is, as defined by Zittel, all those
individuals or all fragments which present certain common
characters and form a circumscribed group, independent of
geological range or geographical distribution, and which may
be linked with allied species by a small number of intermedi-
ate forms. If in the same species certain individuals possess
some peculiar characters which are more or less conspicuous,
they constitute varieties or races of the naturalists. The va-
rieties maintain in some cases the same habitat with the stock
form, in other cases they live in different regions (representa-
tive varieties). It is more difficult for the paleontologist than
for the zoologist to distinguish species from varieties. It
often happens that there are in two contiguous- formations
fossils of the same genus, presenting differences, very slight
but constant, in which case they should be distinguished as
separate species. Fossil species are not always restricted to
either a single geological horizon or bed, nor are they con-
fined to the same geographical region.

Varieties. — The same fact applies in some measure to vari-
eties. Those slight differences, observed upon comparing the
representatives of a species coming from different strata or from
different regions, are considered to be varietal, and not spe-
cific, in case the differences consist in unequal degrees of
modification of the same part or parts, so that the several
specimens may be arranged in a continuous series connect-
ing the extremes by intermediate forms. When such a series
of forms of one species exhibits the differences in connection
with geographical distribution only, the degrees of modifica-
tion are defined as varietal, and those prominent in a particu-
lar locality may then be called distinct varieties.

Mutations. — -When the modification of form is observed to
be associated with succession of their appearance, the differ-
ences are called mutations — a term proposed by Waagen.
Thus modifications of specific form, when contemporaneous,
are called varieties or variations; when successive in time
they are called mutations.

The History of Organisms; the Two Methods of its Study. — The

208 GEOLOGICAL BIOLOGY.

history of organisms may be examined from either of two
points of view, (a) We may examine the embryonic and
ontogenetic course of differentiation of the individual, and,
adding the theory of descent with modification, apply the
laws of individual development to the building of a theoreti-
cal phylogenesis for the whole series of organisms. This is
the method of Zoology, (b) Or, we may examine the fossiL
remains of organisms which have appeared in geological his-
tory, and by comparative study of their characters, arrange
them in series according to their resemblances and differences,
and thus reconstruct the history of the organisms from the
observed order of their appearance on the globe. This is the
paleontological method.

Embryos or Fossils; the Imperfection of the Evidence. — In the
first case the chief criteria upon which the history is built are
the changes taking place in the growing embryo, minute and,
generally, microscopic, and of great difficulty of study. This
method requires great use of imagination in the interpretation
of rudimentary traces of characters, is based necessarily upon
few examples, and those seen mainly by single observers.
The results are of necessity highly theoretical, and, like all
hypotheses, should be regarded as of no value in the face of
facts to the contrary.

In the second case the chief criteria are fossils, which
are the remains of the hard parts and, in most cases, of adult
forms, imperfectly preserved, presenting a very small per-
centage of the total variety of forms that must have lived.
In this method the imperfection of the evidence and the
fragmentary nature of the fossils are the chief sources of im-
perfect judgment. The hypothetical series erected may be
proven by the actual sequence of the forms themselves. The
species may be arranged in the wrong race, but actual suc-
cession is always distinctly indicated, and the filling of gaps
is readily known to be theoretical. The known affinities of
living organisms are also in evidence here, to prevent wild
hypotheses based upon rare and imperfect fossils.

From either point of view the possibilities of error are
enormous, and the proportion of theory to knowledge is
large ; but at the same time it must be said that the two

CLASSIFICATIONS IN NATURAL HISTORY. 2OQ

methods agree in the general results ; and while there is a
vast amount to learn, to which future theories must adjust,
the general facts in the case, which alone we are considering
in these lectures, are already fairly well established.

Mature Individuals, not Embryos, used by the Paleontologist.—
The chief difference between the two points of view, as they
concern us, is that the paleontological method deals essen-
tially with the matured results of individual development.
It is remains of the mature organisms that he investigates,
and he examines the differences between the mature individ-
uals of the successive periods; while in the other method it
is the rudimentary conditions of individuals that carry the
evidence of the affinity.

Differentiation attained during the First or Cambrian Era. —
The paleontologist asks, To what extent has differentiation
proceeded in the individuals of any particular geological
epoch, and on comparing the fossils of successive epochs, in
what respects and at what rate has differentiation proceeded?
In carrying out this method of study we inquire, first, To
what extent has morphological differentiation reached in the
first geological age of which we have record, i.e., the Cam-
brian? In reply the answer may be briefly given in terms
of abstract scientific nomenclature, by stating the numerical
relation existing between the number of the branches and of
the classes of the Animal Kingdom which are known to have
lived in Cambrian time and the total known number in each
•category.

On page 206 is given a table of the branches and classes of
the Animal Kingdom of which record is preserved in the rocks,
with their known geological range. In this summary we
may omit from consideration the branches Tunicata and Ver-
tebrata, of which we have no evidence in Cambrian time ; and
the Protozoa may be omitted from the consideration because,
although it is altogether probable that they were well repre-
sented, traces of them are almost entirely wanting on account
of the minuteness and simplicity of their forms. We may
also omit the consideration of such classes as the Holothuri-
oidea, of which no evidence is found in a fossil state. And,
finally, taking all the other branches, classes, and orders,

210 GEOLOGICAL BIOLOGY.

known in fossil condition, the answer to the question is as
follows : Of the six branches of the Animal Kingdom all six
were differentiated in the Cambrian era; 13 classes of the
26 were differentiated in the Cambrian; of the 73 orders, 14
are known from the Cambrian, 14 more are first seen in Or-
dovician time, 4 more in the Silurian ; or before the close of
the Silurian out of a known 72 fossil orders 32 had already
appeared.

Represented in the form of percentages between the num-
bers represented in the early ages and the number appearing
throughout all the geological ages, we find that, of the dif-
ferentiations of the primary and fundamental nature which
distinguish the branches of the Animal Kingdom one from
another, 80$ of all that has ever taken place was already ac-
complished before the close of the Cambrian. It may have
been still more complete, but this amount we know to have
been the fact. Of differences of only second rank in impor-
tance, i.e., those which mark the separate classes of the ani-
mal kingdom, 13 out of a known 23 fossil classes are already
known to have appeared in Cambrian time, or 56$ of the dif-
ferentiations of class rank had been already attained. In the
evolution of orders at least 32 of the 72 fossil orders appeared
before the close of the Silurian, and 14 orders are represented
in the Cambrian era, or 20$ in the Cambrian era and about
40$ of ordinal differentiation had been accomplished before
• the close of the Silurian.

It is probably well within the facts to say that six out of
the nine known branches were already differentiated in the
Cambrian, and that in all probability all the classes of these
six branches were already differentiated before the close of the
Silurian or third geological era, and probably four fifths of
them in the Cambrian era. In respect of ordinal differentia-
tions, it is probably true that, of the total ordinal differentia-
tion known in these six branches, one fourth, and probably
more, took place before the close of the Cambrian, and one
half before the close of the Silurian. If we recur to the time-
scale, described on page 54, bearing in mind that the rocks
of the Cambrian system may not and probably do not con-
tain records of the earliest organisms that appeared upon the

CLA SSIFICA T ION'S IN NA TURA L HIS TOR Y. 211

earth, but only the earliest records we have of distinct organ-
isms, it will be seen that the statistics given above mean that
at least three quarters of the total evolution of the grander
distinguishing characters of organisms are known to have been
completed before the close of the first quarter of their re-
corded history. The percentage would be much smaller if
the generic and, particularly, if the specific characters of all
known organisms were to be considered ; but to form a cor-
rect idea of what the statement means it is necessary to con-
sider that these latter characters are, both from the point of
view of importance of the characters in the economy of indi-
vidual life and from the point of view of the degree of spe-
cialization to particular conditions of environment, far less
important than those whose differentiation was so rapidly
culminated.

Nature and Extent of the Elaborations. — In order to form a
definite notion of the extent of the differentiation thus early
attained in the evolutionary history of organisms, we may
next consider what structures and functions had been elabo-
rated in each of the several branches of the Animal Kingdom
in the Cambrian era.

In the Cambrian system are found traces of six, at least,
of the nine branches of the Animal Kingdom, and when we
are looking at organic form, of either the morphology or
physiology of organisms, this means that the characters by
which these various branches are distinguished were differen-
tiated before the close of the Cambrian era; and in most
cases there is evidence to show that it was before the "close
of the lower division of the Cambrian. As has been noted,
this statement applies also to a remarkably large proportion
of those characters by which the different classes and even
orders of these branches are distinguished.

Recurrence of Characters accounted for by Descent. — There
follows as correlative to the fact that these characters have
appeared in the Cambrian, that their reappearance in succes-
sive organisms up to the present time is to be explained by
the ordinary laws of heredity. Regarding them no evolution
is observed. Whatever evolution is necessary to explain
their appearance in the world took place prior to the Cam-

212 GEOLOGICAL BIOLOGY.

brian era. It is difficult to appreciate how far back in the
world's history this shifts the great events of evolutional
elaboration, and how little it leaves to be accomplished with-
in even the immense periods of geological time of which we
have the least trace of the history of organisms.

Modern Zoology applicable to the Fauna of the Cambrian Era.
— During the preparation of these pages the writer took occa-
sion to examine the details of form and structure discussed in
the lectures of a well-known professor of Invertebrate Zoology.
It was found that, so far as the evidence is preserved, the
great majority of the differentiations which are considered in
such a course of lectures were actually present in the Cam-
brian era. What has taken place since is differentiation in
respect of less important characters. In other words, a pre-
liminary course of lectures on Invertebrate Zoology (eliminat-
ing the animals adapted to aerial and pure fresh-water envi-
ronment) would be adapted to the fauna of the Cambrian
era. This statement will probably surprise the reader to
whom it comes now for the first time. It is certainly a most
remarkable fact that the great plan-work of structure of all
the invertebrates was so fully elaborated at such an extremely
early period, and that since that time, for the millions of
years that have followed, the modification in organic forms
has been so slight. It is more impressive than the fact that
several genera of Brachiopods (Lingula, Discina, etc.) living
to-day were represented in the Cambrian by forms separable
from them only by the closest scrutiny.

Characters whose Origin is Traced Back to Cambrian Time. —
Assuming the correctness of the above statements, the in-
quiry may be made, What are the characters, expressed con-
tinuously up to the present, which made their first appearance
in Cambrian time ?

First, there is a branch, called Protozoa, all the animals of
which are relatively minute, some of them truly microscopic ;
their bodies are composed of a jelly-like substance, called
protoplasm, without cellular differentiation, and void of per-
manent specialization of function. They show great bodily
activity, but in no permanent direction. The whole sub-
stance of the body seems transiently to be experimenting in

CLASSIFICATIONS IN NATURAL HISTORY. 213

the elementary functions of motion, sensation, digestion, and
reproduction. The one differentiation, which at least numer-
ous kinds of the Protozoa have accomplished, is shown in the
secretions with which they surround themselves, constructed
in definite, forms, but of almost infinite variety.

Second, the next stage of differentiation is seen in each
•of the remaining types of animals, inclusively called the
Mctazoa. In all of these animals (i) there is the localization
of the digestive functions in the interior of the body, the
gastro-vascular cavity, (2) a mouth leading to this cavity, and
(3) the location of the motory functions on the outer side of
the body. In the second branch of the Animal Kingdom,
the Ccelenterata, there is little more of specialization of the
digestive functions than this, i.e. (4) there are two elemen-
tary tissues differentiated, and in this simplest type (as in all
higher) the tissues are formed in the course of individual de-
velopment by the segmentation (40) of the primitive cell (4^),
the formation of numerous cells, and then a (4*:) specialization
of some of these cells as tissues for one function, others of
them for other functions. This process, which is called de-
velopment, may be regarded as a specialization of the gen-
eralized function of reproduction. In the Protozoa repro-
duction takes place by simple fission and gemmation. In this
lowest branch of the Metazoa, the Ccelenterata, the inte-
grality of the body is continued after the separation into
parts, and what constitutes the whole of the reproductive
function in the Protozoa here constitutes but the segmenta-
tion of the contents of the egg, which differentiates the two
layers of tissue — the Ectoderm (4^), or outside layer, and the
inner, or Endoderm (d.e). The fundamental function of the
Ectoderm is motory, the primitive function of the Endoderm
is digestive and assimilative. In the sponges there is de-
veloped between these two layers the Mesoderm (4/), in
which a rudimentary type of skeletal parts, in the form of
horny fibres, or silicious or calcareous spicules, is deposited.
The sponge has differentiated a digestive or gastro-vascular
cavity, but the mouths are several and indefinite, and the
cells within, by their ciliary motions, perform the functions
of motion as well as digestion, thus not exhibiting the full

214 GEOLOGICAL BIOLOGY.

elaboration seen in the true Ccelenterata, but rather con-
stituting a colony of Protozoa-like individuals. The true
Ccelenterata, as illustrated by the corals or Anthozoa, are
elaborated a step further; in their gastro-vascular cavity a
certain polarity (5) of the body is differentiated, of which the
mouth is the centre, the polarity is expressed functionally in
the direction of the currents inward and outward through the
mouth ; in the motor system special (6) motory organs are
developed, radiating from and surrounding the mouth as
tentacles (6a), and the whole of the body, in the higher forms,
also expresses this radial arrangement of parts into compart-
ments (6$), called mesenteries. This radial differentiation is
indefinite in the earliest forms, but there are two modes of
division that are well expressed later, seen in the tetracoralla
(7*7) and the hexacoralla (7^). In the Cambrian only the four-
parted type (Tetracoralla) was specialized. These constitute
the Rugosa; also, the Medusa (8) appeared in the Cambrian,
according to Walcott. In the Mesodermal layer are differen-
tiated both muscular (9) and skeletal (10) tissues, which take
the radiate form of the mesenteries, and in the living forms
there is a differentiation of the sex (n) — a differentiation we
have all reason to believe was existent in Cambrian time. In
the ectodermal or outer layer of the body there is differen-
tiation of a set of cells for offensive and defensive action upon
other organisms; these are the thread cells (12), which are
used offensively (12*2), probably to benumb their prey and
thus aid in the attainment of food, and as defensive (i2#), in
the way of protecting themselves from attack of larger ani-
mals which might seek them for food.

There is no certain differentiation of sense or nervous or-
gans in the Coelenterata, and the above points are about all
that can be said certainly to apply to the organisms referred
to the Coelenteraca for the Cambrian era.

The branch Echinodermata also was present in the Cam-
brian. In them the body presents the radiate (13) type
of structure in the adult, but the parts are normally five
(13^), and there is more or less distinct bilateral sym-
metry (14) exhibited by them in the adult form generally,
or only in the embryonic form in some of the living types*

CLASSIFICATIONS IN NATURAL HISTORY.

In the adult there is developed a more or less resisting integu-
ment (15), either in the form of coriaceous (15^) integument,
with granules (15^), or spicules (15^), or definitely formed
and articulated calcareous plates (i $d). There is elaborated
a peculiar hydrostatic apparatus, called the ambulacral water-
vascular system (16), which subserves the purposes of locomo-
tion (i6#) and the conveyance of food particles (i6£) into the
mouth, and may be considered as a special elaboration of the
elements which are tentacles in the Ccelenterata. In the
Echinodermata there is a considerable elaboration of the ali-
mentary system. There is a closed gastric cavity '(17), separate
from the somatic or vascular cavity (i 8) ; this constitutes a rudi-
mentary stomach. In the more perfect type of the Echino-
derms, the Echinoids, there is a distinct alimentary canal of
several parts, composed of a mouth (19), provided with special
organs for reducing food, five teeth (20), and a differentiated
cesophagus (21) leading to a stomach (22), and a distinct intes-
tine (23) terminating in an anal (24) opening. There is alsa
a pulsatory heart (25), with radiatory vessels, or blood-vascular
system (26). Thus in this higher type of Echinodermata we
find already differentiated organs for mastication, digestion,
nutrition, and distribution or circulation. It is not well
established that the function of respiration is specialized, or
that distinct organs are differentiated for this function. The
Starfish (Asterioidea) do not have distinct teeth, but the
Ophiuroidea do, and the haemal system is present in both, but
the mouth in many cases serves for ejection of fcecal matter.
These two types are developed very early — as early as the Or-
dovician, so that it is evident that all these differentiations of
the Echinoderm type of the digestive system were elaborated
by the beginning of the; Ordovician, and probably in the Cam-
brian. The Crinoids and Cystoids were Cambrian, the Blas-
toids appeared later; the digestive functions were less elab-
orate in them, but the differentiation into a stomach or
digestive cavity, as distinct from the nutritive tract or intes-
tines, was present. The nervous system was also developed
as a ring about the mouth, or oesophagus, and sent out nerves
to the other parts of the body, and there are pigment-cells
developed on the upper side of the Echinoids, which are re-

2l6 GEOLOGICAL BIOLOGY.

garded as of the nature of optic organs, but it is doubtful if
-any such organs were differentiated in the Cambrian type.
The nervous system in this type of animals at that time prob-
ably performed the function of co-ordination of organs. With
the differentiation of the alimentary canal there was probably
a specialization of cells for the particular function of these
several parts of the canal. The reproductive function had its
special organs differentiated, but they were as numerous as
the partitions of the body, and the elaboration of this system
in the Cambrian era had not proceeded far.

The Annelids (which is in our classification a representa-
tive of the branch Vermes, but in Huxley's classification is
placed in a branch Annulosa, distinguished in some particu-
lars from the Arthropoda, but only as a sub-branch, the An-
arthropoda) are represented in the Cambrian. They are the
lowest or less differentiated type of the articulate mode of
body development. There is an elongation of the body, and
in the adult there is a definite division into segments (27) or
metamcrcs (somites repeated and arranged along a longitudinal
axis). A prominent distinction separating the Annelids from
the Articulata proper, as representatives of branch or class
groups, is the absence of jointed appendages articulated to
the somites in this division of Vermes, and Huxley recognized
this distinction in applying the name Anarthropoda (meaning
without joints) to the class, while the Crustacea, Insects, and
allied forms develop jointed and articulated appendages to the
somites. In this type of structure the differentiation of parts
in the first or radiate direction is completed in the strictly
bilateral symmetry. The function of motion has specialized
into definiteness of relation of the motions to the body — a
longitudinal polarity. The direction from which supply of food
•comes toward the body, or towards which the motor system
propels the body, is anterior (280); it is distinctly in front of
the mouth, while the other parts of the digestive system are
arranged definitely posterior (2 8&) to the mouth, along the longer
axis. The parts about the mouth are reduced to their small-
est number, and are determined definitely in relation to a
surface upon which progression takes place ; a ventral side (290)
and a dorsal side (29$) become thus distinguished. The

CLASSIFICATIONS IN NATURAL HISTORY. 217

nervous system is present and surrounds the oesophagus (30),
and expresses the differentiated bilateral symmetry by consist-
ing of a double, ventral, gangliated cord (31), and in some gen-
era there are differentiated distinct optic organs (32) and special
organs of touch (33). The digestive system is differentiated
into mouth, sometimes armed with distinct jaws (34) for mas-
tication, a distinct oesophagus , a stomach or digestive cavity,
an intestine or assimilative canal, and the two openings, mouth
and anal, of the digestive canal are permanent. In the An-
nelids there is a pseudo-haemal system (35), a vascular dis-
tributing system, but not so highly developed as the circulat-
ing system of the true Arthropoda.

In the true Arthropoda, in addition to the elaboration
seen in the Vermes, there is differentiated a distinct system
of motor organs (36), articulated appendages moved by mus-
cle and not by hydrostatic device, and articulated to the
segments. The segments are repeated in more definite num-
bers, but in the Cambrian there was not a permanent selection
as to number. In the Trilobites there was evidently (37) a
selection of number of segments in contrast to the indefinite
number of the Vermes, which in Eunice gigantea, a modern
type, has 400 segments. There was a permanent specializa-
tion of a (38) chitinous exo-skeleton, which is a distinct elab-
oration of the motor skeletal system, and made possible a
number of special differentiations of the motor organs. There
was specialization of appendages for special functions ; that is,
for sense organs (39), for mouth or mandibular masticatory or-
gans (40), for swimming- or locomotion (41), and other sets con-
nected with respiratory (42) function.

The definite differentiation of organs for the respiratory
function {gills or branchice) (43) is a further elaboration of the
alimentary system, but this function was evidently specialized
even in the Cambrian representatives, the Trilobites, which
were the most highly elaborated organisms of that era. In
these Trilobites we find thus a thorough differentiation of
special organs for each of the systems of functions character-
istic of the highest type of animals, viz., Correlation, further
elaborated in the two systems, (a) motory, illustrated by the
muscles and skeletal parts, and (b) nervous system, ganglia,

2l8 GEOLOGICAL BIOLOGY,

nerves and organs of sense ; Sustentation, as exhibited in
organs of alimentation, digestion, nutrition, circulation, and
purification ; Reproduction, with special organs and separation
•of sex.

Insignificance of Characters of Marine Invertebrates Evolved
since Cambrian Time. — When we would speak of evolution of
different kinds of organisms, it is not regarding the evolution
of the differences above described that the geologist has any
evidence ; they were present at the beginning of the records.
All this had been accomplished when we get the first glimpse
of the earliest known relic of an organism. The simplest
types of organisms are living to-day, as are the most elabo-
rated types ; but when we go back to this earliest page of
geological history we find (with the exception of vertebrates)
all the grand types of animals already living together. So far
as these grander differences of organization are concerned, the
millions of years of geological time throw no light upon the
way by which they came about.

When we consider that our knowledge is only of marine
organisms, and how extremely meagre is the evidence we have
of them, it becomes highly probable that for animals adapted
to this environment nothing of branch, class, or possibly of
ordinal rank has been evolved since Cambrian time.

CHAPTER XII.

THE TYPES OF CONSTRUCTION IN THE ANIMAL
KINGDOM.

Records of Evolution expressed chiefly in Generic and Specific
Characters. — From what has been said in the previous chapter
it will be learned that the grand features and the great
majority of the more important details of the structure of any
living organism are of extreme antiquity. Not only so, but
since very early geological time no new types of structure of
.as high as ordinal rank have been evolved in the majority of
the branches of the Animal Kingdom.

In respect, therefore, to a great number of the more im-
portant characters of organisms the development of offspring
has resulted in the repetition, without substantial modifica-
tion, of the characters of the ancestors. This is the law of
Heredity — the repetition in the offspring by generation of
characters like those of its ancestors. Evolution has to do with
the acquirement by organisms of morphological characters which
their ancestors did not possess ; hence we must seek for evidences
of evolution chiefly among the characters of less than ordinal
rank — those of ordinal and higher rank having been evolved
almost at the beginning of the history.

Course of Individual Development supposed to have been Con-
stant.— It is not unreasonable to assume that all the course
and the stages of development, of characters of ordinal and
higher rank in the development of the individual, are repeti-
tions of what has taken place since their first appearance at
the beginning of the geological time-record. In the several
types of organisms now living, the laws of individual devel-
opment, as of the steps by which in each case diversity is
elaborated out of simplicity of structure, may reasonably be
regarded as applicable to all organisms of which we can study

219

220 GEOLOGICAL BIOLOGY.

the history. The reason why the course of development has
been what it is may be no more evident than the reason why
gold is yellow and heavier than sulphur ; in a particular case
the sufficient reason is that it is like that of its ancestors.

Beginning of Individual Life and Development. — In a previ-
ous chapter the stages of development of the individual are-
described. It is there shown how the simple cell is without
distinction of parts, other than as protoplasm with cell-walls ;•
a cell-nucleus, which is of great importance, and regarding
which recent investigations with high power of the microscope
are bringing out wonderful characters and functions ; and a
vacuole, often present, but the function of which is unknown.
From such a cell the individual grows to the state of a com-
plex, independent organism, such as the living Vertebrate,,
seen in its highest representative, Man.

Hypotheses regarding the Phylogenetic Evolution of Races.
— The term Ontogeny has been applied to this development,
and to distinguish it therefrom, Phytogeny, or race-develop-
ment, has been proposed to indicate the analogous passage
from the simplest undifferentiated Protozoan, the Amoeba, or
Monera, through the several stages of increasing complexity
of organization to the most highly differentiated Vertebrate.
Many attempts have been made to construct the history of
the whole organic world on this basis, i.e., to construct
phylogenetic trees of the ancestors of beings now living on the
earth. Haeckel's " History of Creation " is one of the earlier
and most elaborate, and perhaps most artificial, of such
treatises; for as science has developed, our knowledge of the
true genetic relationship in some particular lines of organisms
has greatly increased. When Haeckel's work was published
(1868), the new methods of investigation, so greatly stimu-
lated by the appearance of Darwin's " Origin of Species,"
had only begun to affect the students of fossil remains ; and
it is mainly since that date that the classification of organisms
has been revised on the basis of genetic affinities determined
by comparative studies of structure.

The analysis of organic structure, from the phylogenetic
point of view, is very instructive and suggestive if it be not
overdone, It helps us to attain general notions of organiza-

TYPES OF CONSTRUCTION IN 7' HE ANIMAL KINGDOM. 221

tion, or what we may call the principles of construction of
the Animal Kingdom.

The TJndifferentiated Cell. — From this point of view the
primitive living organism is assumed to be an undifferentiated
cell, having no tissues, no organs, no permanently specialized
functions. If it moves, the motion is spontaneous, irregular,
temporary motion ; if it takes food, it is by attaching the
food to itself; and in a sense such a protozoal cell is all
mouth, all stomach, all everything necessary to living, but
nothing particular in any part of itself is permanently different
from any other part : it is an undifferentiated organism.
The amoeba comes nearest to fulfilling these homogeneous
conditions, but even there appear the nucleus and the con-
tractile vacuoles, which are differentiated, and perform some,
though not well understood, special functions.

!CV

Fia. 51. — Amoeba proteus (after Griiber), greatly enlarged. cv = contractile vacuole, n =
nucleus, ps = pseudopodium.

In the simplest form of themetazoal cell very considerable
complexity is found at the earliest stage in which the cell' is
observed. The steps by which the cell reaches the organic
structure which is characteristic of any of the metazoa when
adult is explained in works on embryology and animal mor-
phology.*

When we look at the progress more rapidly, and note the
steps of progress in function rather than in structural mor-

*See McMurrich, "Text-book of Invertebrate Morphology," chapter ix.,
Subkingdom Metazoa.

222

GEOLOGICAL BIOLOGY.

phology, we observe that in attaining differentiation from this
simple state several systematic groups of differences are
expressed. The first is concerned with general direction of
motion, expressing itself in the arrangement of the body
shape, or in its development.

Polarity. — If we imagine the primitive form to be a globe,
its motion is expressed by assuming polarity of direction —
a definite anteriority, or direction toward which motion ap-
proaches, and the opposite, posteri-
ority, from which it goes. Every
living animal having reached the
first stage of differentiation (seen
in the Metazoa, as the Ccelenterata,
for instance) expresses some degree
of polarity. The longitudinal axis
of the body, in the Metazoa of this
simplest form, is clearly expressed,
and the anterior end is primarily
determined by the position of in-
vagination in the growth of the
embryo forming the gastrula.
(Me- Thus the simple coral polyp is
mature animal representing the
Gastrula stage of embryonic de-
velopment of higher animals.
In Fig. 52 the anterior end of the axis of the body,
AB, is at A, which is the mouth or oral end of the enteron
or digestive cavity. This is the centre of the free end of the
body, and the opposite end, B, is in mature stage often fixed.
Antimeres and Metameres. — As such an organism is sup-
posed to develop parts by differentiation, these parts are
arranged in one of the following three ways: radially, or
around the axis, when they are called Antimeres ; or one
after the other in the direction of the long axis, when they
are called Metameres ; or, third, without repetition of parts,
except to express bilateral symmetry and a dorso-ventral
opposition of parts.

Radiate Structure, Bilateral Symmetry, and Actinimeres. — The
primary axis (AB in Fig. 52) is the one which is longitudinal

FIG. 52.— A simple coral polyp

tridium tnarginatuin Les.), rep-
resenting the gastrula stage of dif-
ferentiation, in which the posterior
end of the body B is attached and
the anterior end A is free.

TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM. 22$

to the body, and the secondary axis is at right angles or
transverse to this. In the course of growth repetition of
parts is first noticed as evidence
of elevation of rank, and the or-
ganism which has no duplication,
or multiplication of parts, is lower in
the scale, because less differentiated,
than one in which there is multiplica-
tion of parts. Where there is multi-
plication of parts the simplest mode
of arrangement is around the longi-
tudinal axis. When each of the
parts about the axis is alike there is

radiate Structure (see the tentacles, /, FIG. S3-— Coral animal and its cal-
careous base, Asteroides caly-

Of Fig. 53). This is the Case in the *«/,»•« Lmk. Longitudinal sec-

tion, j. cd — calcareous skeletal

coral animal, or in the starfish, and
the separate parts are called anti-
meres ; thus the tentacles of the coral,
or the arms of the starfish (Fig. 54),
are antimeres, or opposed parts. When .there is difference

of the colony ; _/ = chambers be-
tween the mesenteries. (After
Steinmann and Doderlein.)

FIG. 54.— A typical radiate, Starfish, Asterias areniccla,. (After Agassiz.)

among these parts, and there are series of parts opposed to
each other, the differentiation has progressed one step higher,

224

GEOLOGICAL BIOLOGY.

and we have bilateral symmetry. When there is multiplicate
division, whether there is symmetry or not, the rays thus
formed are called actinimeres, or ray parts. This mode of
differentiation is characteristic of the Ccelenterata and Echi-
nodermata (omitting from the former branch the sponges), and
suggested to Cuvier the name Radiata (see Figs. 14—19).

Somites, Arthromeres, and Diarthromeres of the Arthropods.—
Another large and diverse group of organisms is character-
ized by repetition of parts in the direction of the longitudinal
axis. The technical name for body without
its parts is soma; the repeated parts which
are longitudinally multiplied are called mc-
tameres, somites, or segments (see Fig. 55).

The annelids represent the simple metam-
eric type, without appendages to the sepa-
rate metameres or segments. In the higher
class, the Arthropoda, including the Crustacea,
Myriapods, Insects, etc., the so-
mites are provided with lateral
appendages which are jointed in
regular manner (see Fig. 56, also
Fig. 50).

In the Arthropoda, such as
the common lobster, and in an
insect, these separate somites
form a single ring enclosing the
interior organs ; but in the Ver-
tebrates the somite is double, the
FIG. 56.— A me- special system of correlation ly-

tameric animal ... . ,

with jointed ing m the upper arch, the organs

appendages, .

Scoiofendreiia of assimilation or auxiliary lunc-

iminaculata. . • « i

Leunis.) tion lying in the cavity below.
To distinguish these two forms
of the metameres the first is called
a joint part, arthromere ; the corresponding part in the verte-
brate structure is called a two-joint part, diarthromere. The
joints of the appendages of a rnetameric part, as the joints of
the legs of a lobster or the several bones in the limbs of ver-
tebrates, are illustrations of multiplication of parts by division

type, a diagram of a
typical annelid, m =
mouth ; ce = cerebral
ganglion ; n = ventral
nerve:cord ; pr x head
(prostomium) ; a =
anus. The body
(soma) is composed of
twenty-five segments
or metameres.

TYPES OF CONSTRUCJ^ION IN THE ANIMAL KINGDOM. 22$

in a transverse direction. The technical name for this mode
of repetition of parts is antimeric.

Distinctive Characters of the Metazoa. — All the higher tissue-
bearing animals, or Metazoa, differ from the Protozoa by the
possession of the following characters, viz. :

Metazoa. — Reproduce by developing egg, or ovum, which
passes through the stages of (a) nucleated cell, (b) segmenta-
tion, (c) blastosphere or morula, (d) gastrula; tissues differen-
tiated into (e) ectoderm, (_/) endoderm, and (g) mesoderm ;
(/i) alimentary cavity, or enteron permanent, and (t) sexual
differentiation the rule and almost universal.

Molluscan Type of Structure. — The third type is that in which
neither metameric nor antimeric repetition is carried on, but
bilateral symmetry and simple antero-posteriority and dorso-
ventral polarity are more or less conspicuous. In this type
of organisms (the Mollusca) differentiation is expressed in
the relative positions of the organs in the body-cavity, and in
the relative development or importance of the different or-
gans or regions of the body.

Development of Organs and their Taxonomic Rank and Value.
— In the molluscan type is seen in its simplest form that
relative development of the several systems of organs which
marks the rank of the stage of progress in differentiation in
each particular case. Thus of the several systems of organs
sustentation is more fundamental, and may be regarded, if
prominent in relative development, as indicating primitive
or low rank. Organs of correlation, when more specialized
and according to the degree of differentiation of the special
organs, imply specialization, hence high rank. Thus among
the Mollusca those which are simply digestive sacs, with no
specialized organs of sense, or of definitive motor organs, are
low in rank (the Lamellibranchiata). The specialization of
sense-organs anterior to a mouth and of the muscular system
for giving definiteness to the motion, indicates higher rank
(the Gastropoda and Pteropoda). Special tactile organs, and
high development of sense-organs, all in front of the oral
opening, show still higher rank (the Cephalopoda).

This principle of differentiation in the development of
organs throws light upon the rank of particular organisms in

226 GEOLOGICAL BIO LOG Y.

the phylogenetic line of their evolution, and relatively in
each line those expressing greater differentiation in the gen-
eral development, or higher specialization of the more de-
pendent or secondary characters, are necessarily of higher
rank, on the theory of acquirement of characters by direct
descent only.

The Principle of Cephalization. — The relative development
of the organs of correlation, especially of the organs of sense,
has been recognized for many years as indicative of grade of
rank among animals.

James D. Dana has written much on this subject under
the name of Cephalization.

In discussing the principle of Cephalization Dana wrote:
"Such growth or progress in the brain and nervous system,
the seat of power in the animal, is accordant with, and conse-
quent upon, the great fact that this is the part of the struc-
ture which comes into actual contact with outside and inside
nature. It is the means in the animal by which communica-
tion is had with the outer world, and also with its own inner
workings and appetites; that which takes impression, which
feels whatever inspires energy, prompts to action, exhilarates,
or exalts; the part, therefore, which must grow whenever
circumstances favor progress, and, at the same time, fail to
grow or dwindle under unfavorable circumstances ; which
communicates whatever it receives to the being to which it
belongs, and in each case to the part or parts responding
to its condition ; which reaches every part of the system and
dominates in all action and growth, and hence must cause an
expression of its own condition in some way on the structure ;
which, moreover, must ordinarily produce correlative changes in
correlative parts, if any, because in its own nature and distribu-
tion the system of correlation has a full expression "

"We may, therefore, believe that in all progress in grade,
upward or downward, there was involved some change in the
animal structure of the kind expressing degree of Cephalization."

"Whatever the types of structure in course of develop-
ment, there was also a general subordination in the changes
to the principle of Cephalization." *

* A.J. Sci., ser. in., vol. xn., Oct. 1876, pp. 245-251.

TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM.

Cephalization one of the Expressions of the General Law of
Differentiation. — Cephalization may be regarded as but one of
the expressions of the general principle of differentiation.
Differentiation concerns the whole organism, because increase
in the specialization of function of one organ always involves
the provision, through the activity of other parts, for the
supply to that organ of resources which it fails to supply to
itself.

Meaning of Homology and Homologous Parts. — When animals
are compared there are some terms which are applied to the
relationship noted in the parts compared ; a few of the terms
are the following:

Homology and homologous are applied to the organs or
parts of different organisms which correspond in type of
structure. Thus the secondary joints of the appendages of
Arthropods are homologous parts, and one appendage may be
used as a swimmer or a claw, another as a mandible, and
therefore be constructed in different form ; but the parts,
although of different form, are said to be homologous, be-
cause modifications of the same element of differentiation
(see Fig. 50).

Another example is the case of the forearm of a bird
and the forearm of a bear. When the bones are compared
they are found to possess corresponding parts — a shoulder-
blade, a humerus, a radius, an ulna, a carpus, metacarpus,
and finger-bones. Although the arm in one case is adapted
to the function of flying in the air, in the other to walking on
the ground, and the shape of each bone is different, the
several parts are homologous, because bearing the same rela-
tion to the structure of the whole, and representing the same
typical part of the primitive structure.

Analogy and Analogous Parts. — Analogy is used in a different
sense. Two parts or organs of different animals are said to
be analogous when the likeness has to do with the functions
or adapted usage of the parts; and not to either the mor-
phology, or the relationship to other parts of the organic
structure of the animal. For instance, the leg of a fly and
the leg of a mouse serve the same function — walking or loco-
motion, but they differ morphologically, i.e., in form; they

228 GEOLOGICAL BIOLOGY.

differ also in their structural relation to the whole organism
of which they are parts. Homology may be said to be based
upon morphological unity, and analogy is based upon func-
tional or physiological unity.

Differentiation Illustrated in the Case of Motor Organs — To
illustrate this mode of analysis of the organic structure from
the point of view of extent of differentiation of parts, or in
order from homogeneity to heterogeneity of structure, a
study may be made of the devices developed for the execu-
tion of motion or locomotion, in the various branches of the
Animal Kingdom.

Organic motion, in its simplest form, is contraction, the
bringing together of two ends of a contractile' tissue, as
muscle, with no hard parts, no specialized organs : this is what
is seen in the lowest forms of the Protozoa, and expresses
itself in change of form of a globule, drawing in of a part, or
pushing out of another part.

Two Directions in which Differentiation Proceeds In dif-
ferentiating the mechanism of motion, elaboration may take
place in two directions.

(A) By subdivision, or multiplication of the moving parts,
and increasing the rapidity of the contracting: this results in
ciliary motion, and the specialized organs thus elaborated are
called cilia.

(B) The second is by concentration, or massing of the parts
of motion, and thus increasing the energy expressed in a
single motion : this leads to the construction of muscular
tissue, and the expression of specialized muscular motion.

In (A) the direction of the motion is indefinite, in (B) it is
definite in direction and united in time, or period of action.

Ciliary Motion. — The real function of ciliary motion is seen
in an augmented state in the special organs called tentacles,
which act by muscular methods, but whose function is vibra-
tile. These may add the functions of ingestion and prehen-
sion to those of simple ciliary motion. But ciliary motion
itself is fundamentally applied for the ingestion of food.
This is accomplished in minute organisms either by causing
the organism itself to move in its medium towards the food,
or by setting up currents in the medium and thus causing the

TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM. 2 29

food to flow to the mouth of the organism. The tentacle is
an enlarged cilium in so far as the function is concerned.
The Ccelenterata exhibit this mode of elaboration of the motor
organs in a "typical way.

Water-vascular System of Echinoderms. — In the Echinoder-
mata a higher elaboration of this kind of action of muscular
tissue is expressed in the water-vascular system. This is a
peculiar adaptation of simple muscular contraction.

Cilia in Molluscoidea and Mollusca. — The Molluscoidea have a
system of ciliary motion drawing the food particles to the
mouth-opening by setting up currents in the water. Some
of the Mollusca have a similar method of producing currents
by means of cilia on the edge of their mantles. In the Gas-
tropod and Cephalopod motion is accomplished by speciali-
zation of muscular contraction. Various types of modifica-
tion of the foot are elaborated in the different classes of
these interesting forms.

Skeletal Parts. — In the Arthropoda and the Vertebrates
organs of motion are more highly elaborated by the addition
of hard parts acting as levers, and thus giving special direction
and change of direction to the simple contraction of the mus-
cles passing between two articulated parts. The general dif-
ference between the motor systems, or the modes of motion,
in these two grand divisions of the Animal Kingdom, is seen
in the different relation which the contracting part (the mus-
cle) bears to the mechanism, or skeletal part. In the Ar-
thropod the muscles are attached on the inside of hollow
skeletal elements. In the Vertebrates the muscles are out-
side and around the levers which they move, and in these two
groups of organisms motion, and both the muscles and the
machinery of motion, reach a high degree of elaboration.

Multiplication of Like Parts Preceding Specialization of their
Functions. — The course of differentiation is from simplicity
or homogeneity of parts, first, to multiplication of the parts
possessing like functions and often uniformity of form, and,
second, to the specialization of function of these parts, their
division into groups, their consolidation, and, finally, definite
ness in number and precision of use or function. Hence
division of labor follows the multiplication of parts and does

230 GEOLOGICAL BIOLOGY.

not precede it. Multiplicity of laborers is a condition neces~
sary to the division of labor, and the organic co-operation of
separate parts.

Comparison between Embryonic Development and Succession of
Ancestors. — Prenatal or embryonic development of higher ani-
mals may pass through stages similar to those expressed in
the mature form of lower animals which are supposed to be in
the line of descent of the former: as an example, the Mam-
malian embryo develops gill-arches, which are characteristic
of the mature stage of fishes: but in the embryo of the mam-
mal this feature appears in the earlier embryonic life, and is lost
as development proceeds. Much has been made by embry-
ologists and also by systemists of this embryonic calling back
to supposed ancestral characters; but in deriving conclusions
from these facts it must be remembered that, since the organs
are neither fully co-ordinated nor completed for action in the
individual embryo, and that not until the natal stage is past,
the likeness of these characters to the mature parts of sup-
posed ancestors is rather a likeness in the plan or course of
development than in the results of the development. The
course of the development may be alike in two organisms ;
that is, the steps by which the morphological features may be
attained may be according to the same plan, and indicate a
fundamental affinity, which is less evident or quite lost in the
mature animal. However, it is not a necessary inference that
in the embryonic development we will be able to recognize
the relationship to an ancestral mature form. Changes, such
as abbreviation, or a different course of development, of the
embryo, can be assumed to be indicative of phylogeny only
in case environment was the determining cause of their origi-
nal appearance. If there be an evolution in these modes of
differentiation, as there is an evolution in the final product,
the resultant differences may be determined by other laws.

Muscular Motion or Specialized Motion, and Locomotion. —
The preparation for motion of the organism in definite direc-
tion is exhibited in the differentiation of the head as the
oral end of a moving organism. Next, it is seen in the differ-
entiation of the assimilating cavity into a tube, the enteron,
with separate entrance and exit, for the materials of assimi-

TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM.

lation. A third stage is represented in the elongation of the
body in such a manner that it may move in part without any
actual locomotion, the one end becoming sessile, or attached,
as in the case of the Ccelenterata. In this case no specialized
organs of locomotion are developed, but the mouth-parts
are moved in relation to the body, and are moved also
in relation to the source of food. In Vermes there is loco-
motion, but no special articulated parts are developed in
this lower type. In the Arthropoda articulated organs
subserving the function of locomotion are developed ; associ-
ated with the specialization of motion, as local motion, we
find a specialization of the poles of the body into an anterior
and posterior end, relative to the direction of the motion.
The first mode of differentiation spoken of has not to do with
locomotion, but rather with relation to reception of food.
An oral end of the alimentary canal was established through
which food reached the interior of the organism. The polar-
ity was a polarity between the oral end of the alimentary
canal and the excretory end, or rather between the approach
of food and the discharge of effete results of digestion.

In the Ccelenterata the oral orifice serves also for the dis-
charge, and therefore the oral and aboral poles are brought
together, typically, at the same point. In the Echinoder-
mata, in some cases, the aboral corresponds to the oral pole.
In other Echinoderms there is a distinct separation of the two
ends of the alimentary canal. With the setting up of the
antero-posterior polarity of the chief axis of the body, and
of these specializations of locomotion, there was expressed a
decided advance by the appearance of sense-organs at the
anterior pole.

Differentiation of Nervous System a Concomitant of Locomotion.
—Motion, bringing about a change of place, implies the selec-
tion of better conditions of environment, and the guidance of
the locomotion toward such favorable conditions. Thus the
differentiation of the nervous system follows, or is intimately
associated with, the specialization of motion into locomotion.
Again, we notice that the head, being thus specialized, is only
one of the kinds of differentiation. Thus the metameric
mode of development first makes possible heteronomy of

232 GEOLOGICAL BIOLOGY.

parts, i.e., the specialization of functions, along the digestive
tract. In such an organism as the lobster, for instance, we
find a definite arrangement of specialized functions with dif-
ferentiated organs, distributed along the line of the axis from
the antennae to the extreme posterior end of the body.

Differentiation Along the Digestive Tract. — Without con-
sidering the skeletal parts, but looking at the organism in re-
spect to its digestive tract alone, we find the following series
of differentiated parts :

First, the detection of food. Provision for this is made by
special organs of sense, antennae, eyes, organs of smell and
of taste, and, finally, those of hearing.

Second, \h& prehension of food. For this purpose jaws and
teeth and other apparatus are provided.

Third, the breaking or gross reduction of food for diges-
tion. For this purpose the teeth and jaws are brought into
action.

Fourth, the digestion of food. In this process several spe-
cial organs take part, the most important of which are the
stomach and the secretions which are furnished at that point
in the enteron ; but there are, in addition, in higher organ-
isms, numerous specialized glands, secreting digestive fluids
with differing properties.

Fifth, the absorption of food. For this function the intes-
tine and associated organs are differentiated.

Sixth, the distribution and application of food-products.
To this group of functions are applied the organs of circula-
tion, and auxiliary to them are those of respiration, and the
corresponding organs.

Seventh, elimination of effete matter. The organs for
this function are at the anal termination of the enteron, and
auxiliary organs are found associated with the circulatory sys-
tem, as, in the higher animals, renal organs; and even the
skin subserves the same function, in part, in perspiration.

Differentiation of the Motory System into Muscular and Skeletal
Organs. — This principle of differentiation might be traced in
relation to the skeletal framework of the body ; but these
relations are not fundamental, and the organs are adjusted to
them to conserve convenience and compactness of arrangement.

TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM, 2$$

The motor organs, however, express their differentiation in
the skeletal parts and in the form of the body. There are
two types of differentiation of the motory system resulting in
the construction of what may be called muscular and skeletal
systems, or parts. Muscle and skeletal parts are correlative
to each other. Hard parts of some kind, to which the general
name skeletal is applied, are essential to specialization of
the direction of motion, and contractile muscles are just as-
essential to the motion of these skeletal parts themselves.
The relationship between these two elements of the motory
system is as intimate as that between steam and machinery
in the steam-engine.

Archetypal Structure. — The further elaboration of this
method of analysis of organic structure may be pursued only
by tracing the elements of structure to their specific charac-
ters in many separate types.

Sufficient may have been said to emphasize the fact that
there is a logical foundation for the idea of archetypal struc-
ture, so much insisted upon by Agassiz, explained embryo-
logically by Von Baer as early as 1828, and expressed in
Cuvier's classification of the Animal Kingdom, in the four
general plans upon which the various kinds of animals were
constructed. Cuvier wrote in 1812-

" . . . On trouvera qu'il existe quatre formes principales,
quatre plans gencraux, si Ton peut s'exprimer ainsi, d'apiea
ses quel tous les animaux semblent avoir ete modeles," etc.

Cuvier's Classification. — Although, later, more minute stud-
ies have produced modification in the systematic classificatioa
of the Animal Kingdom, Cuvier's division of animals into-
Radiata, Articulata, Mollusca, Vertebrata, expresses the most
profound distinction exhibited by these organisms ; and what-
ever criteria we take as the basis for classification, and with
slight modification due to increased knowledge, these grand
divisions of the Animal Kingdom stand out as pre-eminently
the most important groupings that can be made.

To say so much is not an acceptance of the philosophy of
the earlier naturalists as final. That there are a few general

*" Ann. des Mus d'Hist. Naturelle," vol. xix., Paris, 1812, quoted by Agas-
siz in ' An Essay on Classification," London, 1859, p. 309.

234 GEOLOGICAL BIOLOGY.

plans of structure upon which the multitudes of animals were
built, does not carry with it any theory as to the reasons for
the differences, or as to the mode by which the several types
of structure came to be carried out in such multitudinous
fashion.

The fact is beyond dispute that there are a few types of
•construction to which the animals of the whole kingdom con-
form, and these are expressed in the mature forms as well as
in the course of the individual development. Cuvier, when
lie considered the mature results, found them to be four; no
one since has found reason to dispute the validity of three of
them. The branches Ccelenterataand Echinodermata, consti-
tute one of them — the " Animalia Radiata;" the Arthropoda
typify another — the " Animalia Articulata;" and the Verte-
brates include substantially the same organisms classified by
Cuvier under the type " Animalia Vertebrata."

Von Baer's Embryological Classification. — Von Baer, from a
study of the course of embryologic growth, thought he had
found a more intrinsic reason for the types in the modes of
their embryonic growth than in the gross result. He defined
them as the " peripheric type," with evolutis radiata (i.e.,
the Radiata); 2d, the (( massive type " (Mollusca), with " evo-
lutis contorta;" 3d, the "longitudinal type" (Articulata),
with evolutis gemina, or production of symmetrical parts on
both sides of an axis; 4th, " doubly symmetrical type " (Ver-
brata), "with evolutis bigemina" i.e., the development pro-
ducing symmetrical development on both sides of a median
axis, and also developing two cavities, one above and one
below the central axis.

Fundamental Divisions of Classification discerned by Earlier
^Naturalists. — If we throw the light of more recent investiga-
tions upon the matter, we find it necessary to make expan-
sion of those divisions; but very little alteration in the funda-
mental classification, thus early recognized, is required in
order to express the scientific classification of present usage.
Looking upon classification from this point of view, we first
divide the Animal Kingdom on the fundamental character
of cell-growth. When cell-growth proceeds no higher than
the multiplication of cells, having no differentiated and

TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM. 235

complementary functions, the individual is not a compound
organism, but is always cellular. All such animals are
grouped under the one division, Protozoa.

When cells divide, so that as cells they are separate, but
remain in close association, with division of labor, one func-
tion played by some, another function performed by others,
the result is tissue and organ, and the individual is an organ-
ized individual ; this constitutes the group of Metazoa.
Classifying these Metazoa on the basis of the direction along
which development of the specialized parts of the body pro-
ceeds, we find the same grand division appearing prominently
before us as types of construction of the complex organism.

The Polymeric Type. — There are two fundamental direc-
tions along which their development proceeds. Taking the
mouth, the opening or entrance to the enteron, as the centre,
multiplication of parts may be around this centre (radiate),
or it may be from it in the direction of the axis of the
enteron (longitudinal) ; and besides these two ways there is
probably no other direction of multiplication of parts. When
the multiplication is indefinitely radial, it produces the antim-
eres of the coral polyp, having chambers and tentacles dis-
tributed about the mouth as tentacles, and extending back-
ward as septae in the body-cavity. The Ccelenterata ex-
press this mode of construction in its most indefinite manner;
the Echinodermata express it with definiteness of number of
radiations but with a tendency to the following type, and this
may be named the polymeric type of construction.

The Dimeric and Monomeric Types. — The second path is by
the specialization of the polymeric types with limitation of mul-
tiplication to repetition in two opposite directions, forming
bilateral symmetry. This is seen in some of the Mollusca,
as in Fig. 35, representing the idealized primitive Mollusk.
This may be called the dimeric type of construction.

In a third case there is no full duplication of organs,
although the motion and the form are as in the dimeric type.
This may be called the monomeric type.

The Cephalopoda are in part polymeric, but in the main
monomeric. The Molluscoidea are polymeric and monomeric
in different parts of the body. Thus the branches Ccelen-

GEOLOGICAL BIOLOGY.

terata, Echinodermata, Molluscoidea, and Mollusca are all
associated together by the fact that their development of
separate parts is in a direction radiately, or in circle about the
mouth, and hence they are antimeric Metazoa.

The Metameric and Diarthromeric Types. — The Vermes and
the Arthropoda are, on the contrary, metameric ; the de-
velopment adds parts by repetition longitudinally along the
median axis. In the Vertebrates, in which there is added,
as Von Baer already saw, the diartJiromeric separation of a
dorsal and ventral cavity, with specialized parts distributed in
each, the arrangement of specialized organs is on a monomeric
plan, as in some of the Mollusca.

Meaning of Typical Structure and Types in Modern Zoology.
—It may be remarked that the difference between the old
and the more modern use of such classifications, as above
made, consists in the theoretical value placed upon them.
Cuvier and Agassiz considered such " types " as in the nature
of "ideal plans" which all animals for some reason were
obliged to conform to, and departure from the "type plan"
or " arche type" was an abnormality, or required tneoretical
adjustment to the plan.

In modern Zoology by the " typical " structure of a group
is meant a generalized statement of the most conspicuous
features observed in the members of the class under con-
sideration, and departure from the type in individual cases is
evidence not of aberration in the particular case, but of imper-
fection of the description. The fact that there is develop-
ment along one or another line is important : the generaliza-
tion of the law so as to cover the principle and omit the
details aids the formation of clear notions ; but the notions
are not the things, and the latter have to be constantly recti-
fied to express the increase of knowledge of the former. The
four types of Cuvier had their representatives in nature, but
all organisms did not stick closely to a particular type of con-
struction expressed by his formulation of characters of the
types.

CHAPTER XIII.

PHYLOGENESIS IN CLASSIFICATION.

Principles of Classification Illustrated by the Mollusca and
Molluscoidea. — In order to reach a closer view of the meaning of
the relationship of organic form to the place in the time scale
at which it appears, we must examine more particularly the
principles underlying the classification of organisms.

The groups of organisms from which examples will be
chosen are the Mollusca and the Molluscoidea; chiefly for the
reason that they present hard parts which are abundantly
preserved in the rocks, and therefore afford more satisfactory
records of their geological history than any furnished by any
other class of organisms. A second reason for selecting them
is the fact that the statistics, regarding the relation of their
forms to conditions of external environment, are so satisfac-
tory as to be at least equal to those regarding any other
group of animals.

The Author's Philosophy Reflected in his Classification. — From
what has already been said it will have been perceived that
form and function are both regarded in the classification of
organisms ; but hitherto the fact has not been emphasized that
the classification of organisms, i.e., the description and or-
derly arrangement of the characters which one is supposed to
see in particular examples of organisms, is affected by the phi-
losophy of the classifier. At best the classification expresses
only the author's interpretation of the laws of association of
different things ; hence if we know the theory by which the
association is reached we are better prepared to learn truth
from the resulting classification.

Effect of Theories of Phylogenesis upon Classification. — Much
is found in the modern literature about phylogenesis as a
basis of classification, and it is supposed to supersede quite

237

238 GEOLOGICAL BIOLOGY.

entirely the classifications which were made by the earliest
naturalists who believed in the original creation of all species.

The difference between the two methods is quite simple,
and may be explained in a few words. Cuvier and his school
observed the morphological characters of organisms, not al-
ways knowing the exact physiological function, and compared
them together, and then wrote descriptions of the differences
they observed. They separated organisms into distinct spe-
cies and genera by the different characters they observed in
each, and thus their method of classification is based upon
observed differences in form. The new school of naturalists
is intent, first of all, upon the discovery of the affinities of
each kind of organism studied. Their point of view is di-
rectly the reverse of their predecessors. Their descriptions,
and finally their classifications, are based upon the points of
resemblance which can be detected upon comparing different
organisms with each other.

Analytic and Synthetic Method of Classification. — These two
schools differ as to the kind of characters which they consider
to be of chief importance in classification, and, as a general
effect upon classification, the one school is apt to overesti-
mate imagined resemblances, not to be seen by the ordinary
observer, and the other may err on the side of making too
much of external, often trivial, characters.

Irrespective of the way by which the two methods of clas-
sification arose, both methods are now in use and both are
useful.

In order to give them names, free from any accidental as-
sociation connected with their origin or application, the first
may be called the analytic method of classification, the second
may be called the synthetic method; and for the purposes of
illustration Zittel's classification of the Molluscoidea and Mol-
lusca may be selected as examples of the analytic method,
and Lankester's classification of the Mollusca may be taken
as an example of a synthetic classification.

Here, as elsewhere in this treatise, the reader must be left to learn the
full meaning of the descriptions, only outlined, by a study of the objects
themselves. No possible description of natural objects, particularly or-
ganisms, can convey to a student impressions which he has never before
experienced. And the best way for any one to gain a true notion of the

PHYLOGENESIS IN CLASSIFICATION. 239

meaning of the distinctions pointed out in these pages is to take a lot of
mollusca of different kinds, to be found on the sea-shore, and with the de-
scriptions of the expert zoologist at hand attempt to identify and classify
them.

And the naturalist who may possibly look into these pages will appre-
ciate, no more keenly than the author, the great difference between such
an introduction as is here attempted, and the earnest investigation of the
history of organisms by a study of the organisms themselves.

Mollusca and Brachiopods as Illustrations of Evolutional History.
— The Mollusca and the Brachiopods present a peculiar inter-
est, because, having no skeletal parts, the mode of action, —
the result of adjustment to environment, — the adjustment of
several parts and organs to each other in body structure, and
the marks of stages of growth are all concentrated in an ex-
ternal, hardened, and therefore preserved, single, or rarely
more than two-parted shell.

To the extent to which such a shell can express the char-
acters of the organism, the perfection of its preservation, and
the fact that so much of the individuality of the species and
so large a number of individuals are accessible to study, give
to this kind of fossil its great value in illustrating the problems
of evolutional history.

Zittel's Classification of the Branch Mollusca,* — The classifica-
tion of the Mollusca proposed by Zittel differs in some re-
spects from that of Lankester, Gegenbaur, and many of the
stricter modern zoologists.

In his branch Mollusca were included as sub-branches —
A. Molluscoidea (which is a branch in Gegenbaur's
classification),
with the Classes I. Bryozoa •

II. Tunicata (which is raised to
the rank of a distinct branch
by Gegenbaur);
and III. Brackiopoda.
(Gegenbaur places Bryozoa with the Worms

* Note: This classification was taken from the " Handbuch," published
nearly twenty years ago. In the author's " Grundziige " (1895), the Mol-
lusca and Molluscoidea are relegated to separate branches, in accordance
with present usage. The above passage is left as originally written
because it well illustrates the point under discussion. See Am. Jour. Sci.y
ser. HI, vol. L, p. 268.

24O GEOLOGICAL BIOLOGY.

(Vermes), and treats of Brachiopoda as a distinct
class, but allies it on certain accounts with the
Vermes.)
B. Mollusca (proper).

Class i. Lamellibranchiata.

2 . Gastropoda.

3. Cephalopoda.

The embryologists make greater point of resemblances
observed in the early stages of development, and hence the
distribution made of Bryozoa and Brachiopoda next the
Worms, and Tunicata next to Vertebrates; but when the
mature animals are studied and compared the Brachiopoda
are found' to possess structures closely resembling in impor-
tant features the Mollusca proper

The embryological resemblance of Bryozoa and Brachi-
opoda to worms is lost when the adult stage is reached.
Hence, for the geologist particularly, the association of the
two is not suggested by any apparent similarity of characters.

Points of View of the Embryologist and of the Morphologist—
In studying the philosophy of natural history it is interesting
to note this difference in point of view between the strict
embryologist and the pure morphologist. They compare
animals on a different basis, and therefore there results in
some cases a different classification.

The embryologist classifies animals primarily on the theory
of phylogenetic relationship; the student of adult morphology
classifies them according to the nature and extent of differen-
tiation attained in the adult. Here, too, the two examples
selected will illustrate the differences. The two modes of
classification differ much as the classification of houses might,
viz. by considering them, either, according to the styles or
schools of architecture, as Norman, Roman, Queen Anne,
etc., on the one hand, or according to the materials of con-
struction— brick, stone, or wood, on the other.

The line of descent, through which any particular organ-
ism has come to be what it is, is all-important if it can be
discovered by the study of embryology ; but of no less impor-
tance is it to distinguish the different and similar structures
which have been developed for the accomplishment of the

PHYLOGENESIS IN CLASSIFICATION. 24!

same functions in organisms, whether of near or distant ge-
netic relationship.

Embryological Likeness of Organisms whose Mature Characters
are Diverse. — In grouping the Mollusca with the Molluscoidea
it is not denied that they may differ in origin — even that
in their earliest stages of development Brachiopods may be
akin to Worms and Echinoderms; and what animals are not?
The adult modes of life and construction of hard parts of
the Brachiopods presented greater resemblance to the Mol-
lusca in the Cambrian than they did to Worms or Echinoderms,
and it is not ignorance alone which has led the paleontolo-
gist to compare them in studying the faunas of geological
time. On the other hand, when we go back to the primi-
tive steps of development of the germ it is to be expected
not only that two branches will show likeness of develop-
ment, but if we should go back far enough we shall meet
with no visible distinction between the germs of all the
Metozoa; in fact all animals, if we go back far enough, may
be supposed to present no differences. On the ground of
embryology, the Tunicates are akin to the Vertebrates; the
Worms are akin to the Echinoderms, the Molluscoidea, and
the Vertebrates: but the differentiation took place very far
back in geological time.

Evolution not Traceable between Different Classes. — The ar-
rangement into branches, therefore, is from a structural point
of view highly artificial; and for purposes of tracing the his-
tory, or even from a taxonomic point of view, it is of little
importance to deal with characters more ancient or of higher
rank than the class characters.

It may be convenient to associate the classes together into
larger groups; but to reach the point of real union of their
characters, in order to associate two or more classes in 3
common group, leads us far back into the uncertain mists of
the earliest geological time, and into the similar mists of em-
bryonic homogeneity. It is impracticable in the present
stage of science to trace the evolutional history of classes.

The Mollusca and Molluscoidea are of particular interest
because, lacking internal skeletal parts, and developing a
single or two-valved shell, there is concentrated on this shell

242 GEOLOGICAL BIOLOGY.

everything recordable of the characters of the whole organism.
These shells from their imperishable character are preserved
in the rocks in great numbers, so that variations are found
for comparative study. The particular consideration of these
hard parts and the study of the marks upon them, which
have determined the classifications of the paleontologist, can-
not be overlooked when it is an historical study we make of
organisms.

Having in view the importance of the characters of these
hard parts, of parts which can be examined both in living
and fossil condition, Zittel has described and classified them
according to the characters which they exhibit in their ma-
ture condition after their development and whatever of evo-
lution has taken place in their history, are complete. The fol-
lowing is a translation of Zittel's description of the Mollusca:

General Character of Mollusca. — For paleontologists, and
particularly for geologists, the Mollusca present a peculiar
interest; for all these classes, except the Tunicates, furnish
numerous fossil remains. Principally the shells of the Bra-
chiopods, of the Lamellibranchiates, the Gastropods, and
Cephalopods, are so widely distributed in the formations of
all the periods of the earth, that one chooses them in prefer-
ence as characteristic molluscs (" Leitfossilien "), wherever
the attempt is made to determine the age of the different
sedimentary formations. It is quite evident that it is only
the calcareous shells, their moulds in stone, or their imprints
which are at the service of the geologist. But as these fos-
sils are ordinarily distinguished by their characteristic form
and by their varied ornamentation, as the classification
within the several classes is essentially based upon the char-
acters of the shells, there is established a special science,
Conchology, which the geologist particularly cultivates.
Moreover, although the characters presented by the shells
are so insignificant, they are often deceptive; as in the case
where the animals of quite different organization (Patella,
Ancylus) are able to produce shells absolutely similar: so the
classification of shells requires, as in the other divisions of
the Animal Kingdom, a firm zoological basis, and the deter-
mination of species should be made according to zoological

PHYLOGENESIS IN CLASSIFICATION. 243

principles. On account of the relative ease in determination
of species in Conchology, the molluscan fossils have always
had particular favor with mineralogists and geologists. In
any other division of the Animal Kingdom it is impossible to
collect, describe, and figure fossil remains in such great abun-
dance; and besides, it can be said that the major part of
the bibliography in Geology and Paleontology is devoted to
shells — not always, it is true, in an ideal manner. If, indeed,
the insufficient knowledge of living Mollusca is a great cause
of frequent errors in the determination of genera, so too the
determination of species is at present in an almost chaotic
condition. As each author, according to his own views,
extends or contracts the limits of species, it happens
that one rarely finds in the works of different authors the
identical fossils of the same fauna described in the same
terms in the definition of characters. A chief cause of this
unfortunate state of affairs comes from the vertical range of
fossil Mollusca. Very frequently in a series of superimposed
beds of different age one meets with a characteristic type of
which the specimens from each of the different formations
(although presenting minor differences) preserve a special
fades throughout. In older works all the mutations of such
a series of forms were considered as belonging to one and
the same species, while more recently the inclination is con-
spicuous either to raise the smallest differences of their kind
to the rank of different species, or to distinguish them from
one another by application of trinomial names. Mollusca are
in major part aquatic. Of these classes, the Tunicates, the
Brachiopods, and the Cephalopods, live exclusively in the sea.
The greater part of the Bryozoa, the Lamellibranchs, and the
Gastropods are found in salt and in fresh water. The class
of Gastropoda alone presents representatives living in salt, in
brackish, and in fresh waters, and terrestrial species. All the
classes capable of preservation appeared in the Lower Silurian
(probably all in the Cambrian). The Brachiopods attain the
maxim of their development in the Paleozoic age, the Ceph-
alopods in the Mesozoic; the Lamellibranchs and Gastro-
pods appear to have continued their differentiation and ex-
pansion quite up to the Tertiary or recent period.

244 GEOLOGICAL BIOLOGY.

Mollusca. — An animal presenting bilateral symmetry; soft
body, non-segmented ; possesses neither internal skeleton
nor external skeleton; shows digestive organs very well
developed, and a nervous aesophageal collar, with three pairs
of ganglia in the highest types. Very many Mollusca secrete
in a fold of the skin, called mantle, a calcareous shell with a
single or two valves; others are entirely naked, and develop
no solid formation. Respiration is mainly effected by gills
or branchiae, more rarely by lungs or folds of the skin. A
circulatory system imperfectly closed, with a pulsating organ
driving its contents to the periphery, exists in Mollusca,
except in the lower types. Reproductive organs, differen-
tiated into sex organs, sometimes hermaphrodite and some-
times separate individuals, and in Bryozoa by budding and
formation of colonies, and of various forms. All these animals
now called Mollusca were ranked with Worms by Linne.

The Molluscoidea are particularly characterized by a cal-
careous shell, horny integument, or cellulose tissue. Respira-
tory organs often in front of mouth, as tentacles or appen-
dages; central nervous ganglion, between mouth and anus;
besides sexual reproduction, often also budding. All aquatic,
and mostly marine.

Bryozoa. — Small animals, increasing by budding, and united
into colonies, branching like moss (hence the name), and form-
ing incrustation, etc. Animals enclosed within membranous
or calcareous cellules, and possessing at the anterior extremity
of the body a mouth surrounded by tentacles; no heart;
intestine; well-developed body; anal opening near mouth;
hermaphrodite.

Tunicata, — Sac-like animals, free-swimming or fixed,
united into colonies; hermaphrodite; furnished with an enve-
lope (mantle) having the consistence of cartilage or leather,
which completely surrounds the body, and presents only two
openings. Branchiae on the internal part of the cavity
formed by the mantle; mouth in front of the branchial sac;
heart tubuliform [now ranked as a separate branch].

Brachiopoda. — Soft animals, living solitary ; furnished with
a bivalve symmetrical shell, presenting two free lobes of the

PHYLOGENESIS IN CLASSIFICATION. 245

mantle, which secretes the two shells; near the mouth two
respiratory arms, rolled into a spiral; heart present.

The Mollusca (proper). — The Mollusca (strictly speaking)
always multiply by sexual reproduction, never by budding;
respiratory organs either branchia or lungs; a central nervous
mass (brain) with three pairs of ganglia; body enveloped by
a thick mantle, which frequently secretes a shell of one un-
articulated or of two articulated valves; mouth with or with-
out maxillary appendages.

Lamellibranchs. — Mollusca, with an unsymmetrical bivalve
shell, furnished with a large mantle split into two lateral
lobes, upon which the branchial lamellae are developed
equally from one part to another; the two valves of the
shell are united by an elastic ligament, and generally by a
hinge furnished with teeth and sockets; mouth and arms
situated between the branchia in the plane of separation of
the two valves; ordinarily there is a muscular foot.

Gastropoda, — Soft animal, creeping, more rarely swimming,
with a robust muscular foot; presenting a head more or less
distinctly separate from the trunk, and a mantle undivided,
which generally secretes an orbicular shell in form of a low
cone or shield, or spirally enrolled.

Cephalopoda. — Head pointed, separated from the rest of
body ; sense organs, especially eyes, attaining high degree of
perfection ; mouth surrounded by a crown of muscular arms.
Body sac-form; 2 or 4 arborescent branchia, placed in a
cavity formed by the mantle; shell often spiral, of one or
many chambers, sometimes internal, or again entirely wanting.

While the Bryozoa and the Tunicates are scarcely above
the Ccelenterates, and are inferior to the Echinodermes, con-
sidering the differentiation and perfection of their organs,
the Cephalopods should be ranked, without doubt, among the
most elevated of invertebrate animals, and in some respects
they seem superior to certain vertebrates. *

In contrast to this analytic classification of Zittel, in which
the definition and grouping of the organisms is based upon
the visible and generally conspicuous characters distinguishing

* Zittel, "Handbuch der Palaeontologie," vol. I. pp. 571-575.

246 GEOLOGICAL BIOLOGY.

the mature individuals, we turn to the synthetic classification
of the same organisms as presented by Lankester, in which
the distinguishing points are chiefly found in those characters
by which resemblance or relationship to some other different
organism is traced.

In the first case differences and in the second case resem-
blances form the chief criteria upon which the classification is
based.

Lankester's Classification of the Mollusca According to Pro-
fessor Lankester,* whose classification is one of the most rad-
ical and modern, the branch Mollusca includes four classes,
divided into two groups: Class I, Gastropoda; Class 2, Sca-
phopoda; Class 3, Cephalopoda; Class 4, Lamellibranchia.

The Coelomata — Among the Metazoa, to which he applied
the name Enterozoa in distinction from Protozoa, Lankester
recognized two fundamental divisions: (A) the Ccelentera, in
which the enteron or digestive cavity communicates directly
and is continuous with the ccelom or body cavity; (B), the
Coelomata, including the Mollusca and higher invertebrates,
in which the enteron is separate from the coelom which sur-
rounds it and with which it communicates through its tissues,
by osmosis. The products of digestion thus transmitted
into the coelom or body cavity are distributed through a
system of canals and caused to circulate by a contractile
organ which in its more differentiated condition is the heart.
The special advance in differentiation in the Coelomata con-
sists in the separation of the alimentary cavity into a distinct
digestive cavity and an assimilative cavity, the circulative
and purificative functions being auxiliary to the general as-
similative, as distinct from the digestive, functions.

Description of the Mollusca. — The Mollusca are typically
Coelomata. They have also, in common with the other Coe-
lomata, a region in front of the mouth developed as the ex-
pression of the specialized function of forward, as distinct from
rotatory, motion; and in this region, which Lankester calls
the prostomiwn, are differentiated, when present, the chief
organs of sense. As to body form, the Mollusca have differ-

*See article Mollusca, " Encyclopaedia Britannica," gth Edition, vol. XVI.

PHYLOGENESIS IN CLASSIFICATION. 247

entiated a permanent bilateral symmetry, which may be
considered as the final elaboration of the radial type of differ-
entiation which was dominant in the Coelenterata and Echino-
dermata. In those branches the antimeres are numerous, or
at least five, and the bilateral symmetry is only partially ex-
hibited, while in the Mollusca it is a dominant character in
the adults. In the Vermes and Arthropoda the bilaterality of
each somite is further differentiated in the longitudinal direc-
tion by the division into segments. Although the order of
rank exhibited by this mode of differentiation of parts would
lead us to look for Ccelentera with multiple radiate struc-
ture first, then the differentiation of the Medusa, then the
Echinodermata, with symmetrical radiation, then simple bi-
laterality of the Mollusca, to be followed by Vermes and
Arthropoda, we actually find that the Arthropoda are al-
ready abundant in the Cambrian, and in the Trilobites con-
stitute the dominant type of organism.

Digestive System in the Mollusca — As already suggested, in
the Mollusca the alimentary system is differentiated into a
digestive cavity. The products of digestion, finding their
way through the walls of the digestive cavity, are received
into a system of canals with a contractile reservoir, which is
the circulatory blood-system. In connection with the ali-
mentary system is also developed in the higher Mollusca,
and perhaps in all the classes, organs called nephridia, which
are apparently purificative in function, and are primitive
kidneys in differentiation. In all the Ccelomata gonads or
special reproductive organs are differentiated.

Muscular, Nervous, and Motory Systems of Mollusca. — Muscu-
lar tissue is distinctly differentiated, and also nervous tissue,
with the peculiar specialized functions of contractility and
sensibility. The nervous system consists of a gangliated ring
of nerve-fibres around the oesophagus, and in the higher
types of Mollusca special sense-organs are differentiated for
touch and sight. The motory system is developed in a char-
acteristic way in the several classes of Mollusca, in a foot
which is the most permanent and characteristic 'feature of the
branch. Perhaps the simplest way of expressing the rela-
tions of this foot to the structure of the organism and the

248 GEOLOGICAL BIOLOGY.

-development of its organs is to compare it with the radiate
type of the Coelenterata — to suppose a Ccelenterate with
its row of tentacles, specialized in function and differentiated
in structure, so that, first, the contractile function is on the
one side, and muscular tissue is developed in place of numer-
ous tentacles, thus forming a mass of fleshy tissue contracting
in two directions and accomplishing locomotion ; while the
correlative function of sensibility is specialized on the other
side of the mouth into two tentacles, with, in some of the
higher types, distinct and well elaborated eyes for sensation.
In the Coelenterata the tentacles are both sensitive and con-
tractile, they are multiple, and muscular tissue and nervous
tissue are not fully differentiated. The foot of the Mollusca
may be considered as a specialized motor organ, in structure
it is differentiated muscular tissue, and marks the side of the
body as ventral ; while the tentacles, as of the snail, are spe-
cialized sensory organs expressing the differentiation of ner-
vous tissue for the special function of sensation, and mark
the dorsal side. In the Mollusca and in other branches the
gangliation of the nervous cord is co-ordinate with supply of
nerve-fibres to specialized organs: thus as the sensory organs
are more elaborated and specialized there is developed a
large nerve-ganglion on the corresponding part of the ner-
vous ring, and the foot too has its special nerve-ganglia de-
veloped. We find also specialized in the Mollusca tissues
for secreting fluids accessory to the digestive function, and the
differentiated organ for this purpose may be compared to the
liver of vertebrates. The stomach is not, however, highly
differentiated from the digestive tract in the lower types.
The Mollusca have specialized organs for respiration. These
organs — the gills, or mantle fringes — are present in all; but
in the class Lamellibranchia the function is not entirely re-
spiratory, but is also partly ingestive, or has to do with bring-
ing food to the mouth.

Differentiation of the Nervous System. — The nervous system
is differentiated to correspond to the differentiation of other
organs, and in two directions; at an early stage contractility
and sensibility were differentiated. Sensibility, then, has
two essential relations: sensibility as receptive of impressions

PHYLOGENESIS IN CLASSIFICATION. 249

from without, or sensation ; sensibility as active, or excita-
tory of the functions of organs, reflex or motor action. The
differentiation of the nervous system corresponds with this
distinction. In the lower Mollusca the distinction between
the two functions is little seen; but in Gastropoda, for in-
stance, and still more in Cephalopoda, special organs are dif-
ferentiated for sensation, and the nervous system is in com-
munication with each of the differentiated sets of organs,
stimulating and directing their activity. All this differentia-
tion is associated with the distinction of polarity of motion;
the nervous system is essentially co-ordinative, and binds to-
gether the activity of organs in the way of compensating for
the separation of parts due to their differentiation and devel-
opment in size. The nervous system compensates for sepa-
ration of the functional activities of the organism, and the
circulatory system compensates for separation of the physical
parts of the body of the organism, maintaining unity for the
organism co-ordinate with the physiological specialization
and the morphological differentiation.

Branches, Classes, and Subclasses of Mollusca. — Lankes-
ter's division of the Mollusca as a Phylum is first into two
branches:

Branch A, the Glossophora, characterized by a "head
region more or less prominently developed ; always provided
with a peculiar rasping tongue — the odontophore — rising
from the floor of the buccal cavity;" and

Branch B, Lipocephala, of which the characters are
" Mollusca with the head region undeveloped. No cephalic
eyes are present ; the buccal cavity is devoid of biting, rasp-
ing, or prehensile organs. The animal is sessile, or endowed
with very feeble locomotive powers."

All these latter branch characters are practically negative
characters: the Glossophora is a group formed of the Mollusca
which possess in common a few important characters, and the
Lipocephala are those which do not possess those characters.
Only one class is recognized in the Lipocephala, i.e., the
Lamellibranchia. The Glossophora comprise the three classes :
first, Gastropoda, with two subclasses, (i) the Isopleura and
(2) the Anisopleura; second, the class Scaphopoda; third, the

250

GEOLOGICAL BIOLOGY.

class Cephalopoda, with two subclasses, (i) the Pteropoda and
(2) the Siphonopoda.

The classes are chiefly distinguished by modifications of
the foot, as is beautifully shown in Fig. 57.

9

FIG. 57. — Diagrams of a series of Moll asks to show the form of the foot and its regions, and the
relation of the visceral hump to the antero-posterior and dorso-ventral axes, (i) A Chiton.
(2) A Lamellibranch. (3) An Anisopleurous Gastropod. (4) Thecosomatous Pteropod. (5) A
Gymnosomatous Pteropod. (6) A Siphonopod (Cuttle). A, P, antero-posterior horizontal
axis ; Z>, f7, dorsp-ventral vertical axis at right angles to A , P ; 0, mouth ; a, anus ; ms, edge
of the mantle-skirt or flap ; sp, sub-pallial chamber or space ; ff^ fore-foot : mf, mid-foot ;
hf) hind-foot ; e, cephalic eyes; cd, centro-dorsal point (in 6 only). (After Lankester.)

In the Gastropoda the foot is simple, median in position,
and flattened so as to form a broad, sole-like surface (No. 3).

In the Scaphopoda the foot is adapted to burrowing life
in the sand.

In the Pteropod the mid-foot is developed laterally into
paddle-like swimming organs, and the fore-foot may be spe-
cialized into tentacles (Nos. 4 and 5).

In the Cephalopoda the fore, middle, and hind foot parts
are separately specialized, the fore-foot merging with the head
part and developing into arm-like processes, in some cases

PHYLOGENESIS IN CLASSIFICATION. 25 1

beset with hooks or suckers, and the mid-foot is developed
into a tube either closed or with lapping edges (No. 6).

Distinctive Features of the Lankester Classification. — The
distinctive feature of Lankester's classification is seen in his
descriptions of the subclasses. To show the nature of the
characters selected as definitive of the divisions recognized,
the chief characters of the subclass (2), Gastropoda Aniso-
pleura, will be quoted, and for any further details the reader
is referred to the fully elaborated and illustrated article in
the " Encyclopaedia Britannica '' on Mollusca.

The definition includes the following characters, viz. :

"Gastropoda, in which, whilst the head and foot retain
the bilateral symmetry of the archi- Mollusca, the visceral
dome, including the mantle-flap dependent from it, and the
region on which are placed the ctenidia, anus, generative and
nephridial apertures, have been subjected to a rotation tend-
ing to bring the anus from its posterior median position, by
a movement along the right side, forwards to a position above
the right side of the animal's neck, or even to the middle
line above the neck. . . . The shell is not a plate enclosed
in a shell-sac, but the primitive shell-sac appears and dis-
appears in the course of embryonic development, and a
relatively large nautiloid shell (with rare exceptions) develops
over the whole surface of the visceral hump and mantle skirt. . .

" The shell and visceral hump in the Anisopleura incline
normally to the right side of the animal. . . . Atrophy of the
representatives on one side of the body of paired organs is
very usual." (p. 644.)

In these descriptions it will be noticed that characters
chosen as distinctive are based upon comparison of the type
under description with forms from which it is supposed to
have been developed embryologically, or from which it is
supposed to have descended by evolution.

The Gastropoda Anisopleura is conceived of as a Gastro-
pod mollusk which has become modified in a particular way
in the course of evolution.

CHAPTER XIV.

THE ACQUIREMENT OF CHARACTERS OF GENERIC,
FAMILY, OR HIGHER RANK ILLUSTRATED BY A
STUDY OF THE BRACHIOPODS.

IN the foregoing chapters the history of organisms has
been considered in its general principles.

We have noted how organisms are, in general, different
for different periods of geologic time; how the peculiarities
of structure and function, which have led to their classifica-
tion into many different classes, orders, families, genera, and
species, are intimately associated with differing conditions of
environment.

The steps by which the individual organism acquires its
morphological and physiological characteristics have been
examined, and the course of this development for each indi-
vidual has been found to be determined by the ancestry from
which it sprang.

The principles of classification have been discussed, and
from the investigation in this direction it has been learned
that each organic individual develops in the course of its
individual growth not only the specific, but the generic,
family, ordinal, class, and branch-characters of its parents.
These characters have various rank in the classification ; those
which are of higher taxonomic rank are found to be of more
ancient, and those of lower rank of more recent, geological
origin. Therefore we may conclude, as a general law, that
the lower the taxonomic rank of the character the shorter has
been its life-period, i.e., the period of time through which it
has been repeated by ordinary generation.

The various opinions regarding the nature of species have
been discussed. All naturalists find the employment of
species necessary to their science, though the exact definition

252

THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 2$$

of the term and the exact determination of any concrete
species are difficult to accomplish.

The examination has revealed the fact that the funda-
mental difference in opinion regarding species' turns upon the
belief as to the mutability or immutability of species.

The idea that species are mutable is intimately associated
with the inquiry, What is the "origin of species"? In at-
tempting to answer this question the deeper ones arise, i.e.,
What is evolved in evolution? and What is mutable?

The answer was that in any individual case all that is
evolved is to be found in the variation exhibited in those
characters by which it departs from the exact imitation of the
characters of its ancestors, and that evolution consists in the
acquirement of characters not possessed by the ancestors..

We examined the classifications of the Animal Kingdom
particularly, and we found that, looked at analytically as.
composed of avast number of different structures, or synthet-
ically as a multitude of related organisms variously differen-
tiated, and differentiated to various degrees along a few
general lines of evolution, the Animal Kingdom is divisible
into a number of definite groups, marked by definite organi-
zation, all the grander features of which were outlined in the
Cambrian age, and the large majority of all the differentia-
tions of even ordinal rank had been accomplished in the first
quarter of the recorded history of organisms.

It is evident, therefore, that we must read the law of
evolutional history in terms of the genera and species as they
are distributed in families or in orders.

Generic and Specific Evolution Illustrated by the Brachiopoda,
— In order to study the successive appearance of species and
genera, it will be necessary to turn from the more general
characters to the minuter marks distinguishing species from
species, or at least genera from genera. For this purpose no
better group of organisms can be selected than the Brachiop-
oda. In presenting the results of this analysis the paleon-
tologist will miss that elaboration of the facts which would
make the discussion of most practical use to him. The brief
limits of this introductory treatise do not admit of this; and if
the presentation of the facts shall stimulate some such readers

254 GEOLOGICAL BIOLOGY.

to open up the immense field of investigation which is here
suggested, the author's purpose in writing this book will be
fully rewarded.

Brachiopods Thoroughly Differentiated in Early Paleozoic Time.
— When we critically examine a group of organisms like the
.Brachiopods in their historical relations, we find a law of
successive appearance in geologic time of new characters, but
-we are obliged to consider them minutely in order to under-
stand what is the nature of the evolution. The more impor-
tant characters were already present at the earliest period in
which records are preserved.

Both of the orders of Brachiopods (Lyopomata and Ar-
thropomata) appeared in the Cambrian, and they are repre-
sented by numerous individuals and genera; and, according
to Waagen, there are three well-defined suborders of the
Lyopomata, and all of these were expressed certainly as early
as the base of the Silurian. If we take a later tabulation of
the genera and classification of Brachiopods,* we find n
families of the Lyopomata with 55 genera, and 19 families -f-
14 additional divisions recognized as of subfamily rank, with
220 recognized genera belonging to the suborder Arthropo-
mata. Of the total 275 genera, recorded by Schuchert, 50
of the 55 Lyopomata and 139 of the 220 genera of the
Arthropomata, or 189 of the total 275, i.e. about 68 per cent,
appeared in Paleozoic time; and 17 genera of Lyopomata
and 5 genera of Arthropomata are already known in rocks
as ancient as the Cambrian system.

Many of these Extinct since Paleozoic Time. — These figures
will give an idea of the great antiquity of the Brachiopods,
and of the early elaboration of the differences which are ex-
pressed in the systematic classification into genera. Another
fact can be expressed in mathematical form : not only were
the Brachiopods greatly evolved in early geological time, but
many of them have become extinct. Of the 189 genera of
the Paleozoic time 7 lived on to Mesozoic time, and of these
at least 2 genera still live.

Generic Life-periods of the Brachiopods. — Again, although

* Viz., that of Schuchert, in the American Geologist, vol. XL, March, 1893,
p. ii\i , etc.

THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 255

an ancient type of animals, and expressing great persistence
in some lines of succession, they also present very clear indi-
cations of definite succession and limit in their generic life-
periods, as may be expressed again numerically in the follow-
ing way : To express this law we may select a group of re-
lated forms, grouped under three families by Schuchert, the
Terebratulidce, the Dyscoliiday and the Terebratellidce, in which
are known 66 genera. Of these genera 13 were initiated in
the Paleozoic (i Ordovician, I Silurian, 5 Devonian, 6 Car-
boniferous) according to present statistics, 38 in the Meso-
zoic (7 Triassic, 23 Jurassic, 8 Cretaceous), and 10 are known
only in the Recent. Of these 66 genera 41, or about f,
have a recorded continuance of only one era, 1 1 are recorded
from two contiguous eras, 2 from three, 4 from four, 4 from
five consecutive eras.

Climax of Generic Evolution at a Definite Period, — If we go
one step further, and analyze the range of the genera of a
single subfamily, we see the law of evolution expressed with
greater clearness. Using Schuchert's list of genera, we find
that the subfamily of Dallinince (the first division of the
family Terebratellidae) contains 22 known genera; all of these
are known not earlier than the first period of the Mesozoic.
Of them, 3 genera are first seen in the Triassic, 13 first in the
Jurassic, 2 first in the Cretaceous, I first in the Tertiary; 3
are only known as recent species. In this case it is perfectly
evident that the group is a Mesozoic type, that it began to
appear in numbers in the Triassic, that its greatest expansion
was in the Jurassic, that as a subfamily it is not now extinct.

We draw from these facts the conclusion that there was
constant evolution going on, that all along geological time
old types were dying out and new ones were being initiated
or introduced. It is by studying the characters expressed by
these successive genera, and noting their relation to each
other in the order of their succession, that we catch a glimpse
of the actual facts of evolution as they have taken place in
the past.

In order graphically to express the grand facts of the
evolutional history of the various types of Brachiopoda the
following diagram of the evolution-curves of the various divi-

GEOLOGICAL BIOLOGY.

sions of Brachiopods, in terms of the initiation of new genera,
is constructed (Fig. 58).

Paleozic

40

20.

I/

10

Mesozoic Cenozoic
* ^\ ^ — ' — ^~~^ / — *"*'" — N

Ordovloian Silurian Devonian Carboniferou^s, /fri. Jur. Cretaoeoii^ /Tertiary Qy.

UA

t-'L

V

r\

FIG. 58.— Evolution curves of the Brachiopods. The spaces from left to right represent the suc-
cessive geological eras from the Cambrian to the Quaternary-Recent. The curves in the
upper part of the diagram are those of the Arthropomata, the lower curve that of the sub-
class Lyopomata. Starting from the horizontal base-line, elevation above this line expresses
the rate of evolution in terms of the number of new genera initiated in each division during
the era, the cross-lines representing 10, 20, 30, etc., new genera, respectively. The curves con-
nect the points so indicated for each group : a-a' = Arthropomata ; /-/' = Protremata ; b-b'
= Trullacea ; c-c' = Thecacea; /-/' = Telotremata ; h-h' = Helicopegmata ; n-n' = Ancylo-
brachia ; /-/' = Lyopomata.

These facts expressed in numerical form are as follows:

TABLE OF THE NEW GENERA INITIATED IN EACH GEOLOGICAL AGE,
GROUPED UNDER SUBCLASS, ORDERS, AND SUBORDERS (COMPILED FROM
SCHUCHERT'S LISTS).

COS

5 25 37
5 21 18
4 6 5

i 15 13

o 4 19

? 2 3

o i 15

O I I

Arthropomata, 220 Gen.

Protremata,

Trullacea,

Thecacea,

Telotremata,

Rostracea,

Helicopegmata, 52

Ancylobrachia, 67

81
19
62
139
20

D

Cr

T

J

K

Ty

Q

R

34

34

23

33

12

2

4

ii

ii

17

I

6

2

O

o

o

4

o

0

0

0

0

o

o

7

17

I

6

2

0

o

o

23

17

22

27

10

2

4

II

4

2

4

2

I

0

i

I

14

9

ii

2

0

o

o

0

5

6

7

23

9

2

3

10

Its Interpretation. — The two subclasses (if we call the
Brachiopods a class) Lyopomata and Arthropomata are

THE ACQUIREMENT OF CHARACTERS ILLUSTRATED.

thus shown in their strong historical contrast, the former
(/-/' ', Fig. 58) culminating its generic evolution in the Ordo-
vician, while the Arthropomata (a-a') culminates in the
Silurian, but continues to differentiate until the middle of
the Mesozoic ; and the several distinct lines of differentiation
are expressed by the curves for the several suborders, which
are recognized in the classification of Schuchert.*

This classification recognizes the Brachiopoda as a class,
and Arthropomata and Lyopomata as subclasses. The
Arthropomata (a-a') are divided into two orders, (i) the Pro-
tremata (/-/') (Beecher), including two suborders, viz.,
Trullacea (b-b'\ and Thecacea (c-cf) ; and a second order,
Telotremata (t-tr), with the suborders Rostracea, Helicopeg-
mata (h-hf), and Ancylobrachia, (n-n'\

From the irregularities of the curves made by these sub-
ordinal groupings of genera the indications are that the
Thecacea (c-c'} is compounded of three distinct groups, hav-
ing separate courses of evolution culminating in the Ordivi-
cian, in the Carboniferous, and in the Jurassic, respectively;
the Trullacea (b-bf), the Helicopegmata (h-h')y and the An-
cylobrachia («-#') are apparently natural groups; at least
the present evidence expressed in the structural classification
corresponds fairly well with the classification based upon the
rate of differentiation of genera within each group.

The Trullacea are the earliest forms of articulate Brachiop-
oda known and their development was earliest of the fami-
lies— too early for the exhibition of good evidence of actual
differentiation ; but the Helicopegmata (/*-//) show the begin-
ning of differentiation as late as the beginning of the Silurian
era, and as their history has also an ending at about the
middle of Mesozoic time, a study of their history should
throw some light upon the problems before us. The Ancy-
lobrachia (n-nr) beginning its differentiation about the same
time, and continuing to increase, reaching its culmination in
the Mesozoic, presents descendants of the old genera and
also some new genera even in Recent time. This group of
genera may be studied in detail on account of the fuller

* A Classification of the Brachiopoda, by Charles Schuchert, The Ant. Geolo-
gist, vol. xi. p. 141, March 1893.

258

GEOLOGICAL BIOLOGY.

records, the greater time-duration covered by its differentia-
tion, and because living forms have been carefully examined
and their structure and course of embryological development
are well known.

Majority of Characters of Living Brachiopods traceable to Cam-
brian Ancestors. — From this tabulation of the range of the
Brachiopoda it is evident that a great
majority of the characters which any
individual Brachiopod exhibits (as a
specimen of the Terebratulina sept en-
trionalis, now living in large numbers

uo

nu

FIG. 59.

FIG. 61.

FlG. 59. — Terebratulina scptentrionalis. View of the internal structure, the pedicle valve being:
removed (x 2). pe — pedicle ; rm = retractor muscle ; j = shell of brachial valve ; m =
mantle ; am — adductor muscle ; / = intestine ; / = liver lobes ; // = lophophore ; ne = ne-
l>h riil in in ; 011 — ovary. In this figure the pedicle end is the lower.

FIGS. 60, 61. — Shell of a Terebratula. AB = antero-posterior axis; CD = horizontal axis;
V — ventral or pedicle valve ; D = dorsal or brachial valve • / = pedicle ; f = foramen ;
c = cardinal slope ; a = umbo ; u = umbonal slope ; dp = deltidial plates.

off the coast of Maine) (Fig. 59; see also Figs. 60 and 61) are
of very ancient date, and can be accounted for by descent
without modification through direct ancestors running back
to the early Cambrian time.

These characters may be enumerated in the following
manner. The earliest Brachiopod possessed all the charac-
ters essential to each of the following taxonomic divisions,

viz.

A. Organism. — All the characters which it presents, distin-

THE ACQUIREMENT OF CHARACTERS ILLUSTRATED.

guishing it from matter in an inorganic state, were different!'
ated before the Cambrian.

B. Animal. — All the characters distinguishing it from
plants.

C. Mollnscoidea. — The special characters of this branch
were fully differentiated in the Cambrian era. These are
(to follow Claus and Sedgwick) : animals attached, as distin-
guished from moving or perambulatory organisms; the de-
velopment of bilateral symmetry; the absence of metameric
division — they are unsegmented; the differentiation of cili-
ated oral appendages ; enclosure in a calcareous shell, with
differentiation of organs into the various physiological
systems of the Metazoa, viz., digestive, motor, neural, ex-
cretory, and those of reproduction.

D. Class Brachiopoda (and not Polyzoa). — This distinc-
tion includes the characters of two spirally rolled buccaL
arms; the development of bivalve, equilateral, dorsal, and
ventral shells; the development of several ganglia connected
by a pharyngeal ring. There must be included here also all
the characters which are necessary to carrying on the func-
tions of the different parts, mentioned under groups C and D.

E. Lyopomata and Arthropomata. — All that distinguishes
these two orders was fully evolved, certainly, before the
Cambrian era was far advanced, for we find several dis-
tinct families of the one and five of the other already in the
Cambrian rocks. These differences are seen by comparing;
specimens of Terebratulina with a Lingula — both recent
genera. The differentiation includes, in respect of intestine,
a long and open intestine, with anal as well as oral orifice,,
and short, with postero-ventral end closed; in the shells the
distinction between free-sliding valves and hinged, articulated
valves, and the associated modification of muscular apparatus
to move them laterally upon each other in the first case, and
to open and shut them with a hinge in the other.

Perpetuation and Repetition of Characters a Common Law of
Generation. — The more sharply distinguishable characters are
mentioned above, but they include more than the ordinary
observer would notice if handed a specimen and asked to-
describe what he saw — more, I say, but not all the characters-

26o GEOLOGICAL BIOLOGY.

that he would notice; for the ordinary observer will notice
some of the specific characters more readily than he will the
more essential characters of a strange object.

Before leaving the first era of our time-scale we have
still more characters of family rank, others of subfamily rank,
and enough elaboration of them to call for classification into
fifty-five genera of Lyopomata and five genera of Arthropo-
mata, and all these are found in the species now known from
Cambrian rocks.

On the theory that the organisms now living are de-
scended from ancestors in the past, the characters once hav-
ing appeared in the ancestral line are most simply accounted
for by supposing that they have been transmitted without
change by the laws of ordinary generation. However the
characters may have been originally produced, or came
about in the first place, having once appeared in the Cam-
brian their continued reappearance in later stages of geologi-
cal history calls for no other processes than those we see
taking place on every hand, i.e., the successive reproduction
•of offspring by regular generation: no action of evolution is
required. The preservation by continued generation of these
ancient ancestral characters is no less remarkable than the
:slight modifications which have taken place in the course of
geological ages.

Evolution Accounts for Divergence, not for Perpetuation or
Transmission. — This familiar law of heredity will account for
the continuance, as long as they appeared, of the families and
genera of the Cambrian; the appearance of new families
new genera, and new species requires on this theory the
assumption of some other process. When we examine the
length of recurrence of these Cambrian forms, of them we
find only three genera of the Lyopomata and one genus of
the Arthropomata are known from rocks above the Cambrian,
and they are from the next succeeding system. Of the
family characters of the Cambrian Brachiopoda, six Lyopo-
mata, two Arthropomata families lived after the Cambrian :
two of these lived on to the Carboniferous, two of them
reached the Silurian, and only three reached the Ordovician.
There were, however, four families and two genera that ap-

THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 26 1

peared in the second era, the Ordovician, which lived on to
the present time, and it is not improbable that these types
of differentiation may have taken place as early as the Cam-
brian.

Brachiopods Ancient Types and Early Differentiated. — From
these facts we learn that the Brachiopods are very ancient
animals; that at the first geological period they were very
greatly differentiated in structure, and that, except in a very
few cases, the forms that lived in later ages, though suppos-
edly descended from the earliest types, suffered changes in
their specific, generic, and in many cases family characters.

It is also evident that if we wish to study the history of
Brachiopods we must read the evolution in terms of their
specific, generic, and only in slight degree in any characters
of as high rank as family, and not at all in characters of
higher than family rank.

A glance at the range of the families and genera of the
Lyopomata shows them not only to have been ancient, but to
have reached their climax of evolution by the second geologi-
cal period of time — the Ordovician. After the Ordovician no
new families of Lyopomata are initiated, and the new genera
fell from twenty-two new ones in the Ordovician to three in
the Silurian, six in the Devonian, and after that seven new
genera up to the living forms. This slight continuance of ex-
pansion may be driven much farther back by later discov-
eries.

Laws of Evolution Gathered from Study of the Early Families.
— With such an early expansion of the suborder it is evi-
dent that the range of instructive history is limited to the
earliest periods of geological time, and the few forms
that still exist among the recent faunas are very slightly
modified from the ancient types. In the case of the other
suborder, Arthropomata, the evolution was continued to a
later period. Family and subfamily differentiation was
greatest in the first two geological eras, nine new families
appearing in the Ordovician; but two or three new genera
in each of the following eras, except in the Cretaceous and
Tertiary, when the present information records only a single,
new subfamily in each.

262 GEOLOGICAL BIOLOGY.

Genera making their Initial Appearance in each Era. — The
generic expansion kept up with greater force, as the number
of genera making their initial appearance testifies. For the
ten successive eras the initiations of new genera recorded
up to the present time are as follows, viz. : 5,25, 37, 34, 34,
24> 33» I][> 2> 4 f°r Quaternary and n for Recent. The
greater number of recent genera not known in fossil state
may be discounted by the vastly greater knowledge we have
of recent organisms than of the faunas of any, even the most
recent, extinct fossil faunas. The evolution kept up its differ-
entiation of genera well into the Mesozoic time, when it
began to lessen rapidly, and from the Jurassic to the Creta-
ceous dropped from 33 to n in the number of new genera
appearing during the periods, and only two new ones ap-
peared in the Tertiary.

Comparison of the Rate of Evolution of Generic, Family, and
Ordinal Characters. — We may select this division of Brachio-
pods for more minute study of the historical laws expressed
in the evolution of its successive forms. A study of the
curve of results of this series of steps of evolution shows us
at a glance that there are, at least, two nodes in the evolu-
tion, one culminating in the Silurian and one culminating in
the Jurassic. Analysis of the structure of the forms reveals
the fact that the evolution has taken place along several sub-
ordinate lines, which are expressed in taxonomy by division
of the Arthropomata into two primary divisions, called by
Beecher orders (Protremata and Telotremata), and these
again into two groups of families, the Trullacea and Thecacea
in the first order, and into the three groups Rostracea, Heli-
copegmata and Ancylobrachia of the second order, Telotremata.

Evolution Curves for the Several Families. — Each of these
subdivisions was differentiated as early as the Ordovician, or
second era, and their climaces are at somewhat different
points in the time-scale.

The first group of families is the Trullacea ; there were
no new families of this type initiated after the Ordovician,
and no new genera after the Devonian, and the whole group
became extinct with the Paleozoic.

The second group is the Thecacea. Our curve of rate of

THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 263

differentiation shows such irregularity that we are led to sus-
pect within it three well-defined and separate courses of evo-
lution, one of which culminated in the Silurian, one in the
Carboniferous, and the third in the Jurassic. The presump-
tion is that this group is not well arranged ; the classification
will need revision.

The third group is the Rostracea, and this is characterized
by having a very long geological range ; the chief family is
the Rhynchonellidae, which appears to extend from the Cam-
brian to the present, with its characteristic family-characters
the same.

The fourth group is that of the Helicopegmata of Waagen.
This includes the spire-bearing Brachiopods; the history of
this group is well defined in families and in genera. The
culminating point for both is in the Devonian, when the total
number of forms is considered; but the greatest evolution of
families is in the Silurian, new genera continuing to appear
up to the end in the Jurassic.

The fifth group includes the Ancylobrachia. Although
the first family of the group appeared in the Ordovician, the
evolution of this type was very slow, but continuous to the
very end in recent times. From its first appearance each suc-
ceeding era has seen the addition of a new family. The curve for
generic differentiation is also emphatic, but it shows the evo-
lution of this type of Brachiopods to have been late in geolog-
ical time. Instead of being in the Paleozoic, the culmination
of generic differentiation was in the Jurassic, when twenty-
three new genera made their first appearance. The first fdur
family groups of the Arthropomata had their culmination in
the Paleozoic, and the fifth had its culmination near the
middle of the Mesozoic.

Conclusions from Study of Generic Evolution Curves of the
Brachiopods. — The examination of these differentiation or
evolution curves of the generic and family life-histories of the
Arthropomata can leave no doubt in our minds on a few im-
portant points:

I. The geological record, although imperfect, and not at
all exhaustive in its declarations, reveals the fact that some
types of organisms lived in one geological era, others in

264 GEOLOGICAL BIOLOGY.

another era, and leaves us in no doubt as to the general order
of succession of the various genera.

2. Although it is not improbable that in almost every
case the genera and the families will be found to have been
initiated somewhat earlier than they are now reported, and
new families and new genera will undoubtedly be discovered,
nevertheless, the outlines of the differentiation curves are so
emphatic in most cases that we have no reason to doubt
that we already have the fundamental outlines of the history
of each particular group of organisms clearly before us.

3. We have here unmistakable evidence that every genus
and family had a definite time of initiation, and that this time
of initiation for each has definite relationship to the time of
initiation of other genera and families.

4. Another conclusion may be drawn from an inspection
of the curves: the family differentiation for each grouping of
higher rank, suborder or order, had its evident initiation,
culmination, and decrease; also the generic differentiation for
each family had its point of initiation, its period of rapid
activity and culmination, and its period of decline; and in
many cases the actual cessation not only of expansion, but
of all appearance of the genus, is expressed.

CHAPTER XV.

WHAT IS EVOLVED IN EVOLUTION ?— INTRINSIC AND
EXTRINSIC CHARACTERS.

Laws of Evolution indicated by History of Brachiopods. — We
have now gained a sufficient knowledge of the characters of
Brachiopods to enable us to consider the question, What is
indicated regarding the laws of organic history by these facts?

It is evident, first, that the history exhibits evolution.
Evolution of what ? We have been considering the time re-
lations of the genera in families of Brachiopods: is it evolu-
tion of genera ? Tables have been given of the phylogenetic
relations of the families of hinged Brachiopods: have we
been considering the evolution of families ?

Before taking up these points a few words may be said
on the question, " What is evolved?" In general, we may
say, the history of organisms reveals a progressive evolution
of the morphological characters which distinguish the succes-
sive organisms. Classes, orders, families, and genera are not
the things which are evolved. These are names for the
divisions of the classification we make of the evolved or-
ganisms. The classification, when historically considered,
expresses the evolution; but the things classified are the indi-
vidual organisms, each of which has its characters distributed
through all the whole range of categories of the classification.
Therefore it is incorrect to speak of the evolution of one or
other of the categories — as a species or a genus: it is this or
that character of the individual that was acquired by evolu-
tion, as contrasted with other characters acquired by natural
generation from its parents.

Magellania Flavescens Examined as an Illustration In illus-
tration of this proposition we may take, for instance, a Magel-

265

266 GEOLOGICAL BIOLOGY.

lania flavescens, obtained from the seas about Australia; we
examine its shell; we find that it is a bivalve, equilateral
shell, the two valves articulating, and the one larger than the
other, and exhibiting a perforation through the beak for the
protrusion of a stem-like peduncle for its attachment. All
these characters are peculiar to the class Brachiopoda. They
distinguish this individual organism from the organisms of
every other class in the Animal Kingdom (Figs. 59, 60, 6 1 , 62).
Evolution of the Class Characters. — Whence does the Ma-
gellania derive these characters ? We at once say by descent
from the parent Magellania from which it sprang. How did
it attain the characters? By ontogenetic growth from an egg
which expressed none of them. The law of heredity ex-
plains the appearance of the particular characters in this in-
dividual organism, and the law of ontogenetic growth ex-
plains the formation in the individual of these characters.
But how did they come to be at all ? or, to put the idea in
another form, Why is it not a clam-shell ? Heredity explains
why it is like its ancestral predecessors; but what explains
the fact that it is unlike organisms of all other classes ? In
answering this question we are led backwards, and find in
the Tertiary Period forms presenting the same characters;
and because there is thus traced a succession of forms with
the same characters, we assume that descent will account for
the succession. . Still further back, in the Cretaceous, in the
Triassic, in the Devonian, in the Silurian, and even in the
lowest beds of the fossil-bearing series, the Lower Cambrian,
we find fossils possessing the essential class characters of our
living Magellania. There at the first stage of appearance of
Brachiopods the difference is obvious between the Orthisina
and the species of any other class than Brachiopods. We
can go no further for facts. We have to confess that we
have no knowledge of the origin of the class characters of
Brachiopods; we only know that they were evolved as far
back as Cambrian time, and that they have ever since been
transmitted by ordinary generation.

Evolution of the Ordinal Characters. — In the same way we
notice on the hinge margin the production of two processes
each side of a triangular fissure which we call teeth and del-

WHAT IS EVOLVED IN EVOLUTION? 267

thyrium. These we call ordinal characters; the Magellania
is of the order Clistenterata, or hinged Brachiopods.

But these characters have a continuous succession back
to the Lower Cambrian. Again, we notice on the smaller
valve two plates, called teeth sockets, producing with the
outer part of the hinge margin a groove or socket into
which the teeth fit, and at the base of them a pair of calci-
fied processes, called crura; but these too are traceable back
to the Lower Cambrian (Fig. 62).

Calcified Loops which are Subordinal Characters were Evolved
between the Cambrian and Silurian Eras. — The Magellania dif-
fers from some hinged Brachiopods in having, in addition to
the crura, calcified bands of a peculiar form looped back
upon themselves, which are technically called loops. These
are characters of a part of the hinged Brachiopods, and they
are called subordinal characters, separating the suborder An-
cylobrachia from all other suborders of Brachiopods. But
these loops cannot be traced backward further than the base
of the Silurian; they are not known in the Ordovician or Cam-
brian. Regarding the characters of the specimen of as high
as class and ordinal rank, we have no evidence regarding their
origin save the law of hereditary transmission by ordinary
generation ; but Magellania has loops which it could not have
gotten by the law of heredity, i.e; considered as a law of the
transmission of like characters from ancestry to progeny. If
we assume that the law of hereditary descent will satisfac-
torily explain the reappearance on successive organisms of a
character which has once been formed, then we have the ex-
planation of the class and ordinal characters of such a speci-
men as far back as to the Lower Cambrian, but its subordi-
nal characters can by this means be accounted for only back
to the Silurian. In other words, we are led by this train
of reasoning to the conclusion that this Magellania had
ancestors which did not possess its subordinal characters,
among which are the calcified loop of a particular shape.

Each Case of Evolution a Case of the Appearance in some Indi-
vidual of a Character not possessed by its Ancestors. — In the same
way we learn from embryology, or the ontogenetic growth of

268 GEOLOGICAL BIOLOGY.

this individual Magellania, that in the course of its individual
life it has developed from an embryo condition in which its
mature characters were not exhibited. By analogy we infer
that these other characters of the loop were evolved from an-
cestors in which they did not appear; but before asking howr
we observe that since the Silurian time the loop has ap-
peared on successive forms up to the present time, exhibiting
no greater differences than the ordinal or class characters in
the same line have exhibited. The first specimen which
exhibited a loop was distinct from previous forms by that
character, and this, wijh other characters, caused it to be
classed in a distinct suborder from all other forms. We use
the term evolution to express the idea of appearance of such
a character at first. All the various families of which we
speak, and all the various genera, whose history we mark by
range of life-period in the geological scale, had thus a place in
the scale when their first known representatives appeared ;
and whatever the characters may have been (in the present
case it is calcified brachial loops), every case is a case of first
known appearance of such character, that is, it did not ap-
pear before, and the evolution consists in its coming into
appearance on some organism whose supposed ancestors did
not exhibit the character.

Evolution of Fundamental Characters Relatively Rapid — The
facts, to be sure, may be considered as very imperfect, but
if we lengthen our lines backward we but lengthen the period
in which the character has been repeated by ordinary genera-
tion without modification sufficient to upset our classification.
Or if we extend the evolution over a hundred or a thousand
generations it merely reduces the amount of the increment for
each stage of the evolution. Thus we see that so far as
the evidence testifies, the evolution of those characters which
mark the differences between separate classes, orders, sub-
orders, and even some families of organisms, has taken place in
a relatively short period of time; taking as measure either the
rate of general progress in the differentiation of organisms, or
the length of the life-period of each particular genus or fam-
ily. This is in harmony with a law of evolution formulated
by Hyatt as given in a subsequent chapter (Chap, xviii.).

WHAT IS EVOLVED IN EVOLUTION? 269

This Rapid Evolution difficult to Account for by any Working
of Natural Selection. — Thus far it is the evolution of morpho-
logical characters with which we have been dealing. Genera,
or species, are often spoken of as being evolved. When
language is used in this way we mean that there is an or-
derly succession of genera. This orderly succession of forms
we can readily conceive; but a genus is a group of spe-
cies which possesses certayi common characters of higher
than specific rank. It is one thing to speak of the succes-
sion of the different forms and another thing to speak of
the attainment by offspring of characters not possessed by
ancestors. We are accustomed to the explanation by Dar-
win that the method of this attainment of new characters is
by the gradual accumulation of varietal characters which are
considered as arising spontaneously. Taking the case before
us, we can imagine the form of the loop of the Magellania as
having been acquired before its calcification ; but the differ-
ence between the presence of a calcified loop and its absence
was brought about within a brief period (geologically con-
sidered), while the modification of the loop, as indicated in
the several genera of the Terebratulidae, can be conceived of
as having been produced gradually in the geological sense.
Thus, when we consider evolution as applying to the produc-
tion of differences, great difficulty is found in accounting for
the structural differences, which are the basis of our classifica-
tion into groups of family and higher rank, by the slow pro-
cesses required for the working of natural selection upon nor-
mal variations.

What is Evolved? — Hence, in reply to the question " What
is evolved?" it is evident that morphological characters are
evolved — -not species, genera, or any kind of groups of organ-
isms. There is an evolution of the characters of the individual,
and because this evolution takes place in many individuals at
the same time, we recognize the evolution by the appearance
of the modification in the many individuals, and group them
into new genera or families, on account of their differences
from other forms.

How Does the Evolution Proceed? — How does the evolution
proceed? Not by the ab initio construction of a new organ-

2/O GEOLOGICAL BIOLOGY.

ism, or a pair of them in each specific case, from which all
the other representatives of the genus spring by natural gen-
eration without change — which is the old creational theory of
origin of species ; but by the individual assuming a different
course or extent of ontogenetic growth from the course or extent
of growth of its ancestors, including acceleration in the growth
of a part, or of an organ, with increase or specialization of its
function.

Intrinsic and Extrinsic Development, and Intrinsic and Extrin-
sic Characters. — This brings us to the consideration of the
twofold nature of the morphological and physiological char-
acters possessed by organisms. There are two fundamen-
tally different ways in which we recognize the characters as
differing one from the other when looked at from the evolu-
tional point of view. When we mark the course of develop-
ment from the egg to the adult chick we observe that there is
a gradual building up, first of tissues, then of definite organs
made of those tissues, from simple uniform cells; or, going
further back, from the original nucleated, unsegmented cell
itself. This is a process of differentiation of parts, as has
been already defined, and with specialization of functions.
But it is a process of the increase of parts and functions by
division of labor, and is an expression of one of the funda-
mental laws of the organism as a whole. This kind of growth
we may call intrinsic development : intrinsic, because it has
to do with the expansion or development of the organism as a
whole, and involves the internal adjustment of the organism
itself, and not simply the modification of one of its parts.
There is another kind of elaboration of organs and functions
which consists in the multiplication of like parts performing like
functions, and results in difference in the size, the proportion,
or the number of the morphological parts. This kind of growth
we may call extrinsic development, because it appears to be
definitely correlated with the nature and amount of the ex-
ternal supply of materials for growth, and with the outward
demands upon the activity of the functions concerned.

The distinction thus established in the mode of origin of
the characters furnishes the basis for the classification of the
characters into intrinsic and extrinsic characters.

WHAT IS EVOLVED IN EVOLUTION? 2JI

Example of an Intrinsic Character — To take an illustration :
the character which distinguishes the Spiriferidae from the
Terebratulidae and the Rhynchonellidae, called the brachid-
ium, is fundamentally an intrinsic character, because in the
fixation and rigidity of parts there is implied an adjustment of

FIG. 62. — Brachial apparatus of (i) Rhynchonella, in which only the crura are developed ; (2) Ma-
gellania, showing the crura with the looped bands of the brachidium ; and (3) Athyris, with
no loops but the brachidial bands extended in spiral coils.

the other parts of the organism to these conditions; and in the
apprehension, the distribution, the deposition, and the sup-
ply of materials for constructing the apparatus, there is implied
an adjustment of the whole organism to the work of con-
structing this new part. Even though the soft parts were
essentially the same in the Orthis and the Spirifer, the modi-
fication in the Spirifer is a radical one, involving the whole or-
ganism, and not merely the particular part concerned.

Example of an Extrinsic Character, — On the other hand, the
character distinguishing the spires of the Atrypidae from
those of the Spiriferidae is the permanent turning of the point
of the cone toward the centre of the valve in the Atrypa, and
toward the upper outer angle of the shells in the Spirifer.
This is a matter of adjustment which may involve a slight
rearrangement of the relations of parts, but may involve no
more; the difference in the shape of the shell itself may
occasion such adjustment, as a tight shoe might distort the
shape of the foot (Figs. 63, 64).

27?- GEOLOGICAL BIOLOGY.

Characters Early and Rapidly Evolved were Chiefly Intrinsic
Characters. — It will be observed that almost all of the charac-
ters, which have thus far been considered in tracing the dif-
ferences distinguishing different classes, orders, families, and
to some extent genera, are intrinsic and not extrinsic char-
acters.

Application of the Terms Intrinsic and Extrinsic to the Elabo-
ration of Machinery. — To illustrate the fundamental nature of
this distinction we may call attention to a purely mechanical
contrivance, the steam-engine, and the machinery run by it.
The force here concerned is heat, which is transformed from
burning wood into expansion of water into steam. The
simple process is the transfer of elevation of temperature into
enlargement of the space occupied by the steam. This expan-
sion is in every direction. The engine is a device for concen-
trating the direction of expansion in one line, i.e., that of the
axis of the piston-rod. So long as no greater elaboration of
the mechanism is made in the engine, it is necessary to take
the effect of the stroke upward only ; the production of a hinge
in the rod, and an attachment of the rod to a lever, make
the walking-beam engine, which could, at the other end,
work a pump; but the differentiation, which turned the link
into a crank, causing continuous revolution of the wheel, was
an intrinsic elaboration of machinery, involving a coadaptation
of all the parts of the machine. Improvement in the way
of elongation of the lever, or change of the relative size of the
parts, in the modification of the wheel, in the shape and rela-
tive size of its parts, was purely extrinsic. Again, for the
transfer of motion a belt and flat wheel was modified into a
wheel with cogs, or the reverse — I do not know which ; this is
expressive of intrinsic elaboration of the device, while the
increase of cogs in number, or size, or shape, or change of
relative motion by different number of cogs on the two
approximating wheels, is of the nature of extrinsic modifica-
tion.

Summary and Conclusion, — From this illustration it becomes
evident why it is rational to expect a different rate in the
process of organic evolution from within, or intrinsic evolution,
from the rate of the evolution from without, or extrinsic evolu-

WHAT IS EVOLVED IN EVOLUTION? 2/3

tion. Both are at work at the same time, and every organism
has its specific, its generic, and family characters, and those
of higher order. Varietal characters in the process of ex-
trinsic evolution may become invariable, and be ranked as
specific accordingly; but when a character becomes fixed it is
no longer variable, and because one species differs from an-
other, and one genus from another, it does not follow that a
specific character has by degrees become of family or ordinal
rank. On the contrary, the cessation of plasticity which
results when the varietal character becomes transmitted with-
out change, and thus characterizes the species, makes it logi-
cally impossible to account for the difference in rank of the
characters of an organism by any evolutional process. Rank
of characters of the organism, as expressed in their place in the
classification, is inherent in their use; and the same laws which
are engaged in the origin of specific characters must also
account for the origin of ordinal characters. The specific
character does not become of ordinal rank, but whenever an
ordinal character arose it must have first appeared as a variety.
Herein consists the great importance of the facts of variation.

The accumulation of varietal modifications of parts or
their intensification, their growing larger or smaller, stronger
or weaker, is a matter fundamentally of addition or subtrac-
tion in the component units of lower order. Given a tissue
made up of cells and performing a given function, and the
modification of its form is but an expression of increased
growth at one place or diminished growth somewhere else. It
is easy to imagine conditions of environment, use, and dis-
use, adaptation to existing conditions or the opposite, as
resulting in the modification of the form of the organ.

It is not difficult to imagine the same kind of phenomena
working a selective discrimination among the variable degrees
of such adaptation, and -resulting in the preservation of cer-
tain variations and the elimination of others in the struggle
for existence. The theory of origination of species by natu-
ral selection applies to cases of extrinsic evolution; but it is
difficult to imagine how natural selection can operate in the
production of the differences in structure which must be
already differentiated before their relative fitness or unfitness

274 GEOLOGICAL BIOLOGY.

to the conditions of environment can be tested. It is reason-
able to expect, therefore, that all modifications of organic
structure, which imply strictly intrinsic differentiation of the
co-ordinated structure and function of the organism, were
evolved by processes vastly more rapid than those of the ex-
trinsic modification of structures already present in the race.

We have seen how Brachiopods furnish us with the data
with which to trace the laws of the historical evolution of the
more important characters exhibited by any particular Brach-
iopod. These characters have fallen into natural divisions,
or groups of various rank, which are scientifically recognized
as class, ordinal, subordinal, etc., characters. We have seen
how the characters which we call subordinal, when they are
arranged in the order of their morphological affinities, present
a series of forms whose elaboration is as complete by the
beginning of the Upper Silurian as it was at any later time;
therefore we drew the conclusion that so far as the subordinal
characters and those of higher rank are concerned, the differ-
entiation expressed by these characters took place in the
lower half of the Paleozoic time. As far as the facts are in
evidence, we find that the characters of this kind were rapidly
introduced: rapidly in relation to the degree of differentia-
tion indicated by the characters, and rapidly in comparison
with the length of time they persist without apparent modi-
fication. As two ontogenetic forces are at work in the
growth of the individual, to which respectively we apply the
terms heredity and variability, so we recognize upon analysis
of the facts of the phylogeny two kinds of evolution : (I) a
progressive evolution which operates from within and is asso-
ciated with pre-existing conditions ; this is called intrinsic evo-
lution ; (II) another kind of evolution, observed to be more
intimately co-ordinate with external conditions, which may be
regarded as fundamentally a process of adjustment or adapta-
tion of the organism to its external environment ; and this is
extrinsic evolution.

In the ontogenetic development of the individual there is
a rapid elaboration of those typical features of the organism
which express its class, ordinal, and subordinal characters, the
whole framework and plan of structure being elaborated

WHAT IS EVOLVED I AT EVOLUTION? 2?$

before the individual comes into contact with external envi-
ronment, while it is out of reach, so to speak, of the contests
which are called struggle for existence. It is conceived that
there were in like manner in evolution intrinsic modifications
of internal structure, requiring for their functional operation
adjustments of the whole mechanism of the body, and that
these operations were relatively rapid, because they were the
expression of evolutional force working from within, and in
the determination of which the local and immediate conditions
of environment hand little or no part. As, for instance, in the
plant, the special modification of ordinary tissues to produce
the flower, and its complication of floral parts, relatively to
the life-history of the plant is rapid, and the opening of the
flower may in some sense be said to be occasioned by heat,
sunshine, or, in general, by external conditions; but in a
much more important sense it is true that the production of
the flower is intrinsic, and is determined by ancestral, pre-
existing conditions, and not by those present only at the time
of flowering.

CHAPTER XVI.

THE MODIFICATION OF GENERIC CHARACTERS, OR
GENERIC LIFE-HISTORY.

IN the last chapter the conclusion was reached that evo-
lution, which is the acquirement by organisms in the course
of individual growth of characters not previously appearing
in their ancestors, maybe distinguished as of two kinds: one
intrinsic, and expressing steps of progress in the differentia-
tion of function and organization of the organism as a whole,
working from within outward ; the second extrinsic in nature,
and expressed in the modification or adjustment of characters
already differentiated to local and immediate conditions of
environment.

We observed that as the particular characters examined
are of higher and higher rank in classification they are more
intensely intrinsic in nature, not only now, but were so in the
earliest organisms of which we have any knowledge. And
still further, that these more essential characters were earlier
evolved, and the evidence seems to prove beyond doubt that
their evolution was by steps more rapid than would be in-
ferred from the relatively slow progress in the succession of
the lesser characters, generic and specific.

Having noted the general laws of evolution respecting the
more important characters of each individual, we next turn
to an examination of the laws of evolution of the less im-
portant generic characters.

In the generic characters there appears to have been a
rapid attainment of the total limk of modification expressed
anywhere in the family, with a long persistence of the more
widely divergent characters. When we examine the specific
and varietal characters we observe a much slower rate of
modification in individual race-series, but even here a re-

276

THE MODIFICATION OF GENERIC CHARACTERS.

markable degree of expansion of the main features of the
variable characters appears very early in the history of each
genus.

As an illustration of the rapid appearance of the full
quota of extrinsic modifications of a new intrinsic element of
structure we may examine the history of the spiral brachial
appendages in the suborder Helicopegmata.

Statistics of the Life-history of the Spire-bearing Brachiopods
(Helicopegmata). — The earliest trace of the spire-bearing
Brachiopods is in the Ordovician, in a single simple form, the
genus Zygospira.

At the next faunal stage, the base of the Upper Silurian
system, there were representatives of each of the families into
which the known Helicopegmata are divided (Atrypidae,
Spiriferidae, and Athyridae); and of the twelve subfamilies
into which the seventy-three recognized genera are distributed,
nine are also known from as early a stage as the Upper Silu-
rian (viz., Zygospirinae, Dayinae, Atrypinae, Suessiinae, Tri-
gonotretinae, Rhynchospirinae, Hindellinae, Athyrinae, and
Meristellinae). Of the others, Uncitinae, first appearing in
the Devonian, has the same kind of brachidium as the sub-
family Suessiinae; and the loop of Diplospirinae, appearing
first in Kayseria of the Devonian and having several genera
in the Triassic, is rather to be considered as an extreme differ-
entiation of the Athyroid type; and so far as the brachidium
is concerned, Koninckinina of the Mesozoic is also an extreme
differentiation of the same Paleozoic type.*

The Rapid Appearance of the Different Modifications of the
Brachidium. — For the present discussion it matters not whether
the calcification of the spirally-terminated brachidium of the
Helicopegmata is a modification of that seen in the loop of
the Ancylobrachia, or whether it arose from a form in which
there was no calcified support ; for both of the suborders, so
far as evidence is at hand to show, first appeared in the Or-
dovician.

One intrinsic character distinguishing these suborders from
all the previously existing Brachiopods is found in the presence

*In this discussion I have followed Schuchert's "A Revised Classification of
the Spire-bearing Brachiopoda," Am, Geol , vol. xm. p. 102, etc., Feb. 1894.

2/8 GEOLOGICAL BIOLOGY.

in the former of the calcified supports, the brachidium, and it
is the sudden or rapid appearance of modifications of struc-
ture of this brachidium which is under discussion.

TABLE SHOWING THE TAXONOMIC RELATIONS OF THE HELICOPEGMATA..
Branch: MOLLUSCOIDEA

Class: ^Polvzoa

( BRACHIOPODA

Subclass JLyopomata

( ARTHROPOMATA

Order: ^Protremata

TELOTREMATA

Rostracea
HELICOPEGMATA

( ATRYPID^
Suborder:^ _ \ „

Fam.:-< SPIRIFERID^E.

( ATHYRID^E
^Ancylobrachia

The above table is given to show the method of selection
of this particular group of Helicopegmata for study. All the
differentiation represented by the characters distinguishing
the particular class, subclass, order, and suborder must be sup-
posed to have already arisen before family characters of this-
particular suborder could take place.

I have adopted Dr. Beecher's ordinal classification, and take the order
Telotremata, which appears to be the most fully differentiated of the orders
of Brachiopods. The distinctive characters are found in the degree of
differentiation of the delthyrium, or pedicle opening, and its covering, and
of the brachidium or arm support. (" Pedicle opening shared by both,
valves in nepionic stages, usually confined to one valve in later stages, and
becoming more or less limited by two deltidial plates in ephibolic stages.
Arms supported by calcareous crura, spirals, or loops."} The distinctive or-
dinal characters I have italicized in this definition.*

It is within this order that we find the forms with special calcified parts-
called deltidial plates, crura, and brachidium, either loops or spirals. The
subordinal distinctions are based upon the degrees and mode of elabora-
tion of the brachial supports.

Rostracea is a new ordinal name proposed by Shuchert for the family
Rhynchonellidae of Gray, somewhat emended. It includes the genera with
rostrate shells, no spondylium, and the presence of crura.

The Helicopegmata is the group proposed by Waagen to include the
genera with two, calcareous, simple or double, spirally enrolled brachial
supports, which may or may not be attached to each other by a variously
constructed band or "loop."

The third suborder is Gray's Ancylobrachia, slightly emended by

* Beecher, " Development of the Brachiopoda," Pt. I. Am. Jour. Set., vol.
XLI. p. 355, 1891.

THE MODIFICATION1 OF GENERIC CHARACTERS. 279

Schuchert, characterized by the possession of a calcareous loop for the
support of the brachia.*

Three Families of the Helicopegmata. — In the classification
of the Helicopegmata into families Mr. Schuchert's simple
classification into the Atrypidae, Spiri-
feridae, and Athyridae, based upon the
essential structure of the brachidium,
is adopted. His definitions are:

1. In Atrypidce the primary lam-
ellae are directly continuous with the
crura, diverge widely, and have the
spirals between them (Fig. 63).

2. In the Spiriferida the primary FIG. 63 -The jbrachidium of the
lamellae are also directly continuous

with the crura, but lie between the

spirals, thus holding a position the reverse of that in the

Atrypidce (Fig. 64).

3. In the Athyridcs the primary lamellae differ in direo

.

Atrypidae ; Zygospira modesta^
enlarged ; view of interior from
the side of brachial valve, which
has been removed. (After Hall.)

FIG. 64. FIG. 65.

hidium of the Spiriferidae, Uncites gryphus Defr. ; intern
dicle-valve side.

FIG. 65. — The brachidium of the Athyridae, Rhynchospira evax, enlarged, and viewed from the-

FIG. 64. — The brachidium of the Spiriferidae, Uncites gryphus Defr. ; interior of brachial valver
viewed from pedicle-valve side.

. 5. — e racum o e t
pedicle-valve side. (After Hall.)

tion from those in the other families in being more or less
sharply recurved dorsally near their junction with the crura
(Fig. 65).f

* Schuchert, " A Classification of the Brachiopoda," Am. Geol., vol. xi.
141-167, 1093.

t Schuchert, "A Revised Classification of the Spire-bearing Brachiopoda/*
Am. Geologis , vol. xm. p. 102, 1894.

GEOLOGICAL BIOLOGY.

Geological Range of the Families. — The following table of
the geological range of the families, subfamilies, and genera
will help to give a notion of the time-relations of the forms
under discussion.

TABLE REPRESENTING THE GEOLOGICAL RANGE OF THE FAMILIES AND
SUBFAMILIES OF THE HELICOPEGMATA, WITH THE NUMBER OF GEN-
ERA AT PRESENT RECORDED FOR EACH ERA.

C

o

s

D

Cr

T

J

K ,

Ty

QR

HELICOPEGMATA.
Families :
Atrypidse

2

Spiriferidae
Athyridae

ii

8

4

ATRYPID^E

___

Subf.: Zygospirinae
Dayinas . . .

2

i

Atrypinae. . .

— — —

3

Subf.' Suessiinae

2

Uncitinae

I

Trigonotretinae

5

9

5

2

i

Subf.: Rhynchospirinae
Hindellinae

4

4

4
3

6~~

i

p-

Athyrinae

i

i

5

7

Diplospirinae

i

?

4

Koninckininas

6

i

Meristeilinae

?

Description of the Structure of the Brachidium. — The ele-
ments of the brachidium in the Helicopegmata are seen with
considerable elaboration in the genus Athyris.

In the interior of the brachial valve are seen in the apical
region, proceeding forward from the hinge-plate, two stiff pro-
cesses called the crura (Fig. 79) ; attached to the crura, and
in Athyris making a short twist toward the base of the crura,
proceed two ribbon-like bands toward the wall of the shell,
and thence along parallel to its inner surface toward the
front : these are thus far called the primary lamellce. At the
front, and continuously with these lamellae, the spiral coil
begins by curving toward the opposite valve, thence upward
parallel with its inner surface to near the crura, thence turn-
ing again toward the wall of the brachial valve, and in the

THE MODIFICATION OF GENERIC CHARACTERS. 28 1

case of Athyris proceeding onwards parallel but outside the
primary lamella, the second ribbon of the spiral running a
parallel course, but with each spiral diminishing the size of
the coil, and finally stopping at the apex of the spiral cone,
one of which is on each side of the median plane of the valve.
The various volutions of the coils on each side are thus called
primary, secondary, etc., lamellae of the spiral coil of the
brachidium.

Between and uniting the primary lamellae of the two coils
is developed a band, variously complicated in different genera,
called the loop, saddle or jugum.

In Athyris the jugum has at the centre a process extend-
ing upward towards the space between the crura, which is
called the stem of the jugum: this stem forks in the present
case, and the two branches (Fig. 79, b) are called arms of the
jugum (Fig. 79, /); they proceed on the outer side of the
primary lamellae almost in contact with them, forming acces-
sory lamella (Fig. 79, b\ In the genus Kayseria the accessory
lamella are continued along the face of the lamellae of the
spirals to form on each side a secondary or accessory spiral
coil.

Indirectly connected with the modifications of the brachid-
ium is a calcified plate, arising from the interior walls of the
brachial valve along the median line, to which the jugum or its
processes are attached or come in contact ; this is the median
septum. A median septum may also be developed from the
corresponding position in the interior of the pedicle valve.

Recent students of Brachiopods have found the structure
of the brachidium of great value in classifying the species into
generic groups ; and we are indebted to the work of Glass,
Whitfield, Bittner, Beecher, Clark, and others, that our knowl-
edge, systematized in the hands of the veterans Davidson
and Hall, is so full regarding these delicate parts of the
Brachiopod structure.*

* For illustration and description of these characters of the Brachiopods the
student is referred to "An Introduction to the Study of the Brachiopoda," by
James Hall (published in the Reports of the State Geologist for 1891 and 1892 ;
Albany, New York) ; to the elaborate final Report on the Brachiopoda, vol. vin.
of the Paleontology of New York, by the same author ; to Dr. Oehlert's appendix

2%2 GEOLOGICAL BIOLOGY.

Significance of the Facts. — By turning back to the table
representing the geological range of the several genera and
families of the Helicopegmata (p. 280) it will be seen that
the total life-range of all the representatives of the group
extends over eleven periods of the time-scale (from the
Neo-ordovician to the Jurassic). In the Neo-ordovician there
appeared a few small representatives of one of the families,
but in the next period (Eosilurian) all three of the families are
represented. In other words, all of the family differentiation
was attained in, we may say, the first decade of the life of the
suborder, and there were in the Silurian 5 genera of Atrypi-
dae, 5 genera of Spiriferidae, and 1 1 genera of Athyridae.

All the essential extrinsic characters of the brachidium
which ever appeared had arisen at the very outset or initial
stage of the history of the group of organisms possessing the
brachidium.

When we consider that in evolution the real increment
in any case is seen in the acquirement of differences in the
morphological characters of organisms, and it is not a new
species or genus or order that is evolved, but it is the develop-
ment by individuals of some part of their organization in a
different form from that seen among their ancestors, the sig-
nificance of this observation is apparent.

After this initial stage there are no representatives of the
whole order Helicopegmata in which the relative position of
the loop is not found to be of generic value in taxonomic
classification, and there is no case in which the modification
of this character surpasses the limits attained at this initial
stage of evolution.

The Loop of the Ancylobrachia and the Brachidium of Heli-
copegmata.— This was in all probability near the time of
divergence of the Ancylobrachia and Helicopegmata, and as
has been suggested,* the fundamental difference between
the calcified brachial supports of these important groups of

to Fischer's "Manuel de Conchyliologie " on " Brachiopodes ;" to Zittel's
" Handbuch der Palaeontologie," vol. I., and to Davidson's classic treatise on the
"British Fossil Brachiopoda."

* " On the Brachial Apparatus of Hinged Brachiopoda and on their Phy-
logeny," Proc. Rochester Acad. Sci., vol. II. p. 113, etc., 1893.

THE MODIFICATION OF GENERIC CHARACTERS. 283

Brachiopods (see Figs. 66-72) consists in the fact that the
loop or jugum connecting the primary lamellae in the Heli-
copegmata sets off from the sides of the lamellae before they
have begun to reverse their direction in forming the volution,
and the continuation of these lamellae is supplied with a cal-
cified spiral support ; while in the Ancylobrachia the connec-
tion does not take place till after the primary lamellae have
reversed their direction and are proceeding backward toward
the crura. For them there is no calcified continuation of
the lamellae, but the brachial arms, although still preserving

FIG. 66.

FIG. 67.

FIG. 68.

FIG. 69.

FIG. 70. FIG. 71. FIG. 72.

FIGS. 66-72. — Diagrams expressing the relationship between the brachidial apparatus of Ancylo-
brachia and Helicopegmata. 66-6g. The loop of the Ancylobrachia. D = brachial valve ;
V '= pedicle valve ; c = crura ; / = primary lamella of the brachidium ; / = the connecting
bar of the loop corresponding to the jugum of the Helicopegmata (b in the lower diagrams) ;
a = the fleshy spiral arms, not supported by calcified lamellae. 70 = Brachidium of Zygo-
spira, 71 of Anazyga, 72 of Dayia, seen from the side ; the lettering the same as above, except
b = jugum and j = spiral coils of the brachidium.

the spiral form, from the angle of the loop are entirely fleshy,
and therefore not preserved in the fossil state.

If we examine in detail the kind and extent of modifica-
tion exhibited in the various genera, in their relations of the
time and order of appearance in the geological faunas, we gain
a close view of the actual fact of evolution of new characters,
as seen in the following particulars :

Relation of Jugum to the Primary Lamellae. — i. The posi-
tion of the jugum in relation to the point of outset of the

284

GEOLOGICAL BIOLOGY,

primary lamellae from the crurae and the point of their turn-
ing back to form the first volution of the spiral, is perhaps
one of the most fundamental differences, as it affects the
whole mode of elaboration and position of parts of the brachid-
ium. The extremes possible are for the jugum (i) to join
the lamellae immediately at their origin from the end of the
crurae, and (2) to be sifuated at the extreme front of the shell

FIG. 75. FIG. 76.

FIGS. 73-76. — Zygospira modesta. (After Clarke.) Showing the variation in the position of the

jugum.

joining the lamellae where they begin to turn back to make
the first volution of the shell (compare Figs. 75 and 74).

The rapidity with which the differentiation of structure in
this particular took place is seen in a remarkable way by the
examination of the earliest representatives of the Helico-
pegmata, as illustrated by the diagrams of the form of the
brachidium of Zygospira modesta and of the closely allied form
Z. putilla prepared by Mr. Clarke.*

The position of this jugum (or loop) is regarded by Hall
and Clarke as of less than specific value. They say, " This is
* Pal. N. Y., vol. VHI. pt. 2, fasc. i. pp. 155 and 157.

THE MODIFICATION OF GENERIC CHARACTERS. 285

not a specific character, but a matter of variation among indi-
viduals of a given species;" and remark further, " This mo-
bility in the loop of Zygospira is without parallel among
other genera." *

This case of the Zygospira loop is a striking example of
rapid evolution. It has the appearance of being an insignifi-
cant feature, only a variation, because of the presence of all
the intermediate variations at the initial stage.

Relation of the Primary Lamellae to the Crurse, — 2. A
second example is seen in the modification of the direction of
the primary lamellae after they set out from the end of the

FIG. 77. FIG. 78.

FIG. 77. — A Spirifer, showing part of the brachial valve, the brachidium with the primary lamella,

the jugum, and the spiral coils.
FIG. 78.— Cyrtina, the brachial valve removed, showing the brachidium with the spiral coils

turning upwards into the produced umbonal part of the pedicle valve.

crurae. There are two ways in which this direction differs:
(a) The lamellae may proceed directly toward the front of the
shell away from the crurae, as in the case of Spirifer and
Cyrtina (Figs. 77, 78) ; or they may, immediately after their
origin, take a sudden bend upon themselves, making a twist
or double bend before proceeding along parallel to the inside
surface of the shell, as in Athyris (see Fig. 79); observe also
the brachidium in Figs. 64, 65 (p. 279). The latter is re-
garded as a characteristic of the family Athyridae, and,
although the family as a whole is the more differentiated and
later to be dominant, there are several well-marked genera

* See 1. c. p. 156.

286 GEOLOGICAL BIOLOGY.

showing the sudden reflection and twist in the origin of the
primary lamellae among the first species of the Eosilurian (viz.,
Dayia, Hindella, Merista, etc.), while the Atrypidae and Spir-
iferidae, in which the lamellae are directly
continuous with the crurae, are fully ex-
pressed at the base of the Upper Silurian.
^e second particular in which difference
*s exm°ited is seen in (b) the direction away
--> from or else parallel to the plane of a me-
A "AT dian sePtum- In one extreme (see in Zygo-

' V\ spira, Figs. 73-76) the lamellae diverge at

a right angle (or less) from the extremity of

FIG. 79.— Athyns, showing S & \ J

ff twVcreuriraciidisi^- t^le crurae toward the lateral borders of the
twisted pit 2Khep£ri- shell, and curve outward and thence down-

/unSg ward along this outer border to the front;

hVthe and as they reflect in the course of the first
volution, turn inward toward the centre.

follow the direction and T , , . . •• • 1 i_ j-t, •

lie upon the upper part In this type the spirals have their apices

of the primary lamellae. , . , 1 , A

directed more or less inward. Atrypa pre-
sents these characters, and Schuchert has adopted the charac-
ters of its brachidium as a mark of the family Atrypidae (see

p. 279).

In Spirifer the lamellae proceed with almost no divergence
in two nearly parallel lines, from the extremities of the crurae
directly toward the front along the inner surface of the
brachial valve, and at the front curve directly toward the
pedicle valve, and in making the first volution of the spiral
return nearly to the starting-point at the end of the crurae.
The spiral thus formed has its apex directed outward toward
the lateral border of the valve, and it is in this type of
brachiopods that the great production of the lateral wing of
the shell takes place, and the apices of the spires penetrate
into the pointed extensions of the shells (see Figs. 77, 78).
These two extreme types, however, first appear near together
at the veiy base of the Upper Silurian.

The Number of Volutions of the Spiral. — 3. Another diverg-
ence is in the number of volutions of the spiral. The earliest
known Helicopegmata are generally of small size, and the
volutions are not numerous: it is not improbable that the

THE MODIFICATION OF GENERIC CHARACTERS. 28/

primitive form of spiral was with few volutions; but if this be
the fact, the rapidity of their increase to the extreme, found
in Atrypa and in some of the Spirifers (Fig. 77), was early
reached in the basal fauna of the Upper Silurian, and it is
observed that the embryonic forms have fewer coils to the
spiral than the adults (Beecher). (Compare Protozyga (Hall)
and Cyclospira with Atrypa reticularis or Spirifer.)

FIG. 80. — Diagram representing the various positions of the spiral coils in the brachiaium of the
Helicopegmata. The diagrams are drawn as transections viewed from the beak of the shell,
the brachial valve being the upper and the pedicle the lower lines of each figure. A , the posi-
tion with apices of the cones directed outward, as in Spirifer ; B, apices directed toward the
pedicle valve ; C, apices directed toward the centre of the pedicle valve ; Z>, apices nearly
meeting on the median plane ; £, apices directed obliquely inward toward centre of brachial
valves ; /% apices directed toward the pedicle valve with subparallel axes.

Direction of the Axes of the Spiral Cones. — 4. Among the
earlier representatives we have also every position of the
spirals, so that the direction of the pointing of the axes and
the apices of the cones reaches its full elaboration very early.
In Zygospira the apices are directed obliquely toward the
centre of the brachial valve (Fig. 80, £)• in Atrypa and
Atrypina toward the deepest part of the brachial valve (F),
while in Spirifer and several other genera they are directed
toward the outer margin of the two valves (A); in Coelospira

288 GEOLOGICAL BIOLOGY.

and Dayia outward toward the lateral slopes of the pedicle
valve (the position is intermediate between A and £)• in
Catazyga toward the median plane just below the surface of
the brachial valve (£7); in Glassia toward the centre, and the
apices nearly meet at the centre of the internal cavity (Z7) ; in
Cyclospira they are coiled nearly parallel to the vertical axial
plane, and the apices are slightly introverted.

Although in lines of species (which in their combination
of characters show them to have close affinity and hence are
grouped in generic groups) the direction of the axis of the
spiral cone is pretty constant, we see that whatever impor-
tance may be attached to the different position of the spirals
in relation to the other parts of the body, the differentiation
of these features was quickly attained.

FIG. 81 — Diagrams of the various forms of the jugum in the Helicopegmata. a = Atrypina ;
t> = Spirifer ; c - Hindella ; d - Hyattella ; e = Retzia ; / = Whitfieldia ; g = Meristina ;
h — Athyris ; * = Kayseria ; j = Meristella.

The Form of the Loop. — 5. The character presenting the
greatest degree of divergence in the structure of the brachid-
ium is the form of the loop or jugum. In the paper above
referred to, Mr. Schuchert has suggested that the nature and
complexity of the loop which joins the spirals are the more
important characters for subfamily differentiation.

In Spirifer proper (Fig. 81, b) the loop is a simple band,
about the size of the primary lamellae, joining the two lamellae
together; in some cases in adults this was partly absorbed,
leaving only two calcareous processes facing each other on the

THE MODIFICATION OF GENERIC CHARACTERS. 289

sides of the opposite lamellae. In Zygospira the loop is sim-
ple, but arched or forming a double bow-like curve (Figs. 73—
76). In Dayia there is a simple process from the centre of
the saddle running toward the base of the crura (see Fig.
8 1 , c). There is added a bifurcated end in Whitfieldia (8 1 , /).
In Athyris (81, Ji) the ends of the branches are curved over to
partly cover the primary lamellae of the spirals. In Kayseria
they are continued along parallel to the lamellae of the spiral
coil (8 1, t). This extension is only seen in the late Mesozoic
forms. In Meristella (/) the branches of the process recurve
and join together, forming on each side a loop, resembling the
handles of a pair of scissors.

In this series of modifications the extreme degree of elab-
oration is met with among the Meristellinae, and this subfam-
ily was well represented among the Eosilurian faunas.

Characters of the Brachidium found to be good Distinctive Char-
acters of Genera, — It has been acknowledged by all the more
advanced students of Brachiopods, that the modifications of
the brachidium are the most important characters to be found
for determining the generic and higher affinities of these in-
teresting forms, and great and most painstaking labor has
been expended within the past ten or fifteen years in working
out the structure of their delicate parts.

We may interpret this experience of systematists to mean
that the various degrees of modification observed in these
parts are found to be constant among species which by like-
ness in other characters are associated together into groups to
form genera.

Plasticity a Characteristic of their Early Initial Stage. — We
have already seen by analysis of the characters that almost
without exception the plasticity of the characters, and the ex-
pression of the widest range of possible differentiation in each
particular direction, were characteristics of the early stage in
the history of the Helicopegmata. By the beginning of the
Neosilurian the expansion of differentiation had reached its
extreme in almost every particular.

Evolution of the Characters of the Brachidium Relatively Rapid.
— When we consider that we have knowledge of only a few
small types of this whole order earlier than the Eosilurian, and

2QO

GEOLOGICAL BIOLOGY.

that the Helicopegmata lived on to the middle of the Mesozoic,
and, third, that most species have a life-period of a third or
half of the duration of the whole Silurian time, it is no exag-
geration to say that the evolution of these modifications of
the brachidium was, relatively to all laws of organic change in
geology, extremely rapid.

Rate of Initiation of the Genera of Helicopegmata. — If now we
reduce the facts of generic differentiation to graphic form, we
find that the sudden or rapid differentiation is a fact, and is
not due to imperfect evidence. Considering, as in previous
cases, classification to be a mode of expressing degrees of dif-
ference, we may rely upon the mathematical relations of initi-
ation of the groups of equal rank as an expression of the rate
of initiation of new characters in general, or an approximate
measure of the rate of geological evolution.

TABLE EXPRESSING THE RATE OF EXPANSION OF THE FAMILY, SUBFAM-
ILY, AND GENERIC CHARACTERS OF THE HELICOPEGMATA.

HELICOPHGMATA.

Families,

Subfamilies.

Genera

c

o

s

D

Cb.

T

J

K

Ty.

Q.R

»—

—

2

20

16

10

16

2

The Helicopegmata as a suborder is found to be repre-
sented in three family types of structure : one of these ap-
peared first in the Ordovician, in a single subfamily, a single
or possibly two genera, and but few species. At the opening
of the next era, the Upper Silurian, the other two families
appear, and seven out of the known twelve subfamilies were
initiated.

THE MODIFICATION OF GENERIC CHARACTERS. 2QI

If we consider the actual total number of generic types
for the whole suborder, and some of the later of these genera
are based upon very slight modification of characters, we find
76 in all. The rate of their initiation is: Ord. 2, Sil. 20,
Dev. 16, Carb. 10, Trias. 16, Jur. 2; or by the time of the
second stage in which any of the suborders are known one
quarter of the total generic differentiation had taken place,
and differentiation did not cease till six eras had been passed
and the suborders became extinct.

Representing these facts in whatever way we may, they
are positive in testifying to a rapid and early expression of
the differences in structure which have served as the means of
distinguishing different families, subfamilies, and genera ; and
a close inspection of the figures seems to indicate that in
proportion to the higher taxonomic rank of the characters,
the earlier or more rapid was their initiation.

General Law of Rate of Initiation of Generic Characters In

general terms, the scientific fact here noted, irrespective of
any theoretical explanation, is that, relative to the known
geological range of species of the Helicopegmata, the grander
differences in structure were very early to appear, and that
the progress of differentiation after this early stage was largely
in respect of varietal and specific characters and proportion-
ally small in characters of higher rank.

The Life-period of Genera and the Initiation of a New Genus.
— We have now examined some of the laws of genera as ex-
hibited in the case of the Helicopegmata. The characters
which are found to be of generic value, such as the particular
structure of the calcareous framework supporting the brachial
arms, have a definite history. Examining all the known
Brachiopods, from the beginning of geologic time to the pres-
ent, it is found that the structural characters peculiar to this
suborder are confined to the time extending from the Lower
Silurian to the Triassic or Jurassic era. As a particular ex-
ample, for instance, the arrangement of the brachidium char-
acteristic of the genus Meristella (see Fig. 81, /, with the
complex loop forming two lateral rings, and the cone of the
spirals pointing to the lateral margin of the shell, as in Fig.
80, a) begins in the Silurian, and is never seen after the Devo-

GEOLOGICAL BIOLOGY.

nian. The genus is said to be characteristic of that period ;
and not only in America, but in Europe, in China, in South
America, wherever Paleozoic rocks are known, Meristella is
found characteristically in the Upper Silurian, running rarely
a little below, but more frequently above, into the De-
vonian.

There comes a time in the history of organisms of a par-
ticular line of descent, when a certain definite arrangement of
the parts of the organism becomes conspicuous, as this partic-
ular loop of the Meristella; the occurrence of individuals
developing this peculiarity is limited below and above. This
arrangement differs from that of the corresponding part in
any other animals of the same time ; and all the animals ex-
hibiting this character may be considered as closely allied
genetically, because in other characters they also show strong
resemblance. This state of things is evidence of the beginning
or initiation of a new genus.

If all the specimens known possessing this new character
were examined and classified, they would be found to have
minor differences of form, surface marking, etc., which furnish
criteria for dividing them into several distinct species. Geo-
logically, one of these species is the first to appear; it lives
but a short time relatively, or it may continue to live during
several periods. It is peculiar to one country, or it is the
same throughout the world wherever the genus appears ; but
whether there be many or few species, the character which is
called a generic character begins at some particular time :
during a certain period it is frequently met with ; after a time
it ceases, and is never known to appear again. The particu-
lar combination of characters on some one organism consti-
tutes its generic characters, and we may say that the genus so
characterized has a certain definite life-period.

During the Life-period of the Genus its Characters Constant. —
While other characters may vary, these generic characters do
not change sufficiently to be noticed as of importance to the
paleontologist. Not only the generic, but the family and
the ordinal, characters, which are associated together under
the generic name Meristella, are thus constant for all the
specimens examined.

THE MODIFICATION OF GENERIC CHARACTERS. 293

A Culminating Point or Acme in the Life-period of a Genus. —
Again, we observe that the fossil specimens which present the
characters (of Meristella for instance) are most abundant
along the middle of this period; for the Meristellas it is about
the Neosilurian ; also in that period they are more frequently
met with in distant parts of the world ; and where they are
most abundant the characters which serve to distinguish them
into separate species are more numerous ; and both before that
epoch and afterward there are fewer and fewer, until we reach
both ends, where the species are very rare.

Summary of the Geological Characteristics of a Genus. — To
generalize the above observations, it may be said that the
genus practically has a time of beginning and a time of ending.
Practically, that is, according to the knowledge we possess,
there was a geological time, represented by a particular horizon
in the geological series of strata, when each genus began;
there was a particular period, of shorter or longer extent,
during which the genus was freely propagated, and abundant
individuals flourished, leaving their remains in the strata,
wherever the conditions were appropriate for their preserva-
tion. The genus had a period of decadence, or of growing
old, the species became fewer and fewer, the individuals more
rare, and finally the genus died out, and, so far as our knowl-
edge goes, became extinct. These laws apply to Meristella,
and in substance they apply to all genera we know of. The
period from the initiation to the extinction of the genus is the
life-period of that genus.

CHAPTER XVII.

THE PLASTICITY AND THE PERMANENCY OF CHARAC-
TERS IN THE HISTORY OF ORGANISMS.

Races in Paleontology — During the life-period of a genus
constant changes are found to take place among the represent-
atives of the genus as we follow them upward from stage to
stage of their geological succession. The forms appearing at
the first epoch, in the life-period of a genus, are generally
found to be of different species from those occurring later;
and in many genera there are enough specimens collected, and
sufficient knowledge regarding them accumulated, to enable
the paleontologist to recognize a series of forms regularly
succeeding one the other, presenting slight modification from
one stage to the next, but those of each stage showing closer
resemblance to those immediately preceding them than to any
other species of the same genus. The series of forms thus
resembling each other may be called races, because of the
very evident genetic relationship existing between the later
and the earlier representatives of the series.

Phylogeny of the Race. — When we examine the details of
form in such a series of succeeding forms or races of a genus,
comparatively, it is often apparent that the changes under-
gone in respect to each character are progressive or of an
accumulative nature, and thus they resemble the changes
which the individual undergoes in ordinary growth. The
technical name proposed by Haeckel for this morphological
history of the race is Phytogeny > contrasting it with Ontogeny
or the history of growth or development of the individual,
from its relatively homogeneous condition in the ovum to the
more or less differentiated adult organism.

Mutability and Phylogeny. — The Cuvierian school of natu-
ralists believed in the immutability of species, and for them

294

PLASTICITY AND PERMANENCY OF CHARACTERS. 2$$,

the principle of racial evolution or phylogeny was barred out.
But Geoffrey St. Hilaire and Lamarck with their idea of mu-
tability of species laid the way for a consistent theory of
phylogenetic evolution, although in their time the knowl-
edge of paleontology was not far enough advanced to furnish
actual phylogenetic series of organisms. It was, however,
not till Darwin had constructed a working hypothesis for
the steps and manner by which new types of organisms can
arise, that evolution became an accepted mode of explana-
tion of the course of biological history.

The great advance which the present generation has wit-
nessed in the interpretation of the science of organisms is the
change in belief, which all naturalists have more or less thor-
oughly undergone, from the doctrine of immutability to that
of mutability of species. Some theory of evolution and
phylogenetic origin of species is the necessary outcome of
this new doctrine. Darwin more than any other single man
was the means of producing the change of conviction in re-
gard to this point.

The Phylogenetic Theory of Evolution. — The phylogenetic
theory of evolution is logically an expansion and application
of the principle of organic growth, already recognized in the
development of individual characters, to the evolution of spe-
cific and more fundamental differences. It is a recognition
of an organic correlation between separate individuals. As
growth takes place in the individual by the segmentation
and separation of cells, with specialization of functions, first
for different cells and finally for the complex structures
called organs, the whole showing its organic unity by the
mutual cooperation of all of the parts in the life of the
whole, so the phylogenetic theory recognizes in the species,
or the race of species, an organic unity of a higher sphere, in
which the individuals play the part of mutually adjusted and
cooperating parts in this greater organic whole.

The theory goes one step further, and includes the propo-
sition, that as the principle omne vivum ex ovc is true in the
life-history of individuals, so each species postulates a pre-
existing species. This is the philosophy of the theory, but
it must be observed that the concrete facts illustrating these

296 GEOLOGICAL BIOLOGY.

laws are always found together in the same organism. Each
individual organism is the source and record of those facts
which we separately interpret as evidence of cell-growth, in-
dividual growth, the differentiation of organs, and the phylo-
genetic evolution.

Thus there are series of organic forms succeeding each
other in some regular order, known or unknown, which are
bound together by organic, and in this case called particularly
genetic relationship. The changes in form observed upon
comparing the individuals at different points in the line of
succession are accounted for by some law of evolution, and
the origin of the different members of the series is said to be
by generational descent, the later arising from the earlier.
On account of the mutability of form in the process, species
presenting different form, different function, and incapable of
organic fertility are supposed to have arisen originally from
a common parentage.

Mutability the Fundamental Law of Organisms ; the Acquire-
ment of Permanency Secondary. — This analysis brings us face
to face with one of the chief inconsistencies in the prevalent
conception of the nature of organisms. While the doctrine
of mutability of species has generally taken the place of im-
mutability, the proposition that like produces like in organic
generation is still generally, and I suppose almost universally,
accepted. It therefore becomes necessary to suppose that
variation is exceptional, and that some reason for the accumu-
lation of variation is necessary to account for the great diver-
gencies seen in different species. Darwin's theory of natural
selection is chiefly concerned in accounting for the accumula-
tion, increase, and perpetuation of divergencies arising by
natural variation.

If we extend the principle of mutability, and instead of
regarding it as an accidental circumstance in the life-history
of organisms, recognize it as the distinctive and fundamental
characteristic of living beings, we escape this inconsistency.

In the physical and chemical world like causes do pro-
duce like effects; but in the organic world like produces like
" with an increment," as Professor J. D. Dana put it. Muta-
bility and variation are evidences of this increment. The

PLASTICITY AND PERMANENCY OF CHARACTERS. 297

increment is the great fact ; the checking and limiting of it is
secondary. The search has been for some cause of the varia-
tion; it is more probable that mutability is the normal law of
organic action, and that permanency is the acquired law.

It is more probable that the use and tested adaptability
of a variable part is the cause of checking the variability and
of the transmission of the character with less or no variation,
than that the variation is increased by this process. Adopt-
ing mutability as a fundamental law of all organic activity,
and the construction of a theory of evolution becomes a simple
matter. We have in that case to account for the acquirement
of permanency of characters. This is found in the principle
of ordinary generation, the instituting of habit, and the more
and longer the species breed together the closer and more
fixed will the characters become.

Early Plasticity Succeeded by Permanency expressed in Geo-
logical History. — Examination of the history of geological spe-
cies suggests the truth of this hypothesis, for it is observed
that many species, which by their abundance and good preser-
vation in fossil state give us sufficient evidence in the case,
exhibit greater plasticity in their characters at the early stage
than in later stages of their history. A minute tracing of
lines of succession of species shows greater plasticity at the
beginning of the series than later, and this is expressed in the
systematic description and tabulation of the facts by an in-
crease in the number of the species.

In order to illustrate this law a special consideration will
now be given to the facts regarding the laws of specific his-
tory as observed by the paleontologist.

Pritchard's Definition in which the Constancy of Transmission
of Same Peculiarity is made the Criterion of Species. — Thus far
we have been considering generic characters — that is, those
characters which are constant for one or more species. The
next question to consider is. What are the laws exhibited
in the history of specific characters? There are various defi-
nitions of species which are more or less theoretical ; but
whatever our theory about the definition, the fact remains
that all naturalists do recognize within slight limits of difference
the reality of groups of organisms called by the name species.

GEOLOGICAL BIOLOGY.

In a previous page are given some of the definitions of
species formulated by early naturalists. Alfred R. Wallace,
who published as early as 1855 an article on the law which has
regulated the introduction of new species (Darwin's " Origin
of Species " was published in 1859), set forth some of the chief
principles of the modern evolutionary conception of the his-
tory of organisms. Wallace made a careful study of species,
and, perhaps as well if not better than any one else, under-
stands the relationship between species and geographical dis-
tribution. In an article of his " On the Malayan Papilionidae,
•or Swallow-tailed Butterflies, as Illustrative of the Theory of
Natural Selection," published in 1864, is found the following
definition of the word species:*

" In estimating these numbers [of the species of Papilionidae] I have had
the usual difficulty to encounter, of determining what to consider species
and what varieties. The Malayan region, consisting of a large number of
islands of generally great antiquity, possesses, compared to its actual area,
a great number of distinct forms, often indeed distinguished by very slight
characters, but in most cases so constant in large series of specimens,
and so easily separable from each other, that I know not on what principle
we can refuse to give them the name and rank of species. One of the best
and most orthodox definitions is that of Pritchard, the great ethnologist,
who says that ' separate origin and distinctness of race, evinced by a constant
transmission of some characteristic peculiarity of organization,' constitutes a
species. Now leaving out the question of ' origin," which we cannot deter-
mine, and taking only the proof of separate origin, ' the constant transmis-
sion of some characteristic peculiarity of organization' we have a definition
which will compel us to neglect altogether the amount of difference be-
tween any two forms, and to consider only whether the differences that
present themselves are permanent. The rule, therefore, I have endeav-.
ored to adopt is, that when the difference between two forms inhabiting
separate areas seems quite constant, when it can be defined in words, and
when it is not confined to a single peculiarity only, I have considered such
forms to be species. When, however, the individuals of each locality vary
among themselves, so as to cause the distinctions between the two forms
to become inconsiderable and indefinite, or where the differences, though
constant, are confined to one particular only, such as size, tint, or a single
point of difference in marking or in outline, I class one of the forms as a
variety of the other. I find as a general rule that the constancy of species
is in inverse ratio to their range. . . . When a species exists over a wide
area, it must have had, and probably still possesses, great powers of dis-
persion. . . . When, however, a species has a limited range, it indicates less
active powers of dispersion, and the process of modification under changed

* " Contributions to the Theory of Natural Selection. A Series of Essays."
p. 141. Macmillan & Co., 1870.

PLASTICITY AND PERMANENCY OF CHARACTERS, 299

conditions is less interfered with. The species will therefore exist under
one or more permanent forms, according as portions of it have been iso-
lated at a more or less remote period."

Permanency of Characters in Living Forms Co-ordinate with
Limitation in Distribution and Breeding. — From these quota-
tions it will be seen that in the conception of an organic spe-
cies the fundamental idea here emphasized is the reproduction
of numerous individuals possessing likeness in all their mor-
phological characters, except in those in which the offspring
of a single brood may differ when compared together. This
specific permanency involves absence of intermixing of the
separate species, if in the same locality, or local separation of
the species. In other words, co-ordinate with the likeness of
form there is assumed to be limitation in breeding and limita-
tion of local environment. This is the extent of the limita-
tion which the study of living forms reveals.

Specific Variability Restricted with each Successive Generation
in Fossil Forms. — When we examine geological species we
find also a limitation in time of the repetition of like individ-
uals. When species are studied historically, the law appears
evident that the characters of specific value — those which
.serve to distinguish one species from another, according to
the rules above formulated and generally practised — present
a greater degree of range of variability at an early stage in
the life-period of the genus than in the later stages of that
period. To express this law in terms of the history of organ-
isms, we say there are periods in the history of particular
lines or races of organisms, of unusual variability or plasticity
of some of the characters, and afterwards the history shows
relatively long periods in which the characters expressing
such plasticity are constant or present very slight divergence.
Further, in this second period of slow modification, or persist-
ence of form, the changes taking place in the phylogeny are
slight, but they increase in a particular direction steadily and
slowly with time.

Illustrations of the Acquirement of Permanency of Charac-
ters.— In order to illustrate these laws the following actual
cases will be described in detail : the Spirifers at the base of
the Silurian, as an illustration of extrinsic evolution; Atrypa

300 GEOLOGICAL BIOLOGY.

reticularis and its allies, as an example of the permanency of
the plastic condition ; the bivalve shell Lamellibranch (Pty-
chopteria) of the Upper Devonian, illustrating the initiation
of the species of a genus; and Mammals, in illustration of
progressive evolution.

The History of the Spirifers. — When we attempt to dis-
cover the laws of phylogenetic succession we are obliged to
consider specific and varietal characters.

As has been already shown, the length of the geologic
time through which the characters of generic and higher rank
are exhibited is, by the Brachiopods at least, measured by
geologic periods : and there are series of Brachiopods extend-
ing through one or more geologic systems in which the ge-
neric characters expressed are alike, the various representatives
from beginning to end exhibiting differences only in the
lesser or specific characters.

For the study of the history of such specific characters
the Spirifers may be taken as examples. The whole family
Spiriferidae begins, according to present knowledge, near the
base of the Upper Silurian, and there are two known repre-
sentatives in the Triassic. The genus Spirifer begins at the
base of the Upper Silurian, and is well represented through
the Silurian, Devonian, and Carboniferous. There are named
2 1 8 species in America. Hall in his * ' Genera of Brachiopods "
recognizes over two hundred species. The species referred
to this genus in the Mesozoic are probably of distinct generic
rank ; a large number besides are defined in other countries.
Among the numerous species assigned to this genus there are
great variations in a few particulars. In the whole genus
there may be three hundred, or possibly four hundred, good
species, or forms, presenting two or more describable char-
acters, of which each is different from any other species.
When we examine the whole genus, and note the characters
which distinguish one species from others, and arrange the
characters into classified groups, as they concern separate ele-
ments of the shell, they may be classified as modifications of
a few elements of the form or structure of the shell.

The Permanent Characters of Generic or Higher Rank. — Exam-
ining the successive forms of Spirifers, we observe that there

PLASTICITY AND PERMANENCY OF CHARACTERS. $01

are long lines of individuals, each running through one or
more geological periods, and repeating without noticeable
change the precise morphologic characters of its ancestors
down to the generic characters, and exhibiting differences only
in the specific or less important elements. The specimens
exhibiting this law we associate together as a genus and call
them by the same generic name, expressive of the fact that
they agree in all their morphologic elements, except such as
distinguish different species of the genus. The characters of
specific value vary during the life-history of the genus, but
the generic characters remain unchanged ; or, to apply a spe-
cial designation to these two facts, the generic characters are
fixed, and the specific characters are more or less plastic.

Characters which are Plastic at the First or Initial Stage of the
Genus. — At the initial stage of the genus Spirifer the generic
characters may be supposed to have become fixed. The
still plastic characters are chiefly seen in a few definite mor-
phologic elements. These are : (I) the contour of the shell, or
in terms of growth, the relative rate of growth from the nu-
cleus outward ; this is seen in specimens with short hinge
lines, in others with produced angles at the extremity of the
hinge and in the intermediate forms; (II) the vertical extent
of hinge area, ranging from low to high area; (III) the del-
thyrium, open to closed ; (IV) the surface, evenly arched
over, or producing a single median fold, or several folds, ex-
tending to the beak, or with intermediate development ;
(V) the surface striations, radiating and concentric, fine or
coarse, continuous or interrupted, uniform or bifurcating as
they develop toward the front.

The Fixation of Plastic Characters in a Generic Series. — It is
the various degrees of modification of these characters that con-
stitute the specific differences upon which the several species
are defined. They are all plastic or variable at the first
stage, and the individual species of the genus present certain
limits of variation of each of the characters.

If we go a step farther and classify all the variable char-
acters of the genus, we may discover in numerical terms the
relation between retention of plasticity and the passage of
time, or the effect of time in limiting the variability of the

302 GEOLOGICAL BIOLOGY.

more or less plastic elements of the genus. The characters
of Spirifers that are of chief generic and specific value are the
following :

A. The form and arrangement of the spiral appendages.

B. The general proportions of the shell.

C. The delthyrium, deltidium, and fissure ; their shape and
development.

D. Hinge area, its length and height.

E. Surface markings; radiating striae, fine and continuous
or coarse and interrupted ; including imbrication.

F. Medial fold and sinus.

G. Plications of surface — simple fold, or many and bifur-
cated folds.

H. Structure of shell — fibrous or punctate.

I. Spines, or setae, or elevations, granular or otherwise.

K. Special development of septa, medial or deltidial.

Whatever evolution has taken place should be expressed
in terms of some one or more of these characters, for these
constitute the differences distinguishing the several known
species.

A. Spiral Appendages * — So far as we know, these varia-
tions during the life-history of the subfamily or genus do not
exceed slight adjustment of position and direction of the coils
to the internal capacity of the shell, and variation in the num-
ber of the coils. Of both of these characters too few statistics
are at hand to enable us to base upon them any law regarding
the rate, or even direction, of evolution ; but the modifications
appear to be all easily explainable by the principle of ex-
trinsic evolution, i.e., adaptation to external conditions in
the process of ontogenesis.

B. The General Proportions of the Shells. — Taking an
average of the extremes of form for the whole of the hinged
Brachiopoda and constructing a medium form, the result
would be an oval shell, with hinge line shorter than the great-
est width, and the pedicle valve larger than the brachial, with
low hinge area ; a deltidium ; no fold or sinus, further than a

* See, regarding this and other details, Paleontology of New York, vol. vm.,
" An Introduction to the Study of Genera of Paleozoic Brachiopoda," pt. II., by
James Hall, assisted by John M. Clarke, 1894.

PLASTICITY AND PERMANENCY OF CHARACTERS. 303

tendency to lengthen the central part of the ventral and
shorten the central part of brachial valve. Both valves would
be convex, but slightly so. Atrypa reticularis is not far
from such a medium form of an articulate Brachiopod. The
Spirifers vary in the following directions in respect to these
characters: The pedicle valve may be greatly developed
about the beak, forming considerable contrast between the
two valves. This variation is noted in species of the earliest
stage, in individuals of most species when contrasted, and in
different stages of growth of the same individual. The varia-
tion is most noticeable among species which are abundam.

C

FIG. 82. — Variations in form of Sflirifer Verneuili. (After Gosselet.) A, outline of the form
Cylindrici, from the upper Frasnien ; Z?, form Hemicycli, from the Frasnien ; Ct form Obo-
vati, from the Famennien ; D, cardinal view of form Elongati^ Famennien ; E, cardinal of
extreme of hemicycli form, from the Frasnien ; f = fold of the brachial valve ; b — apex of
the beak of pedicle valve ; d — delthyrium ; a = cardinal area ; e — lateral extremity of the
cardinal area.

and of wide range, and rare or local species are generally less
variable in this particular (Fig. 82).* In size there is consider-
able variation, but most of the species of the Silurian are
small for the genus, though in this respect perhaps the

* Fig. 82 illustrates some of the conspicuous differences in form assumed
by the Spirifers. The variations are further interesting as occurring all
on the same species, and appearing on specimens selected from the same
geological province, from strata differing a little in age, but all from the upper
half of the Devonian of northern Europe. Similar specimens have been seen
in the corresponding rocks of New York State (see Am. Jour. Sd.t vol. XLIX,
P- 473)-

304 GEOLOGICAL BIOLOGY.

largest species of the Niagara or Silurian is not more than an
average for the whole range of size. If the size is expressed
on a scale of 10, I representing the smallest and 10 the
largest, the range in the Silurian is about I to 4. There is
a general tendency to increase in the size of species of the
genus from their beginning to the Carboniferous. The Silu-
rian species average about 3 on such a scale, the Devonian
species 5, the Carboniferous 7, and the size of the Mesozoic
species would average about 3.

The species which contain the larger individuals for their
period are generally more abundant in numbers. There is
evident adaptation of size and abundance to conditions of en-
vironment, for certain deposits contain abundant and large
representatives of a particular species, while other deposits
contain but few and smalt individuals. The character B, then,
is evidently in its evolution purely extrinsic, the species adapts
itself to environment, and in each race the adaptation is
greater with advance of time up to the Carboniferous, where
the whole race deteriorates, and in most species becomes ex-
tinct, only a few surviving, and those having some specially
developed characters.

C. The Delthyrium and Deltidium. — The delthyrium is the
opening through which the peduncle passes for the attach-
ment of the shell, and its covering is the deltidium. In its
early stage the young shell was always attached, and the del-
therium was open. In some species there was very plainly a
gradual closing of the fissure by a pseudodeltidimn, a covering
of shell growing over the fissure from beak downward. In
others, there is this pseudodeltidium with a slight foramen
permanently running through it (see Fig. 82, A, B,D, and
E,d\

In others there is a permanent open fissure; at least, no
calcified covering is present in the adult. The presumption
is that there was variation in the length of time the individual
was attached, some species becoming free at very early periods,
others remaining attached throughout life. If we express this
character mathematically, I referring to attainment of free
state very early, 10 permanently attached, we find among the
species of the lowest period of the Silurian (Niagara and cor-

PLASTICITY AND PERMANENCY OF CHARACTERS. 30$

responding formations), in each of the chief types, great fluc-
tuation in this character; i-io, perhaps, is not too much. In
later periods there is variability, but each species is subject
to less variation, so that mathematically the species might
be, said to have this character variable in separate cases, 1-3,
2-5, 3-7, 5-9, etc. ; and there are certain lines of forms in
which the general range of this variability continues the same
from period to period.

As to the size of the fissure in proportion to the other
parts of the shell, there is considerable variation, but it is
probably co-ordinate with the development of the area, those
with -high area having narrow fissure, those with low area a
broad fissure. The characters, therefore, of the delthyrium
and its cover show, in respect of evolution, purely extrinsic
modification, the characters reaching extreme range at first,
and afterwards, in the various races, expressing modification
by restriction of variation and adaptation to special or local
conditions.

D. Hinge Area. — This may be very narrow and elongated,
forming a long hinge-line, or it may be very high, forming a
triangular and greatly developed area and ventral beak. I
know of no species, or sets of forms, which express a greater
range of modification of this feature than the two species
called Spir if er plicate lla and Cyrtia exporrecta of the Wenlock
limestone. The specimens with high beak are generally
called Cyrtia; the specimens with moderate or low beaks are
Spirifer. This character ranges from i-io in the earliest
stage. In other species (S. crispus and its associates) there is
a less degree of modification of this character (Figs. 88-91).
In later forms the range of modification for each species is
generally confined within less limits. The extreme extent of
the modification and the extreme forms themselves are gen-
erally met with where the species are most abundant, and the
prevalence of one extreme or the other is expressed in the
later end of a series, which from the close resemblance of the
successive specimens constituting it may be considered to be
a true genetic series or race. Here again we find evidence
that whatever evolution takes place is extrinsic and results,
theoretically, from adjustment to environment, selection in

306

GEOLOGICAL BIOLOGY.

FIG. 83.

FIG. 84.

FIG. 85.

FIG. 89.

FIG. 90.

FIG. 91.

FIG. 92.

FIG. 94.

FIG. 95.

FIGS. 83-05. — Modifications of the surface features of the Spirifers, expressed in the species of the
Niagara period. (After Hall.) 83. Spirifer radiatus Sow. 84, 85. S. plicatella Linn.
86. S. eudora Hall. 87. S. Niagarensis Hall. 88. S crispus His. 89. .S. sulcntus His.
90. S. bicostatus Hall. 91. .S'. tenuistriatus H. 92. Enlargement of the surface of S. cris-
pus. 93. Surface, enlarged, of S. sulcatus. 94. Surface, greatly magnified, of S. Niagaren-
sis. QS. Enlareement of surface of S. eudora.

PLASTICITY AND PERMANENCY OF CHARACTERS. 3O/

breeding and limitation of range of variability by hereditary
transmission.

E. Surface Markings. — The surface of Spirifers, when
well preserved, are almost always covered with fine longitudi-
nal or radiating lines, or these interrupted by concentric lines
or irregular papillary elevations (see Figs. 83—95). Judging
from the structure of living Brachiopods, these are associated
with certain setose prolongations of the edge of the mantle,
or bristles, and their appearance in the structure of the shell
surface may be due to a growing around the individual bristles
of the extreme edge of the shell, so that the striae are of im-
portance. In some (Spirifer fimbriatus, lineatus, etc.) the
size is large enough to show the openings, which are quite
complicated and resemble the opening of a double-barrelled
gun. The modification of this feature is by increase or de-
crease in size of the striae, by interruption regularly or irregu-
larly. When interrupted regularly, it appears to be by a
periodical stoppage of growth, and thickening of the shell
lamellae, forming on the surface imbricated structure, the striae
starting anew at each successive imbrication. The fact that
they are surface striae is also so accounted for, the deposit of
shell filling up all the under side of the striae. In Sp. plica-
tella (Figs. 84, 85) the whole surface is uniformly covered
with these continuous radiating striae. In 5. crispus (Figs.
88, 92) the surface is interrupted by imbrications, and is cov-
ered by regular rows of the interrupted lines.

A comparison of series of successive species, which by
their general combination of characters may be supposed to
have been in direct line of genetic succession, shows a gradual
diminishing in size of the striae, and in case of the continua-
tion and increase of imbrications there results an entire
absence of the striae — at least they fail to be discernible on
specimens.

The particular size and form of these striae seem to be a
very delicate means of tracing the lines of hereditary succes-
sion, or what we may suppose to be such lines; for species
which in other respects are very much alike can be easily dis-
tinguished by this character if the surface be well preserved.
The modification of this character appears to be in two

308 GEOLOGICAL BIOLOGY.

or three directions. In the series in which the striae con-
tinue unbroken by imbrication there is an increase in their
strength until, in Carboniferous times,
the species of this series develop a
spinous extension of the surface with
minute tubes, extending outward from
the shell : these tubes are seen in a
few of the Devonian forms also. In
the race with imbricated surface, where
the imbrication is persistent and regu-
lar, the striate structure becomes entire-
ly obliterated in the course of time

FIG. 06. — An enlargement of the , ~ r ^ . .

surface of Spiriferpseudolinea- (SCC figures OI O. CriSpUS, QO, Q2). In
tusHall. At e the test has been X . , . . . .

partially removed, exposing the others, where imbrication is irregular, in

tubular character of the spines .

below the surface of the shell; at the Devonian and the Carboniferous

a the spines are perfect ; at b,

broken away ; at * they are rep- eras there are species with roughened

resented as weathered, showing
the tubular character of the
spines ; and at d they are broken

the tubular character of the surface, irregular but granular (as 5.

ken

of the Hamilton), and this
indicates a development of part, with
kuk,iowa. (After Hail) ' obliteration of others, of these surface-
reaching ends of the striae. All the modification noticed in
this respect is also extrinsic, and can be accounted for by
processes of natural selection, slowly intensifying the character
with repeated generation.

F and G. Plication of Surf ace and Median Fold and Sinus.
— The next character to be noted is that of the plication of
the surface ; each species is pretty constant in the extent to
which this modification reached, but in the early forms of the
Niagara formation there is extreme range of variation, not
only in the whole set, but in the species which are, in other
respects, less variable.

Spirifer plicatella, variety radiatus (Fig. 83), is generally
lacking in plications ; but in Europe there are specimens (gen-
erally associated with the others) in which the plications are
seen on the margin of the adults (see Figs. 84, 85); a few
plications appearing on each side of the medial fold. In Amer-
ica the plicated form is called 5. Niagarensis (Fig. 87), and is
uniformly plicated to the beak. In the series vS. crispus and
5. sulcatus (Figs. 88-93) we find the same variability, speci-

PLASTICITY AND PERMANENCY OF CHARACTERS. 309

mens showing all grades of modification, from one or more to
what might be represented by number six, on a scale of ten ;
and in plicatella, the variation is one to four. In some
later forms the variation for each species is slight, rarely more
than one or two tenths, using this means of designating the
degree of plasticity. In the Spirifer Icevis, found abundantly
in the rocks at the foot of Fall Creek, Ithaca, N. Y., there
are generally no plications, but occasionally a specimen is
found with the margin for half an inch up corrugated by this
modification. In this species the character is probably the
remnant of a plasticity more strongly expressed in its an-
cestors.

The general development in number of plications is noted
on some lines of species, especially those showing bifurcation
of the plications during growth ; and, as in the case of the
median fold and sinus, this character is developed in the two
directions of increase and decrease, in different races.

In one series increase, by dichotomy of the surface plica-
tions, beginning in adult forms and becoming more and more
early in starting, affects first the centre of the shell, then the
neighboring parts of the side until the whole surface is
affected, but by slow degrees; so that, expressing the evolu-
tion in the same way as heretofore, the rate of development
is approximately as follows: 1-3 in Lower Devonian, 2-4 in
Upper Devonian, 3-7 in Lower Carboniferous, 6-10 in Upper
Carboniferous.

This modification appears to be dependent upon, or ex-
pressive of, the rate of increase of the shell in either the
radial or in the circumferential directions. If the circumfer-
ence of the shell increases more rapidly than the growth in a
radial direction the margin becomes too large for the shell at
its normal distance from the beak, and it is necessarily puck-
ered into folds to accommodate itself to its conditions; thus
as it grows its surface becomes plicated into folds. When the
growth in the radial direction keeps up with the increase in
the circumferential direction the shell remains smooth, and no
plications are developed. Thus the increase in the number of
plications for a given shape of shell is evidently due to the
acceleration or earlier starting of the differentially excessive

GEOLOGICAL BIOLOGY.

growth in the circumferential direction. A general rule is,,
that the coarser plications are more prevalent among Silurian,
forms, while the forms with fine plications are more prevalent
in the Carboniferous. Increase in the actual number of pli-
cations on a shell is, as a variation of the species, due to ex-
tension of the hinge-line and corresponding parts of the shell,
and not to irregularity in the general size or number of the
plications upon a given extent of surface.

H. Structure of Shell. — The shells of true Spirifers are
fibrous in structure ; the presence of punctation characterizes
such closely allied genera as Cyriina, Syr ingot hyr is, and Spiri-
ferina. Cyrtina is present with the genus Spirifer in the
Niagara, and continues about as long as that genus. They
seem to be parallel genera, differing in the constant presence
of this character in the genus Cyrtina; but this peculiarity of
structure, the punctation of the shell, whatever it indicates,
is more conspicuous among the later than among the early
types of the family, and continues longer to be dominant. In
its first or initial appearance, as a character, it seems to have
been evolved intrinsically, among the distinctive differentia-
tions of the family. The modification of structure, which dis-
tinguishes punctate from fibrous structure, appears associated
with other modifications and to involve considerable internal
adjustment. No evidence of the gradual appearance of the
character has been discovered. In Spiriferina or Cyrtina the
punctation is found wherever, among the earlier forms, the
shells are well preserved. The punctate genera are sharply
distinguished from the types with fibrous shell structure.

I. Surface Spines, Granulation, etc. — These are associated,
more or less, with characters marked E, and affect the super-
ficial layer of the shell (the periostrachum) ; their development
is successive and accumulative, and is associated with particu-
lar series, and it appears to be a feature increasing with time,
both as to size and strength of the characters. The characters
develop quite in the extrinsic way in all the races in which
they have been traced.

K. Special Development of the Median Septum. — This
modification in different species of the Spirifers is extrinsic in
its mode of evolution. One case has been traced with pre-

PLASTICITY AND PERMANENCY OF CHARACTERS. 311

cision in a series of specimens of Spirifer mesocostalis Hall,
which, in the Middle Devonian, shows in most specimens no
trace of a median septum in the ventral valve, occasionally a
variety appearing with a mere line representing the septum.
At the base of the Upper Devonian (Ithaca group) frequent
specimens with slight development of the septum are seen ; a
little higher, in the middle and upper part of the Upper
Devonian, the septum is conspicuous and is strongly de-
veloped. In other species the development of septal parts
appears to be varietal ; the older shells, in general, express-
ing fuller calcification of parts which are supports or partitions
between active organs of the animal. Among the Spiriferidae
there are several such lines of species, as the Cyrtina and the
Spiriferina; and in fact the forms which are punctate are all
more or less prone to develop calcified supports or partition
plates.

Evolution of Extrinsic Specific Characters Comparatively Slow,
although their Plasticity is Greater at the Initial Stage, — In all
of these characters, which constitute the specific differentiae
of the species concerned, we observe a relative slowness of
evolution which is quite consistent with the laws of natural
selection, of gradual acceleration or retardation by hereditary
means, and of the perpetuation of favorable characters by the
dropping out of others ; but at the same time we notice at
the early stage of the life-history of the genus, or subfamily,
a marked plasticity in respect of most of these characters-
which is in strong contrast with the fixity and persistence,
without change, of the characters of higher rank which mark
the family, and appear to have arisen at the same time.

Laws of Intrinsic and Extrinsic Evolution expressed in Varia-
bility and Permanency of Characters. — Among the first repre-
sentatives of the family there are family characters which
are repeated thereafter in numerous individuals for several
periods of geologic time without noticeable change, and they
did not appear before. There are also characters appearing
on the first species which vary and show slight change all the
way along thereafter, and are themselves less different from
the characters of previous forms : relatively, one set of char-
acters appears and thereafter a long line of successors follow

312 GEOLOGICAL BIOLOGY.

with the same characters not modified; the other set are
plastic at the first appearance, and only by degrees in the
course of geologic time do they become fixed and permanent.
It is this difference in the law of evolution of the characters,
as traced in historical series, that has led to the distinction of
the two modes of evolution, the one intrinsic, and the other
extrinsic, as defined on a previous page. Intrinsic evolution is
conceived of as normal expansion and differentiation of the
organism itself from within, and is the expression, in some
way, of an intrinsic tendency of the particular race of organ-
isms. The other, extrinsic evolution, expresses the limitation
and selection exerted upon the organism from without. Varia-
bility is thus the morphological expression of intrinsic evolu-
tion, and permanency or the transmission of characters with-
out modification is the morphological expression of the effect
of extrinsic forces.

Hall's Analysis of the Genus Spirifer and Classification of its
Species. — The history of the evolution of the genus Spirifer
may be seen from a somewhat different point of view by an
examination of the classification of American Spirifers by
James Hall, than whom we have no more critical observer of
specific differences in fossils.* Professor Hall recognizes
about two hundred species of Spirifers in the American
Palaeozoic rocks, none of which he considers worthy to be
regarded as even of subgeneric rank in relation to the typical
Spirifer stock. But there are certain groups of species natu-
rally associated together in successive lines which may be
regarded as genetically separate races, each line being char-
acterized by an association of common characters and differ-
ing from the others by the relative development or elabo-
ration of one or other of its characters.

Six such principal groups are recognized, called by Hall,
I. Radiati ; II. Lamellosi ; Fimbriati ; IV. Aperturati ; V.
Osteolati; VI. Glabrati.

Range of Species of Spirifer in American Formations. — In
the following table the lists are arranged in such a way as to
show for each particular race in each group the number of

* " An Introduction to the Study of the Genera of the Paleozoic Brachiop-
oda," — Paly. N. Y., vol. vm, part n, fascicle i, pp. 12-40, 1893.

PLASTICITY AND PERMANENCY OF CHARACTERS. 313

species recorded from each successive formation in the North
American rocks.

fPanciplicati

I. Radiati \ Multiplicati

(, Dupliciplicati. . . .

fSeptati.

II. Lamellosi -j Aseptati

L Submucronatus

[Unici- jCrispus.

III. Fimbriati J spinei| L«vis...

I Duplicispinei ....

Disjunctus

Hungerfordi.. ..

Striatus.

Texanus

Imbrex

Suborbicularis...

Orestes

Divaricatus..

IV. Aperturati

V. Ostiolati fOsteolati.

Acuminatus.. .

VI. Glabrati Aseptati.

Silurian

Devonian

Carboniferous

C

N

L

O

D

H

Q

P

C

Ch

B

K

W

8t

€

Cm

—

—

•••

Each Type of Spirifer shows a Continuous Series of Species.

— After making allowance for the gaps to be expected in our
limited knowledge of fossil faunas, it will be seen, by a glance
at the table, that each of these special types of Spirifer had
a more or less continuous series of species, persisting for two
or three or a dozen formations, represented, not by the same
form, but by mutations of the earlier form which differ suffi-
ciently from it to call for different specific names in the suc-
cessive formations. Some of the species are reported for
two contiguous or for several consecutive formations, but

GEOLOGICAL BIOLOGY.

in these lists the life-period of most of the species is for the
length of a single formation.

Each of the Chief Types Represented at the Initial Period of
the Genus. — It will be noticed also that each of the four prin-
cipal groups had representatives in the Niagara epoch, and
only a single species of Spirifer is reported from a Lower
horizon. The other two groups, Ostiolati and Glabrati, did
not appear till the Devonian, but these both appeared together
in the Upper Helderberg.

Three Epochs of Special Expansion with Slow and Gradual
Change During the Rest of the History of the Genus, — Of the 20

Silurian Devonian Carboniferou

VI. GLABRATI

FIG

neric groups, according to the

. 07. — Table representing the expansion of the Spirifers in subgeneri
classification noted by Hall, and elaborated on page 313.

races into which the known American species are subdivided,
7 are reported from the Niagara; I begins in the Lower Hel-
derberg, 3, Oriskany, 4, Upper Helderberg, I, Hamilton, and
4 at the base of the Carboniferous, i.e., over a third of the
known races began at the first fauna in which the genus ap-
pears in North America (except the one species in the Clin-
ton); 7 more began near the base of the Devonian, and 4
began at the opening of the Carboniferous. This special
rapidity of appearance of new types at the three periods
marking the beginnings of three geological systems points at

PLASTICITY AND PERMANENCY OF CHARACTERS. 315

the same time to the fact that these systems, which have been
recognized as well-established natural divisions in the geologi-
cal scale of formations throughout the northern hemisphere,
were also distinguished at their beginnings by marked change
in the life of the world. Not only do new types of genera
and families appear, but even the specific types of a continu-
ous race of species express the changes incident to the open-
ing of a new period.

Whatever be the explanation, these facts make it evident
that the divergence of a generic type into different subgeneric
expressions was not by slow and accumulative process, but
by relatively rapid expansions, followed by the continuance of
the types with gradual but restricted modification until the
race died out. The divergence of these types from each
other was very early in the history of each race, and in the
present case there was evidently a secondary divergence at
the beginning of the Devonian, and a slight tertiary diver-
gence, in the Aperturati group, at the beginning of the Car-
boniferous.

Characteristics of the Life-history of Atrypa reticularis. —
Atrypa reticularis may be taken as an example of a species
which exhibits scarcely
any trace of what has
been called extrinsic evolu-
tion, but has lived a long
time, been very fertile,
has been distributed all
around the world, and has
shown its adaptability to a
great variety of environ-
mental Conditions, Without FIG. gS.—A, B, Atrypa reticularis Linn. / = fora-

men ; cr — crura ; b = jugum ; sp = spiral coils of
Suffering any appreciable the brachidium. ^.adult, natural size- B} young

specimen, magnified one fifth. (From Steinmann
morphological Change. It and Doederlein.)

began with the initiation of the genus, and lived throughout the
life-period of the genus, which is almost equal to that of the
family or suborder to which it belongs. The species has re-
ceived a great many names, and been referred to many genera ;
but the more careful the study applied to it, the more clearly
does it appear that under all proper discrimination of specific

GEOLOGICAL BIOLOGY.

identity there is under consideration but one species, though
it is constantly variable. The species exhibits constant plas-
ticity of several of its characters, but never reaches that fixa-
tion into separate forms which has been interpreted as the
result of the survival of the fittest by natural selection.

Considerable and Continuous Plasticity of the Species. — -The
width and form of the shell, the number of the striae, and the
concentric laminae constitute some of the more conspicuous
differentiae of the various forms of the species ; but, as David-
son says, " All these modifications can be traced in specimens
from any locality."

In Murchison's " Siluria," (second edition,) is a remark
regarding the species, so pertinent that it is worthy of quota-
tion as it stands: "Among the Mollusca nearly all the
species of Atrypa, Orthis, and Spirifer differ from those of
the Silurian age" (speaking here of Devonian Brachiopods).
" One shell, however, the Atrypa reticularis, must be men-
tioned as an exception to the prevalent rule of each great
group being distinguished by peculiar forms ; for this hardy
species, with which the reader became so familiar in the
Silurian rocks, lived on to the Devonian era, and is as com-
mon in the limestones and shales of Devonshire as in the
older rocks. It even ranges to the farthest known geographi-
cal limits of the Devonian rocks of Armenia, the Caucasus,
and China on the East, and to the Devonian deposits of
America on the West."

Nature and Extent of the Variation. — The variations of this
species interested the acute naturalist Edward Forbes, and he
caused 117 specimens to be critically examined and the ribs
of each to be counted, and also the number of concentric
foliaceous expansions or fringes upon the surface. The
number of ribs, counting those on old and young specimens,
varied from ten to sixty, but there was found less divergence
in respect to the development and frequency of the concen-
tric fringes. Hisinger and Lindstrom, Davidson, Bronn, and
McCoy, among the earlier paleontologists, agreed in consid-
ering the forms with fewer and larger plications, called A.
aspera Schl. to be varieties of A. reticularis, but did not re-
gard them as distinct species. Lindstrom observed " that the

PLASTICITY AND PERMANENCY OF CHARACTERS.

Linnean form " varies like all those species which possess an ex-
tended horizontal and vertical distribution.'" Barrande recog-
nized two varieties of the species var. Verneuiliana and var.
Murchisoniana. McCoy in " British Silurian Fossils," says
11 It varies, firstly, in the convexity of the valves, both as to
degree, distance from the beak (at which it is greatest), and
equality — some small varieties, and the young at all times,
having the valves almost equally and evenly convex ; secondly
in form, some, and particularly the young and small varieties,
being nearly orbicular; others being elongate, and nearly
triangular from the width of the hinge-line and narrowness of
the front ; thirdly, in the number, thickness, and closeness of
the ridges and the scales which cross them, both of which are
often smaller and closer than in the typical variety ;" and Lind-
strom, speaking of the coarse-ribbed specimens in Gothland,
says, " these variations are connected with the finely-ribbed
varieties by every possible gradation and intermediate shape."

These opinions were written by naturalists looking upon
species from the old point of view of immutability, but it will
be noticed that the testimony is unmistakable as to the great
range of incessant variation exhibited by the species.

Hall's Comment on the Variability of the Species, — James
Hall, the veteran American paleontologist, in one of his latest
and ripest publications,* speaking of the genus Atrypa, says:
" Following closely the foregoing diagnosis will result in elim-
inating from this group the great majority of species passing
under the name of Atrypa, and in retaining only those which
conform to the well-known A. reticularis, primarily in the
structure of the brachidium, and secondarily in the expression
of the exterior. Such forms are comparatively few in
number, and most authors have been disposed to regard them
as representing unessential variations from the specific type
of A. reticularis. There is, however, a multitude of desig-
nations which have been applied to contemporaneous varia-
tions or consecutive mutations of this specific type, some of
them unnecessary, but many very useful both to the geologist
and the systematist " (pp. 166-7).

* " Introduction to the Study of the Genera of the Paleozoic Brachiopoda "
(1893).

GEOLOGICAL BIOLOGY.

The species, like the genus, ranges from near the base of
the Upper Silurian to the Waverly or beginning of the Car-

boniferous age.

Almost coincident in time with the

appearance of Atrypa reticularis, in its typical aspect, we
find," writes Hall, " in the shales of the Niagara group shells

Silurian

Devonian

Carboniferous

FIG. QQ.— A is a graphic expression of the nature of the differentiation supposed to have taken
place in the course of the history of the race, individuals of which are called Atrypa reticu-
laris. The lines and their relative position and length represent the divergence in varietal
modification and the continuance in generational repetition of like characters for the race.
B represents the groupings of the individuals at three successive stages of its history; viz., at
the beginning of the Silurian, near the beginning of the Devonian, and in the later part of
the Devonian era. The rather distinct specific grouping seen in the latter case is observed to
result from the dropping out of the intermediate forms as well as by the increasing dominance
of the divergent forms.

which are persistently small, with few and coarse plications,
more or less distinct median fold and sinus, and strong con-
centric lamellae. These shells have been designated as
Atrypa rugosa and A. nodostriata Hall " (p. i?o); and these

PLASTICITY AND PERMANENCY OF CHARACTERS. 319

two types continued on to the close of the Devonian, living
often together, but having an independent existence, and not
reaching a completely specific differentiation till the close.

They are more properly claimed as varieties than as dis-
tinct species, this being chiefly due to the maintaining of
variability and the failure of disappearance of intermediate
forms linking the extreme and typical forms, which thus at
the beginning of the life-period of the genus quite fully ex-
pressed their characteristics.

In the Closing Part of the Life-period of the Race the Extremes
of Acceleration and Retardation Expressed. — In the last few
pages the characters of Atrypa have been described, and it was
pointed out that a certain part of the characters, those of
the species Atrypa reticularis and closely allied species, have
exhibited great persistence of variability. We observed that
this species, or race as we may call it, began at the opening
of the Silurian or possibly in the latter part of the Ordovician,
was conspicuous in the Silurian and the Devonian, but ap-
pears to have become extinct at the close of the Devonian.
At the close of the life-period of the genus the variability in
respect of rate and extent of bifurcation of the surface plica-
tions presents a tendency in two predominating directions.

On the one hand, the bifurcation is rapid and extreme, and
the whole surface of the adult appears covered with numerous
fine plications : this would indicate rapid and continuous
bifurcation during growth, or the character of bifurcating of
the plications shows, in comparison with ancestors, accelera-
tion of development.

On the other hand, there is a well-marked variety which
becomes sharply distinct from the others in the Neodevo-
nian and goes under another specific name, Atrypa spinosa,
which shows the opposite tendency ; the bifurcating has be-
come almost nil. The adult shows no more plications than
does the early stage of growth at the distance of one-fourth
inch from the beak: this is an expression of retardation of
this particular element in the growth of the shell.

Summary. — To define precisely those characters which are
considered in the above analysis, the following summary may
be given : In the geological series of forms described under

32O GEOLOGICAL BIOLOGY.

the name Atrypa reticularis and its varieties, there are ob-
served certain plications of the surface, of indefinite num-
ber, and increasing by bifurcation. The variability or plas-
ticity is observed in respect to the rate and extent of the
bifurcation, which in the early and middle part of the life-
history is indefinite — i.e., there is in the species no fixation
of the law of this bifurcation ; but gradually there is acquired
a tendency to permanency in the two directions of (a) extreme
acceleration and of (U) extreme retardation of the rate of the
bifurcation in the development of the individual, and the
species which may be said to originate by this process, and to
be characterized by the different extent of bifurcation attained,
are thus gradually perfected (see Fig. 99). In a set of Iowa
specimens examined by the author, a well-defined differentia-
tion was noted ; the two species are so nearly distinct that it is
found, by arranging the forms in order of their resemblances
and differences, that there are two well-defined groups, and
the intermediate forms, although they almost touch, are so
separate that careful study decides for every individual case
on which side of the imaginary line it belongs. Thus Atrypa
reticularis is an example of very slow evolution. The family
characters appeared well defined with the earliest representa-
tives of the suborder Helicopegmata; the generic differences
were well elaborated at the first stage of the Eosilurian. This
species was among the earliest representatives of the genus,
and lived nearly as long as we have any trace of the genus.
But the great variability or plasticity of certain characters is
a peculiar characteristic of the early forms up to mid life of
the genus, and might be called a specific character, and the
fixation of this variability is very slowly assumed.

Conclusions Suggested by the Study of Atrypa Reticularis. —
Natural selection is supposed to result in the fixing of variable
characters, and the failure of natural selection to select would
naturally result in a continuation of the variability. It is
rational to conclude, therefore, that a species' which continues
to live without fixing its variable characters is particularly
well adapted to live under a wide range of modified conditions.
The wide geographical distribution of the species here under
consideration confirms this conclusion. That a species does

PLASTICITY AND PERMANENCY OF CHARACTERS. 321

die out in course of time is illustrated by thousands of species
which are represented abundantly in the rocks of some par-
ticular period, but thereafter are never seen again. Varia-
bility in ontogenesis is a necessity of living at all. The organ
which in its minutest characters has ceased to change, has
ceased to live ; and if we extend this generalization to the law
of phylogeny, we might expect to find, not a uniform, contin
uous evolution along all lines, but pulsations, so to speak, in
the activity of phylogenetic evolution of organisms along each
line. Taken as a whole, doubtless there is a gradual read-
justing of parts ; but each part is temporary, and is displaced
by another. So long as great flexibility of any particular
character, or set of characters, prevails, there will be rapid
appearance of new forms ; but after their initial appearance,
the repeating of the characters by natural generation will tend
to their fixation, and with the limitation of adjustability to
environment there will result death upon the slightest mal-
adjustment ; thus, as the variability of the species becomes
more and more narrow, the conditions under which it can
thrive become more and more restricted, and the final result
must be extinction.

Whenever the action of heredity becomes restricted — that
is, when sterility limits the range of variation within which
generation is possible — this condition of fertility must work
toward the final extinction of the race. Thus, according to
this theory, if a species be found breeding perfectly true, we
can conceive it to have reached the end of its life-period, and
likely soon to become extinct. The theory in this respect
can be tested by the facts ; and although statistics as to the
actual fact on this particular point are wanting, it has been
frequently noticed in fossil species which have been care-
fully observed by the author that it is a conspicuous law, that
in respect to those characters which serve as distinctive marks
of species, there is greater general variability in the early
stages of the life of the genus. than in the later stages. The
following fact is an expression of the same law, viz. : the spe-
cies occurring at the early stage of a genus are generally more
difficult to separate, and there are more intermediate links
among earlier than among later species of a genus. After

322 GEOLOGICAL BIOLOGY.

examining and trying a number of hypotheses to account for
these facts, the following definition seems to be fairly satis-
factory : The species in its specific characters shows a greater
degree of variability or plasticity in the earlier than in the later
stages of its history.

Atrypa was an illustration of the remarkable continuance
of the stage of plasticity, but we observe that the particular
limitation of range of the plasticity became thereafter a
specific characteristic of the race. The greatest and the
least number of plications attained by any representative of
the genus are probably met with within what has been called,
in a broad sense, the species Atrypa reticularis. Another
law of specific modification is seen in the gradual narrowing
of the limits of the plasticity — one series concentrating about
the forms with few plications, the other series concentrating
about the forms with many; — the one expressing the law of
retardation of growth for this character, the other the law of
acceleration for the same character.

The Initiation of the Species of Ptychopteria. — Ptychopteria *
is a remarkable instance of variability among the initial rep-
resentatives of a genus. The case is as follows : A genus of
Lamellibranchs, having some well-defined generic characters,
is first seen in the upper sandstones of the Neodevonian in
Western New York and Pennsylvania. A few years ago
the genus Ptychopteria was defined and figured by the
New York State Geologist, f and nearly a score of species,
were described from different localities and, possibly, different
geological horizons. About the time of the publication of
the species a block of sandstone, about a cubic foot in size,
was found in Chautauqua County, fallen from a ledge of the
Panama sandstone, containing many hundreds of specimens
of shells of this genus. These were carefully collected,
sorted, and classified according to the characters by which the
several species defined by Hall had been distinguished. An
analysis of the species already described showed the following

* The facts of the case were briefly alluded to in a paper "On Devonian
Lamellibranchiates and Species making," — referring to species which paleon-
tologists make, and not to the origin of species. Am. Jour. Sci., vol. xxxii,.
p. 196.

f Paleontology, New York, vol. v, " Lamellibranchiata."

PLASTICITY AND PERMANENCY OF CHARACTERS.

to be the distinguishing differences : the chief of them were
certain surface markings, the prominence and the angle formed
by the shell along a line called the umbonal ridge, the
angle formed by this umbonal ridge and the line of the cardi-
nal margin, and the contour shape of the shells. A careful
study of the characters exhibited by all the known species
was made, and instead of rinding the new specimens to repre-
sent a new species, they practically represented the whole
genus. Every specific character which was described for the
known species was expressed in a series of 32 specimens.
One feature, of great importance in producing the shape of
the shell, is the angle formed by the umbonal ridge and the
hinge-line. This character varied regularly in the series from
less than 30° to over 60°, and these were also the limits of
difference in the described species. The geological horizon
in which this set of specimens occurred was probably the
lowest in which the genus has been seen. The specimens
were slightly smaller in size than most of the species de-
scribed from other regions, but the uniformity in size and their
occurrence altogether in a single block of stone, well pre-
served as originally imbedded, are proofs that the specimens
were very closely related genetically, and were not very far
separated from a common ancestor. The variations may be
assumed to have been pure variations, in the strict sense of
the word, that is, of common origin and possessing common
fertility.

This series seems to admit of only one explanation for the
origin of the several species of the genus Ptychopteria — i.e.,
the fixation, by isolation or subjection to various conditions of
environment, of the variable characters of the initial stage of
the genus as it appeared in the Panama sandstone.

The Law of Progressive Evolution of Mammals as Formulated by
Osborne. — The force of the evidence of Brachiopods may be
weakened in the minds of some by the consideration of the
very low rank of these organisms in the Animal Kingdom.
But the same methods of minute analysis lead to like conclu-
sions in the study of mammals, the highest type of organic
structure. Professor H. F. Osborne, at the conclusion of his
recent address, as Vice-president of the American Association

324 GEOLOGICAL BIOLOGY.

of Science, on " The Rise of the Mammalia in North
America,"* in which a minute study is made of the law of
evolution as expressed in the teeth of mammals, says: " The
evolution of a family like the Titanotheres presents an unin-
terrupted march in one direction. While apparently prosper-
ous and attaining a great size, it was really passing into a
great corral of inadaptation to the grasses which were in-
troduced in the Middle Miocene. So with other families and
lesser lines, extinction came in at the end of a term of devel-
opment and high specialization. ... A certain trend of de-
velopment is taken leading to an adaptive or inadaptive final
issue ; but extinction or survival of the fittest seems to exert
little influence en route. The changes en route lead us to be-
lieve either in predestination — a kind of internal perfecting
tendency, or in kinetogenesis. For the trend of evolution is
not the happy resultant of many trials, but is heralded in
structures of the same form all the world over and in age
after age, by similar minute changes advancing irresistibly
from inutility to utility. It is an absolutely definite and
lawful progression. The infinite number of contemporary
developing, degenerating, and stationary characters preclude
the possibility of fortuity. There is some law introducing
and regulating each of these variations, as in the variations
of individual growth." f

* Am. Jour. Sci., vol. XLVI. pp. 379-392 and 448-466. f pp. 465, 466.

CHAPTER XVIII.

THE RATE OF MORPHOLOGICAL DIFFERENTIATION IN
A GENETIC SERIES; ILLUSTRATED BY A STUDY OF
THE CEPHALOPODS.

The Evidence Furnished by the Cephalopods. — Having used
Brachiopods for what they are worth towards illustrating
the laws of evolution, another group of organisms may be
examined in the same way, to ascertain what they testify
regarding the same points of history.

Cephalopoda present some general peculiarities contrast-
ing them with the Brachiopoda. The Cephalopoda are con-
structed on a plan which is shared with two or three other
large groups of organisms. The class Cephalopoda is, with
Gastropoda and Lamellibranchiata, and, according to some
authors, Pteropoda, only one of the classes of the branch
Mollusca. We are able, therefore, to distinguish its class
characters from those of closely allied classes. This we
could not do satisfactorily with the Brachiopoda, which stands
out sharply distinguished from all other classes of organisms
from the earliest geological time. We find the first traces of
the Cephalopoda above the first, or Cambrian, period, i.e.,
we have a well-defined fauna in which no Cephalopoda
existed, so far as our records testify.

Lankester's Schematic Mollusk. — In attempting to introduce
a beginner to a knowledge of the Cephalopod mollusk the
method of Lankester, so admirably expressed in his article
" Mollusca" in the Encyclopaedia Britannica, and afterwards
published with others under the title " Zoological Articles,
etc," presents some excellent features.

Professor Lankester constructs a schematic mollusk as
represented in Fig. 100.

325

326

GEOLOGICAL BIOLOGY.

This schematic mollusk possesses "in an unexaggerated
form the various structural arrangements which are more or
less specialized, exaggerated, or even suppressed in particular
members of the group." It represents, as near as our knowl-
edge will enable us to do, the actual mollusk ancestor from
which the various living forms have sprung, and therefore
does not represent any actually living species of mollusk.
However, the accuracy of the schematic type is evident when

FIG. zoo. — Diagrams showing the arrangement of the organs in an ideal Mollusk. (After
Lankester.) a, tentacle ; 3, head ; <r, margin of mantle ; d, margin of shell ; <r, edge of body ;
y, edge of shell depression ; £, shell ; gc, cerebral ganglion ; gpe, pedal ganglion ; gpl, plural
ganglion ; h, osphradium ; z, ctenidium ; k, reproductive pore ; /, nephridial pore ; m, anus ;
n and/, foot'; r, ccelom ; s, pericardium ; ^, testis ; #, nephridium ; v, ventricle of heart ;
zlt liver.

we attempt to compare with it a living specimen of some one
species of mollusk.

It is an, attempt to give form and definite relation to the

terms of a systematic definition of the characters of the
branch Mollusca. In his diagrams of a series of mollusks the

same method is used to give formative expression to the
characteristics of the several classes.

MORPHOLOGICAL DIFFERENTIATION. $2?

The value of this representation for our purpose is to-
show the extent of structural elaboration which the evolution
of organisms had actually reached at the time when we first
meet with a representative of the class Cephalopoda.

Supposed Characteristics of the Primitive Mollusk. — In this
earliest mollusk bilateral symmetry was fully developed. The
nervous system was expressed in bilateral pairs of ganglia and
nerves. The organs of sense were in pairs : two eyes and two
otocysts were present. The body form was normally sym-
metrical, its spiral coiling or one-sided development coming
as a specialization of growth. The cephalic is sharply distin-
guished from the visceral part of the body. The shell is
associated with the visceral part, and is not auxiliary to the
functions of motion, but is protective in nature. The head — •
anterior part — is distinctly connected with motor functions ;
and organs of the motor and sense functions are separate
and widely differentiated. Motion is elaborated into distinct
organs for offence and for prehension.

The alimentary function is dominated by a single central
canal, with an anterior mouth, about which are the accessory
organs of excretion (nephridia or rudimentary kidneys), and
there is a circulatory system, with a heart and a pair of auri-
cles and one ventricle. Locomotion is a conspicuous func-
tion, and the presence of an enlargement of the mantle as a
foot-organ is one of the most characteristic features of the
mollusk.

The differentiation of this foot-organ is also one of the
most fundamental of the characters distinguishing the classes
of Mollusca, and the adaptation of the part to special modes of
locomotion was developed at a very early stage, as indicated
by the presence of distinct Gastropoda, Pteropoda, Cephalop-
oda, and Lamellibranchiata at as early as Ordovician time.

Differentiation of the Foot-organ in Mollusks. — This dif-
ferentiation is represented in Lankester's diagram of a series
of mollusks to show the form of the foot and its regions, and
the relation of the visceral hump to the antero-posterior and
dorso-ventral axes (Fig. 101). In these figures are seen the
simple continuous flat foot of the Chiton (i), or isopleural
Gastropod, which retains the bilateral structure of the primi-

328

GEOLOGICAL BIOLOGY.

tive mollusk. In the Gastropoda anisopleura, or typical Gas-
tropods, the specialization does not greatly affect the foot,
which is still symmetrical and occupies similar relations to
the rest of the body ; but the twisting of the body coincident
with the spiral shell which is developed as a cover, affects the
proportionate size and vigor of the organs on the two sides,
50 that the organs are in fact not strictly symmetrical in the

(a)

FIG. 101. — Diagrams of a series of Mollusks to show the form of the foot and its regions, and the
relation of the visceral hump to the antero-posterior and dorso-ventral axes, (i) A Chiton.
(2) A Lamellibranch. (3) An Anisopleurous Gastropod. (4) Thecosomatous Pteropod. (5) A
Gymnosomatous Pteropod. (6) A Siphonopod (Cuttle). A, />, antero-posterior horizontal
axis ; Z>, K, dorso-ventral vertical axis at right angles to A , P ; o, mouth ; «, anus ; MS, edge
of the mantle-skirt or flap; sp, sub-pallial chamber or space; jff"^ fore-foot : »*/", mid-foot;
hf, hind-foot ; f, cephalic eyes ; cd, centro-dorsal point (m 6 only). (After Lankester.)

adult (3). The Pteropods have the foot modified for free
swimming into two lateral flappers or wings (Fig. 101, (4)), and
the Cephalopods proper have the right and left lobes, corre-
sponding to the wing-expansion of the Pteropods, folded
under to form a funnel-like tube or siphon, which accom-
plishes locomotion by forcing water outward and forward, the

MORPHOLOGICAL DIFFERENTIATION. 329

anterior part of the foot being differentiated into special
grasping organs auxiliary to the mouth functions.

The specialization of the tubular mode of locomotion and
the differentiation of the foot into a funnel and tentacles are
characteristics of this highest type of mollusk ; and its relation
to the Pteropod wings is seen in the fact that in the Dibran-
chiate order the lateral lobes are fused together to form a
closed tube — the siphon, while in the Tetrabranchiate order
they are only brought close together, and not fused into a
continuous tube. There is also the differentiation of distinct
swimming flappers in some of the Dibranchiates, in addition
to the siphon, which is specialized as an organ for distribution
of ink into the water, and by darkening the water compen-
sates for its slow rate of escape by locomotion from any cause
of danger.

The Structure of the Cephalopods, — Although the purpose
of this volume does not include the detailed description of
organisms, a better understanding of the remarks that follow
may be reached by a brief review of the essential structural
elements of the Cephalopods. For this purpose the following
translation of extracts from Zittel's description will be useful,
and for further details the reader is referred to his excellent
Handbook of Paleontology.*

FIRST ORDER, TETRABRANCHIATA. — Cephalopods with shell; furnished
with four branching gills, or branchiae, funnel formed by union of two
lobes of foot, but not permanently united; no ink sac or pouch. In the
place of arms, numerous tentacles, slender, elongated and without suck-
ers or hooks; shell chambered.

The Animal. — All that we know of the organization of the Tetrabranchi-
ata is based upon the genus Nautilus, the only one of the order now liv-
ing, the shell of which is seen in most museums; but the animal is very
rare, and has been seen alive in only a few instances. The animal occu-
pies the last chamber of the shell, with the ventral side turned outward
(the coiling of the shell thus being toward the dorsal side); the body is
short and thick; the head separated from the trunk by a slight constric-
tion. In place of arms, about ninety contractile filiform tentacles inserted
in muscular sheaths surround the mouth; they are grouped in sev-
eral bundles, and in an order a little different in the male and female.
The tentacles situated on the dorsal side are soldered together to
form a thick muscular lobe which can close the opening of the shell
when the animal has withdrawn into the last chamber. The funnel is a

* " Handbuch der Palseontologie, i. Abtheilung: Palaeozoologie," von Karl
A. Zittel, vol. n, 1881-1885, pp. 332, etc.

33° GEOLOGICAL BIOLOGY.

very thick, enrolled muscular fold, of which the external borders are in.
terlaced one with the other; the tentacles and funnel, as the innervation
shows, correspond to the foot of the Gastropod. At the base of the lateral
ocular tentacles is found on each side a large eye, with short peduncle;
in the midst of the crown of tentacles is situated the buccal (mouth) cavity
surrounded by thick walls, with a fleshy tongue, the root of which is com-
posed of many series of plates and hooks. The jaws, of extraordinary
strength, recall in form the beak of a parrot. The large branchias, or
gills, are found in two pairs at the base of the funnel: they penetrate
freely into the respiratory cavity; between these open the anal orifice
and a little further back, the organs of generation.

The respiratory cavity and head are covered by a thin lobe or mantle,
-especially developed on the ventral side, and secreting the shell of the
outer chamber.

The animal is attached to the shell by a powerful muscle of oval form,
placed below the eyes, and inserted on the internal wall of the chamber

FIG. 102.— Nautilus pompilius. (After Owen.) a — mantle ; b = dorsal aspect of mantle ; c =
hood ; d — funnel ; e = nidamental gland ; h — shell muscle ; o = eye.

of habitation, where it leaves slight impressions. From the rounded
posterior extremity of the animal proceeds a membranous hollow cord,
furnished with blood-vessels, the Siphon, which passes by a rounded
opening through the last partition-wall into the chambered part of the
shell, and continues thus in an uninterrupted manner to the initial chamber.
The Shell. — By the internal chambering or partitioning of the shell
(Fig. 102), which is characteristic of them, the shells of the Tetrabranchiata
are distinguished from all the shells of Mollusca hitherto considered. The
last, distinguished by its greater capacity, serves as the chamber of habi-
tation for the animal; all the rest of the shell is divided into chambers by
transverse partitions, called Septa, which succeed each other at regular
intervals. The chambers are filled with air (gas), and united together by
the Siphon. The exterior form of the shell presents extraordinary varia-
tion; in general, it may be considered as a straight conical tube, aug-
menting little by little in thickness, which continues to incurve, sometimes
in a straight line, and often in a curved line. There are, consequently,

MORPHOLOGICAL DIFFERENTIA TION.

331

FIG. 103. — Ortho-
ceras timidum
Barr.

FlG. \o\. — Cyrtoceras Murchi-
soni Barr.

FlG. 105. — Hamites
rotundus Sow.

•FiG. 106. — Gyroceras alatum Barr.

FIG. 107.— Trochoceras nptntum Barr.

332 GEOLOGICAL BIOLOGY.

shells of straight, staff-like form (Orthoceras, Fig. 103; Baculites), slightly
curved (Cyrtoceras, Fig. 104), hooked (Hamites, Fig. 105), spirally enrolled
(Gyroceras, Fig. 106), or coiled in manner of a snail shell (Trochoceras,
Fig. 107). If the turns of the spirally enrolled tube are
in the same plane, and touch each other, the shell is
disk-formed (Clymenia, Trocholites, Nautilus, Ammon-
ites); if they turn in form of a screw, the shell is heli-
coidal (Cochloceras, Turrilites, Fig. 108). It is not rare
that the last coil is elongated in straight line, and de-
tached from the rest of the anterior by enrolled spiral
(Lituites); sometimes it is curved still more slowly in
the form of a hook (Ancyloceras, Macroscaphites). In
many of the shells spirally coiled in the same plane the
last turn encloses the previous turns either entirely or
in part. If this envelopment goes so far that the pre-
ceding turns are entirely concealed, and that only the
last one remains visible, the shell is called involute. If
the older coils are still visible in the centre, there is
then an umbilicus, and according to the degree of in-
volution the shell is said to have narrow or broad um-
bilicus. In the evolute, or open spiral, the turns do not

t, „, .... touch each other so that one can see between them. By

FIG. 108. — Tumhtes

catenatus d'Orb. their ornamentation also the shells of Tetrabranchiata

show considerable diversity: on the one hand there are
forms of which the surface is covered only by fine striae of growth, and
on the other are forms presenting a rich ornamentation of the surface.
The surface markings are smooth lines, punctate, granulate, and more
or less prominent lines, foliaceous excrescences, rings, protuberances,
simple or bifurcate ribs, tubercles, or spires, isolated or arranged in
series. The ornaments which follow the general direction of the longi-
tudinal axis of the whorl go under the name of longitudinal or spiral
sculpture, while those which are arranged obliquely, or at right angles
to these, are called transverse or radiating ornamentations.

The position of the animal of the Nautilus (see Fig. 102) offers the only
good evidence by which to orient the shell of the Tetrabranchiates As
it turns the ventral side of the animal outwards, R. Owen has designated
the external or arched part of the shell, the ventral side, and the oppo-
site internal part, the dorsal side. All the ancient authors, who occu-
pied themselves exclusively with the shells, called, in the spirally en-
rolled forms, the external side of the shell back, and the internal side
the ventral side of the shell. According to Barrande, the external arched
part of the spirally twisted fossil forms does not always correspond to-
the ventral side of the animal; the convex ventral side of the shell is dis-
tinguished, particularly in the Nautilus, by a depression of the buccal
border. It is admitted, therefore, that always where such a sinus exists
in the buccal border it indicates the position of the siphon, and conse-
quently the ventral side of the animal. According to Barrande, the sinus
is found frequently in fossil Nautilids, sometimes upon the external
arched side, sometimes upon the concave inner side. There are thus,
evidently, exogastric and endogastric shells. In the majority of the fossil
shells of Cephalopoda, and particularly in the Ammonites, data are want-

MORPHOLOGICAL DIFFERENTIATION. 333

ing for deducing the organization of the animal; in that case the terms
internal side and external side are used, which prejudge nothing. A
vertical line running from the external to the internal side gives the
height; a second line, perpendicular to the preceding, gives the breadth,
or thickness of the turn.

In the involute shells, the growth, as was first recognized by Reinecke,
and later verified by Leop. von Buch, takes place according to a definite
law. Moseley and Naumann show that the law of growth corresponds to
a logarithmic spiral; consequently, the height and breadth of all the turns
are in the same proportion; the quotient of the height of two successive
turns gives the rate of growth of the mouth in height; the quotient of
the corresponding breadths gives the rate of increase in breadth; the
quotient of the diameter of the entire shell by the height of the last turn
expresses the rate of growth of the discoid (Scheibenzunahme). The
calculations of Moseley and Naumann were afterwards confirmed by G.
Sandberger and Grabau.

The constitution of the internal partitions (septa) which limit the differ-
ent air-chambers is of considerable importance. Their number varies
extraordinarily in the different genera and the different species, but it is
quite constant in one and the same species; they are at increasing inter-
vals from each other, according to law, proportionate to the growth of the
shell, and it is only the last two partitions (septa) which precede the final
chamber, which are at a somewhat less distance apart. Probably all the
chambers have successively served as dwelling-chambers, and it is only
after a new partition was formed that it was transformed into an air-
chamber, which was no longer in communication, except by the Siphon,
with the last chamber. The mud and the sand were not able, generally, to
penetrate into the interior of the fossil shells when they were buried in-
tact, except in the last chamber, or by the siphonal opening into the last
air-chamber only. This is the reason why the chambers are very often
not filled with rock, but are coated or filled with crystals of calcite, of
quartz, of pyrite, of celestite, of barite, etc., which have been precipi-
tated from the infiltrated chemical solutions.

The line of attachment of the partition to the internal wall of the shell
is called the Suture. (See Fig. 27, p. 106, and Figs. 112-118, p. 346.) It
is not exteriorly visible unless the shell is removed or dissolved. It
is seen more distinctly on the fossil moulds, in which the shell is
wanting. In the Nautilus, and in many of the shells of fossil Tetra-
branchiates, the septa attach themselves to the internal surface of
the shell by a slightly arched sutural line. Moreover, very often the
line of the suture, on account of the undulating curvature and a flut-
ing of the septum, acquires a high degree of complication resembling
the branching of moss. There are all degrees of variation from lines
the most simple to those most complex. Besides, as the lines have essen-
tially the same sinuosity for all the specimens of one species, and on the
contrary show differences quite striking in different species and separate
genera, they furnish thus one of the most important systematic char-
acters. In the Nautilidae the lines of the sutures are generally simple
(Fig. 106); in the Goniatites and Clymenias (Fig. 112) the undulating and
slashed suture forms prominent saddles before and curved sinuses behind,
called lobes. A later differentiation is met with in the Cerat'tes, etc.,

334 GEOLOGICAL BIOLOGY.

(Fig. 114), the lobes being denticulated by secondary notches. In the Am-
monites (Fig. 115) the saddes also, as well as the lobes, are denticulated
in the most varied manner, notched, cut, or ramified, in form of branches,
or foliated. The curvature of the suture line, as well as the formation of
the saddles and lobes, takes place symmetrically in such a manner that
a median line in the direction of the height divides the turns into two
equal parts. The exterior lobe is called the external or siphonal lobe,
when the siphon is on the exterior side. For Leop. von Buch it is the
dorsal lobe, because he called this the back of the shell, but for recent
authors, who consider the external side to be the ventral part, it is the
ventral lobe. The opposite unpaired lobe is the internal lobe (or, accord-
ing to opinions, antisiphonal lobe, or dorsal, formerly ventral lobe). Be-
tween the two are found the lateral lobes and the lateral saddles, situated
on the body of the whorls, and the lobes and saddles concealed between
the line of contact of the contiguous whorls and the internal lobe: among
the former, the saddle which is found on the side of the external lobe is
the external saddle, the two following are the first and second lateral sad-
dles; all the others, up to the line of junction of the two whorls, are the
auxiliary saddles; near the internal lobe is found, generally, an internal
saddle, which is distinguished by its size from the other concealed internal
auxiliary saddles. For the lobes, the first lateral lobe is that which is
between the external saddle and the first lateral saddle; the following
one is the second lateral lobe; all the others are called auxiliary
lobes.

The beautiful researches of Hyatt and Branco have shown that the
complicated lines of the suture of the Ammonites do not attain their normal
form until the animal has developed a greater or less number of the cham-
bers. The first sutures of all the Ammonites are always as simple as those
of the Nautilidae, Clymenias, or Goniatites (Figs. 112, 116); it is only little
by little that the undulating lines become marked by secondary notches,
and the complication of the line of the suture proceeds always from the
exterior to the interior side. The complication of the suture line — which
augments with age, so that the young sutures, more simple in Ammonites,
resemble those of the geologically more ancient Goniatites and Nautilidae,
— shows, probably, that this differentiation indicates at the same time a per-
fection of the organism. It is truly difficult to discover wherein this con-
sists. It is possible that the strongly ramified borders of the septa serve
to increase the solidity (firmness) of the shells; for, in general, the shells
of Nautilidae, provided with simple suture lines, are considerably thicker
than the shells of Ammonites — ordinarily as thin as paper. If one breaks
cautiously, little by little, the enrolled shell of a Tetrabranchiate, there
are distinguished the first whorls, and finally also the initial chamber of
the whole coil. In the fossil evolute, or baculiform, shells this first cham-
ber is, ordinarily, abbreviated or broken, and it is extremely rare that it
is preserved.

According to Barrande, Hyatt, and Branco, there are two kinds of initial
chambers in the Tetrabranchiates which can be distinguished by funda-
mental characters. In the Nautilus, and many of the paleozoic genera,
the initial chamber is in the form of a truncated cone, slightly arched or
straight, enlarged in front; upon the posterior convex wall, which termi-
nates the truncated cone, is observed a depressed cicatrix, linear (Nau-

MORPHOLOGICAL DIFFERENTIATION. 335

tilus), circular (Cyrtoceras), elliptical (Trochoceras, Phragmocus), or some-
times cruciform.

The initial chamber of Clymenia, the Goniatites, and the Ammonites
is formed in an entirely different manner. In all these this spirally en-
rolled chamber has a vesiculous, spherical, or ovoid form, generally a
little depressed and transversely striated; no scar or impression has been
met with, and the siphon begins at the anterior wall. It is not probable
that the initial chambers of the form of a truncated cone of the Nautilidae
are homologous with the spherical enrolled initial chambers of the Am-
monitidae; on the contrary, the presence of a cicatrix makes it probable
that this impression represents either the point of attachment, or the
opening of communication, closed at a later stage, of a frail vesicle, per-
haps membranous, which corresponds with the initial chamber of the
Ammonites. According to this view, proposed by Hyatt, the initial cham-
ber of the Nautilidae should be equivalent to the second chamber of the
Goniatites and the Ammonites.

The Siphon is a tubular prolongation of the skin of the posterior part
of the body; it traverses all the chambers, and in Nautilus begins under
the form of a closed tube covered with nacre, in the initial chamber, or
truncated cone, where it touches the internal posterior wall at the same
place, where exteriorly is seen the cicatrix. In the Ammonites and the
Goniatites the siphon begins with a spherical swelling situated imme-
diately behind the anterior wall of the initial vesicle (nucleus), conse-
quently perforating only the first septum, without penetrating more
deeply into the chamber. According to Hyatt, the part of the siphon
penetrating into the embryonal chamber was, in general, only a depression
of the first partition. Munier-Chalmas has observed in the Ammonites
a particular prolongation of the siphon in the initial chamber which ought
to have replaced the true siphon in the
embryonic stage; this prosiphon, as he
calls it, is attached to the siphon, which
begins in a reflected cul-de-sac, and is
very variable in form. It forms some-
times an enlarged membrane, sometimes
a cylindrical tube; the prosiphon does
not communicate with the siphon, prop-
erly speaking.

In the recent Nautilus the siphon is
a rather solid membranous tube covered
exteriorly by a thin coating of brown
color, earthy, formed of fine calcareous
grains. In the Ammonites (see Fig. ioa)

this exterior Calcareous envelope Seems

to take on a more substantial consist-

ency, so that the siphon is enclosed in a the third whorl, where it is prosiphonate
j i- i * T • or turned forwards. (After Zittel.)

delicate calcareous tube. It is necessary

not to confuse this envelope of the siphon itself with that which is called
the siphonal collar, which is met with always where the siphon penetrates
the septum.

The siphonal collar (Fig. 109) is a reflection or production of the sep-
tum of greater or less length, directed, generally, in Nautilus, backward,

collar and its change of direction on pass-

336 GEOLOGICAL BIOLOGY.

and in Ammonites forward, and possesses the same structure with the
septum. Ordinarily, the siphonal collar has only short length, and forms
in front and behind the septum a sheath in the form of a band or collar
about the siphon; but, sometimes, they pass from one septum to the other
and form there a close continuous tube, or they have the form of an open
funnel, slightly contracted behind, and prolonged to the next following sep-
tum, or even go beyond it, thus implanting themselves one within another
(telescoping, Endoceras). The siphon is found in the median plane of che
shell, and it is only exceptionally that it deviates a little from this plane.
In this plane its position vacillates from the external side to the internal
side in the different genera and the different species.

In the Ammonitidae it is constantly on the external side of the shell.
In the Nautilidae its position does not remain constant in one and the
same genus : it may be external, internal, central, or intermediary.

Numerical Rate of Differentiation expressed in Terms of the In-
itiation of New Genera. — A study of the statistics of classifi-
cation in relation to time will exhibit in this, as it has in-
previous cases, the grand features of the historical differenti-
ation of the cephalopods.

First, we may consider what are the conclusions to be
drawn from the succession of new genera as to the rate and
order of the differentiations of the class Cephalopoda.

The classification itself is expressive of differentiation, as
has been already observed. The division of the class into
two orders is expressive of a very marked differentiation in
structure. The genus is a group of organisms with the same
ordinal and family structure, but exhibiting some particular
characters, such as shape, relative size of parts, or special de-
velopment of some part, which are the same for several dif-
ferent species ; hence we recognize the number of genera to
be a numerical expression of the amount of differentiation
attained in the family at any particular period of time, and
the greater the number of genera in a particular family, at a
particular time, the greater is the amount of differentiation
expressed in the family-history at that period, and the
number of genera beginning or living in each period becomes
a rough indication of the rate of expansion or evolution of
the race under consideration. The total number of genera
in the order Tetrabranchiata is 123 (Zittel). Two grand
subdivisions of subordinal rank are made, including, respec-
tively, Nautiloidea 29 genera, and Ammonoidea 94 genera.
28 genera of the 29 Nautiloidea had appeared in the Silurian.

MORPHOLOGICAL DIFFERENTIATION. 337

One genus, Aturia, is considered to be a distinct new genus
of the Tertiary; 16 genera were already well exhibited in
the Lower Silurian, or Ordovician. Only 8 genera lived
into the Devonian, only 5 to the Carboniferous, and but 2
(Orthoceras and Nautilus, the perfectly straight form and the
tightly coiled form) survived from Paleozoic into Mesozoic
time.

The other suborder, Ammonoidea, has 94 genera; of
these, one genus is known as early as the Silurian (Goniatites),
one new genus (Clymenia) was added in the Devonian, and in
the latter part of the Carboniferous 5 more genera were initi-
ated. Of the rest, all appeared in the Mesozoic, 41 genera
beginning in the Triassic, 28 new genera starting in the Juras-
sic, and 1 8 new ones appearing, for the first time, in the
Cretaceous. Not a single genus of the whole suborder sur-
vived the Cretaceous period. Thus the Nautiloidea are
peculiarly Paleozoic in range, although there is still living the
genus Nautilus, and the Ammonoidea are peculiarly Meso-
zoic, and every genus of this suborder is now extinct.

The other order, Dibranchiata, is less capable of showing
its history : the hard parts were of inferior character and less
in proportion to the fleshy parts, and upon the death of the
animal were much more likely to be destroyed 533 genera are
known, and all are Mesozoic, or more recent. There were 3
genera in the Jurassic, 15, Triassic, 8, Cretaceous, 10, Ter-
tiary and 3 now living.

Second. The lesson, regarding the evolution of the ordinal
and subordinal characters and their generic expansion, which
we derive from these statistics is as follows :

Rate of Differentiation of the Suborder Nautiloidea. — The
Nautiloids (Orthoceras, Nautilus, and their kindred genera)
first appeared in the Ordovician. Before the close of the Silu-
rian this type had reached its fullest expansion, and began in
a very marked manner to drop out of the race ; 5 genera did
not survive from Ordovician into Silurian, and of the 22 Silu-
rian genera only 8 survived into the Devonian. Of this type
the two genera to live the longest were Orthoceras, the simp-
lest expression of the type, and Nautilus, probably the most
differentiated ; and the latter continued to live up to present

338

GEOLOGICAL BIOLOGY.

time. At least, of the structures preserved to tell us the
story these two are the extremes — one, Orthoceras, a simple
slender cone, straight, and with regular septa dividing it into
chambers, and with a central siphuncle ; the other, Nautilus,
a closely coiled disciform shell, with siphuncle also central,

B

FIG. no. — Theoretic sections through the middle of the shells to show the variations in the curva-
ture and coiling of Paleozoic Cephalopod shells. A, Clymenia ; £, Nautilus; C, Nautilo-
ceras ; Z>, £, Aploceras ; /% Orthoceras ; G, Melia ; //, /, Cyrtoceras ; y, Gyroceras ; /T,
Ophidioceras ; L, Cryptoceras ; M, Goniatites. (After Gaudry.)

outer chamber large, and whorls with ventral side out. The
two features which best express in these shells the amount or
degree of differentiation are, the amount and direction of
the curvatures of the shells and the position of the siphuncles.

MORPHOLOGICAL DIFFERENTIATION. 339

The characters which serve most readily to distinguish the
Ammonoids from the Nautiloids are the sutures. In the
Nautiloids, above described, the suture is always straight, or
but slightly curved. In all the Ammonoids the suture is
more or less lobed or notched.

Mode of Curvature of the Nautiloid Shell, — Third. Before
considering the Ammonoids, we may notice the law of varia-
tion expressed by this curvature of the shell. In the Nau-
tiloids there are four types of form expressed in the direction
of growth of the cone :

1. The shell is straight, or nearly so (see Orthoceras,
Fig. no, F).

2. The shell is simply arched (see Cyrtoceras, //, /, D).

3. The shell is discoidal, rolled in a spiral in single plane

(/)•

(a) This spiral may be loose (C or J, Gyroceran) ;

(b) or close-coiled, (Goniatites, M) ;

(c) or involute (Nautilus, B).

4. The shell may be spirally coiled in a screw plane, or
helicoidal. (See Fig. 108, p. 332).

When we separate out for special consideration the mode
and amount of curvature of the shell, we are first struck with
the evidence of progressive modification from the straight to
the close-coiled forms ; but when the relation of these modi-
fications to the time of their first appearance is noted we learn
that forms of the different types of modification occur at the
earliest period (the Ordovician) in which the suborder appears,
and if we were to seek a series representing gradual modifica-
tion from one extreme to the other, we could find them well
expressed at this initial stage of the subordinal history.

The Rate of Initiation of the Orthoceratidse. — For instance,
take the species of America alone, and of the straight or
slightly bent form, Orthoceratidae, there are recorded by
Miller 5 recognized genera and 170 species in the first stage
of this family, Ordovician ; in the Silurian there were 3 genera
and 8 1 species; in the Devonian 3 genera with 145 species;
in the Carboniferous, 2 genera with 43 species.

Rate of Initiation of the Cyrtoceratidae. — The same general
law is seen in the (2) arched forms included in the family

340 GEOLOGICAL BIOLOGY.

Cyrtoceratidae. In America 2 genera (Cyrtoceras and Phrag-
moceras) began in the Ordovician and continue throughout
the Paleozoic, and Miller records of them 72 species in the
Ordovician era, and 42 Silurian, 20 Devonian, and 8 Carbo-
niferous species.

Rate of Initiation of the Nautilidse. — Take the third type
(3), the discoidally spiral form Nautilus, and its various gen-
eric allies. The Nautilidse has in America 5 well-marked
genera. 4 genera, including 35 species, are Ordovician; 4
genera, including 17 species, Upper Silurian; 2 genera, in-
cluding 35 species, Devonian; 2 genera, including 62 species,
Carboniferous. In this case the apparently different law ex-
pressed in the number of genera and their decrease, and in the
number of species and their increase, is due to the combina-
tion in the family of two sets of genera, the one set of which
have their maximum representation of species early in the
Paleozoic ; the other increases in the number of its species as
we ascend. Lituites, for instance, has 15 species in Ordovi-
cian, 7 in Silurian, and then became extinct. On the other
hand, Nautilus has 13 species recorded for the Ordovician, 4,
Silurian, 15, Devonian, 59, Carboniferous; and the genus con-
tinues on to the present time.

History of Trochoceras by Species. — The helicoidal type (4),
including, for America, the one genus Trochoceras of the
family Trochoceratidae, is specifically represented as follows:
Ordovician I, Silurian 7, Devonian 10; and then it ceases.

General Law of Evolution of Shell Cuvature in the Nautiloidea,
— Thus, to generalize, we find that this grand feature of the
Nautiloidea, the form assumed by the shell in its growth,
expresses the fulness of its differentiation among the repre-
sentatives of the first or initial period of the existence of the
race. All the several types of form run along together and
continue nearly, or quite, to the close of the Paleozoic, and
there, with the exception of two genera, become suddenly
extinct.

Rate of Initiation of New Species in the American Region. —
Fourth. As if to emphasize the law above expressed regard-
ing the initiation of new genera, the statistics of the initiation
of species in the American rocks point in the same direction.

MORPHOLOGICAL DIFFERENTIATION. 34!

When we observe the number of the different species of a
genus, recorded in the rocks of each period in which the genus
occurs, we find that the greater number of species of the
genus, as well as the greater number of genera of the family,
are recorded from the initial geological period, which in this
case is the Ordovician ; and the genera and the species gradu-
ally decrease in number for each following period until the
close of the Paleozoic, with the exception of the genera Nau-
tilus and Trochoceras, whose expansion appears to be later
and its life-period longer. Even in these cases, however, the
law is relatively the same.

This, again, is expressive of the general law before stated,
that tJie chief expansion of any type of organisms takes place at
a relatively early period in its life-history. This law was ob-
served in the case of the Brachiopods, and is observed here
among the Cephalopods. There are some modifications or
exceptions to it, which the facts regarding other groups
suggest ; but the general law is sufficiently well attested to
be defined in these general terms.

Hyatt's Formulation of the Law of Rapid Expansion of Differ-
entiation at the Point of Origin of a New Type of Organism. —
Hyatt has given expression to this law in an article on
"Genera of Fossil Cephalopods."* The generalization is
based upon a very exhaustive study of the Cephalopods.
He had access to the collections in the Agassiz Museum of
Natural History, which was the most complete in this
country; and he also visited all the museums in this
country and in Europe where Cephalopods are found, and
made particular examination of every species he could learn
of throughout the scientific world. Speaking of the Nau-
tiloidea and Ammonoidea both, he wrote: "These groups
originate suddenly and spread out with great rapidity, and in
some cases, as in the Arietidae of the Lower Lias, are traceable
to an origin in one well-defined species, which occurs in close
proximity to the whole group in the lowest bed of the same
formation. These facts, and the acknowledged sudden ap-
pearance of large numbers of all the distinct types of In-

* Published in 1883, in the Proceedings of the Boston Society of Natural His-
tory.

342 GEOLOGICAL BIOLOGY.

vertebrata in the Paleozoic, and of the greater number of all
existing and fossil types before the expiration of Paleozoic
time, speak strongly for the quicker evolution of forms in the
Paleozoic, and indicate a general law of evolution. This, we
think, can be formulated as follows : Types are evolved more
quickly and exhibit greater structural differences between ge-
netic groups of the same stock while still near the point of origin,
than they do subsequently. The variations, or differences, may
take place quickly in the fundamental structural characteristics,
and even the embryos may become different when in the earliest
period, but, subsequently, only more superficial structures be-
come subject to great variations. ' ' *

Summary. — If we ask, In what particulars does the structure
of Cephalopods illustrate this law? we may answer in brief,
that we notice it first in the class characters of the Cephalop-
oda. In the description of the class we found the Cepha-
lopods most closely allied to the Pteropods. This is con-
spicuously observed in the difference in structure of the
locomotor apparatus of the foot. In the Pteropod there are
two lateral flaps used like wings, or paddles, for locomotion.
The Cephalopods are modified to form a siphonal funnel
which accomplishes locomotion by forcing water violently out
and forward from this funnel; other structural peculiarities
are associated with this modification.

The Pteropods are abundant in the Cambrian faunas, and
appear to have attained a relative dominance never afterward
held, but in this first fauna there were no Cephalopods. The
Cephalopods of the next (Ordovician) period were extremely
abundant, and the Tetrabranchiata type was expressed by 17
of its 29 genera at the initial Ordovician stage (including here
the Upper Tremadoc, whose fauna seems more appropriately
associated with Ordovician than with Cambrian faunas).

It is seen, secondly, when the Ammonoid type of the
Cephalopods made its appearance in the Goniatites. The
Goniatites came out in full force in the Devonian, with a few
species in beds doubtfully referred to the Upper Silurian but
called Lower Devonian by Kayser. The most characteristic

* See "Phylogeny of an Acquired Characteristic," by Alpheus Hyatt, Proc.
Phil. Soc., vol. xxxn., No. 143, p. 371.

MORPHOLOGICAL DIFFERENTIATION. 34$.

difference in the hard shell is seen in the curved and lobed
suture of the Goniatites as contrasted with the simple suture
of the Nautiloids.

The law is again seen in force in the evolution of the Am-
monites, beginning in the Sicily and India Permian beds; by
the early part of the Trias this new type had expressed a
wonderful expansion. Out of the 92 genera described and
recognized by Zittel, 45 occur in the Triassic, representing 9
out of the 13 known families.

Again, in the Jurassic the great differentiation of type
expressed in the Dibranchiates took place, not in a single form,
but both decapod and octopod modifications appear together.

Thus we find this distinguishing character of the Dibran-
chiate (the consolidation of the siphonal tube, after the tube
with disunited edges had existed from Ordovician time
throughout the Paleozoic) making its first appearance at the
beginning of the Mesozoic, but thereafter continuing on in
successive and various forms until the present time.

In each of these cases, of the initiation of new types of
the Cephalopod mode of organization, there was a rapid
evolution of the chief modifications of the new type near the
period of its first initiation among the geological faunas of the
world.

CHAPTER XIX.

PROGRESSIVE MODIFICATION OF AN EXTRINSIC CHAR-
ACTER; ILLUSTRATED BY THE EVOLUTION OF THE
SUTURE LINES OF AMMONOIDS.

The Ammonoids Illustrate the Law of Acquirement of Differences
T>y Gradual Modification. — The Ammonoids illustrate another
of the laws of evolution in a particularly satisfactory manner.

When we examine the representatives of the same genus,
or family, or order, at the beginning and at the close of its
life-period, it is very common to find the two representatives
differing in one or more characters, which may be described
.as differing in the degree or extent of their development.
The number of parts has increased ; some part which is small
in one is large in the other; some structure which is simple
in the earlier is complex in the later; or parts which are in-
definite in form, or similar in the beginning are definite and
particular in form and structure at the close.

It is rare, however, to be able to collect examples to show
the various stages by which the one was elaborated by degrees
of modification into the other. The famous case of the de-
velopment of the specialized horse foot out of a five-toed an-
cestor is familiar to all, with the beautiful theory of the way
by which the modification came about. This is a case of
relative rather than of direct evolution, since the prominence
of the one toe and line of connecting bones is produced by
the aborting and withdrawal from use, and finally from devel-
opment, of the normal number of parts which were present at
the beginning of the series. The Ammonoids, as we shall
see, illustrate the case of actual increase in complexity, grad-
ually and continuously ; the order of succession in the steps
of progress being clearly and regularly expressed by the actual
appearance of each form at the particular geological stage in

344

EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED.

which it should appear according to the law of genetic evolu-
tion of the characters of the race.

Description of the Characters of the Ammonoids. — In order to
place before the reader a concise description of the characters
of Ammonoids, the definitions of Zittel may again be followed,
furnishing as they do the precise characters needed for an
understanding of the problem under discussion.

Zittel's definition of the characters of the Ammonoidea is
as follows :

SECOND SUBORDER: AMMONOIDEA. Shell generally enrolled or spiral,
discoidal, more rarely spirally coiled, evolute, arched or straight; open-
ing simple or furnished with lateral and ventral prolongations. Suture-
line undulating, notched or with slashed or dentate lobes and saddles;
siphuncle cylindrical, always marginal, without internal deposit; initial
chamber spherical or ovoid, frequently an aptychus or anaptychus.

In the description of the fundamental characteristics of
the sutures and their development we follow Zittel's synopsis.

The embryonal chamber (nucleus, ovisac) of the Ammonoids has a-
spherical or transversely ovoid shape (Fig. 112, a); it is smooth, separated
by a contraction from the rest of the shell, and always enrolled spirally
about an imaginary axis. Its anterior aspect is, in consequence, essen-
tially different from its lateral profile, its sides
having a projection in form of an umbilicus. The
embryonal chamber, of which the height varies
from 0.3 to 0.7 mm., is limited in front by the
primary septum. The constitution of the first
suture gives, according to the beautiful researches
of Branco, excellent basis for classification. In
the most ancient Ammonoids it forms a straight
line, more or less simple, and then resembles the
first suture of Nautiloids; Branco calls these
forms the Asellati (Fig. in, A)

In a second group the first sutural line pro-
ceeds forward to form an arch towards the ex-
terior, and forms a large simple ventral saddle, FIG. in.— Ventral views of the
Latisellati (Fig. in, B\ edges of the embryonal

chamber, representing^ the

The third group is distinguished by the rela- primary septum_of an asel-

tively narrow ventral saddle, on each side of

which is developed a lateral lobe, and generally and C of an angustisellate
also a small lateral saddle, Angustisellati (Fig. gute? Bianco.) (Phyloceras)-
in, C).

While the first suture of all Ammonoids is comparatively simple, more
or less considerable complication is produced by the later development of
the shell. Only a few of the more ancient types possess a sutural line
altogether simple, like that of the Nautilids. Almost always, even in
Paleozoic forms, the suture attained at least the Goniatite stadium, that

346

GEOLOGICAL BIOLOGY.

is, an undulating or notched suture formed of simple lobes and saddles
(Fig. 112).

FIG. ii2. — Development of the su-
ture of Goniatites diadema Goldf.
(After Branco.)

D ~~MJ\f\T\l\/\w

FIG. 114.— Suture of Ceratites nodosus.

FIG. 113.— Sutures of the various tribes of
Goniatites. (According to the Sandberg-
ers.) A = Linguati, G. tuberculosis cos-
tatus ; B= Lanceolati, G. Becker i ; C —
Genufracti, G. sphericus; D = Serrati, G.
saggittarius; .ZT=Crenati, G. intumescens;
F = Acutolaterales, G. terebratus ; G =
Magnosellares, G. retrorsus ; /zr=Nauti-
lini, G. subnautilinus.

A later complication is observed in the Ceratite stadium, in which the
saddles remain intact while, on the contrary, the lobes are notched by
slight denticulations. The more elaborate differentiation is reached in
the Ammonite stadium, in which the lobes and saddles are gashed by
secondary notches in the most variable manner.

FIG. 115. — Suture of an Ammonite, Desmoceras latidorsatum. (After Zittel.)

As the Goniatites appeared, in general, before the Ceratites, and these
in part before the true Ammonites, it is believed that these three genera
may be considered to be the three principal stadia of development of the
Ammonoids. This view is further confirmed by the fact that the suture
line of all Ammonites in the course of the first whorl passes through the
Goniatite stadium (Fig. 116, H to N), According to the researches of
Hyatt and Branco, however, the Ceratite stadium is, in general, passed

EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 34/

over and the Goniatite stadium passes directly into the Ammonite stadium.
The development of the sutural line by folding of the septum advances
from without inwards ; on the contrary, the new lobes and the new
saddles are intercalated, almost always, at the lateral
suture of the whorl, and rarely on the external ridge.

The second suture is distinguished from the first in
almost all Ammonoids by the development of an ex-
ternal ventral lobe, more or less deep, simple or bifid,
which gives rise to two external saddles caused by the
dichotomy of the original simple saddle. It is rare that
it is confined to these three elements; generally, there
is added besides a lateral lobe and a lateral saddle. In
the more simple forms the suture has by that time
acquired its definite shape, and all the later chambers
present the same design at the point of their attachment.
There generally occurs, however, a multiplication of the
lobes and of the saddles, and the external lobe takes part
in it by one small median saddle becoming bifid.

Such is the characteristic development of the suture
in the Goniatites, the Clymenias, and a small number
of the Triassic Ammonites. In the Ceratites and the
true Ammonites there takes place exactly the same differentiation at the
outset as in the Goniatites; but later, when the shell has reached the size
of 3 mm. in diameter, begins the secondary slashing of the lobes and of
the saddles of the exterior and of the interior. (See O of Fig. 116.)

At the size of 4 mm. the Ammonites are generally in possession of these
characteristic suture lines, which from that time on remain constant, or
at least suffer very slight change. In the determination of the several
species it is necessary to compare the suture lines of only the mature
forms. The external lobe does not tend to become bifid in the Goniatites
and Ammonites, the most ancient geologically, as in a stadium of relatively
tardy growth. In the relatively young Angustisellati the division into

116 — Develop-
ment of the suture
of an Ammonite
( Trobites subbul-
latus). G = ist
suture, H — 2d, I
= 3d, L = 7th, MN
— sutures of sec-
ond whorl, O =
definitive suture.
(After Branco.)

FIG. 117.— Suture of Pinacoceras Metternichi. (After Zittel.)

two lobes is distinctly accomplished. In a single form, or even in series
of forms, or in the most closely related species, the geologically younger
representatives generally possess the more differentiated suture lines;
on the contrary, however, it is not possible to deduce the geological age
of an Ammonite from the structure of the suture line alone. In the Trias
there are forms (Pinacoceras, Fig. 117) which present lobes so finely
slashed and so complicated that one can scarcely observe similar ones in

348

GEOLOGICAL BIOLOGY.

the most recent formations; on the other hand, there are known Am-
monites (Buchicera) from the Middle and Upper Cretaceous, the sutures
of which represent the Ceratite stadium (Fig. 118; also compare with Fig.
114) by retrocession, if they be not
quite the same genera. In all
typical Ammonites there is devel-
oped, besides the external ventral
lobe, which, in the forms with an
external siphuncle, is called often
also siphonal lobe, two main lobes
on the side — the first and second
lateral lobe. Besides the external
lobe, there are two large external
saddles; and besides the lateral
lobes, the two primary lateral sad-
dles. The external is almost al-
ways profoundly slashed into two
points by the development of a
secondary median saddle, while
the internal lobe (dorsal lobe) op-
posite ordinarily remains entire.
The external saddles are also able

to be divided sometimes by deep FlG- ««.— Tissotia Foumeli *Rxy\e. Cenoman-

ian, Algeria. (After Bayle.)
secondary indentations. In some

genera (Pinacoceras) the differentiation of the external part of the external
saddle goes so far that there are intercalated between it and the external
lobe a greater or less number of supernumerary saddles and lobes. All
the saddles and all the lobes from the second lateral saddle to the internal
contact suture of the whorl are called external; those which are within the
contact sutures up to the inner saddle receive the name internal auxiliary
lobes and saddles.

The variability in the number and size of the lobes is, generally,
in relation with the form of the shell. If the whorls are circular,
one observes, ordinarily, only a few lobes, and in that case they are of
nearly equal dimensions (Lytoceras); upon a wide ventral side the ex-
ternal lobe and the external saddle acquire considerable dimensions; the
more flat the sides are and the thinner the ventral part, the larger the
size of the lateral lobes and lateral saddles, and the more numerous the
auxiliary lobes.

Two Divisions of the Retrosiphonatae : Goniatites and Clymenias.
— In following the course of evolution of this group, as indi-
cated by the modifications of the suture-line, we begin with
the first division of the Ammonoidea — the Retrosiplionatce of
Fischer. The two groups are the Goniatites and the Clyme-
nias. The fundamental and constant difference is found in
the relative position of the siphuncle. In the Goniatites the
siphuncle is external and in the Clymenias always internal.

The Goniatitinse, of Hyatt's classification, begin in the

EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 349

Silurian and are dominant in the Devonian, and the undis-
puted Goniatitinae are not continuous beyond the Carbonif-
erous. Sagiceras and like forms are Triassic, and are inter-
mediate between this and the true Ammonite type. The
Goniatitidae (v. Buch, emend. Zittel) contain about 300 spe-
cies, all of which are Paleozoic.

Quick Evolution of the Clymeniidae. — Of the Clymeniidae,
about 30 species are known — all from the Upper Devonian.
When, however, the character of the suture is made the chief
means of classification, we find a considerable range of modifi-
cation in the Clymeniidae, and of the other characters: the
shape of body whorls, rounded, angular, tuberculated, etc.,
and amount of involution of whorls, all indicate great modifi-
cation, so that authors have classified even this special little
group of forms into many genera. Hyatt proposes 3 families,
with 9 genera in all, based upon the minute studies of
Giimbel. Hyatt remarks, regarding the Clymeniidae :

"This extraordinary series shows the phenomena of quick evolution
in three series of forms. Cyrtoclymenidse, with a series beginning with an
Arcestes-like form, and passing through discoidal and compressed to quad-
ragonal forms ; Cymaclymenidae, a similar parallel series, but with more
complex sutures; and Gonioclymenidae, also a similar series, but with more
involute forms than the last, and the sutures becoming Ammonitic, with
median ventral lobes and saddles, divided by a pair of marginal lobes." *

When we compare this series of suture-lines with those
of a single Goniatite, at different stages of individual growth
(Fig. 112), the evolution may be expressed as a case of rapid
acceleration, with some variation added.

Classification of the Goniatites. — The attempt to classify the
Goniatites by their sutures has resulted in various systems, in
each of which the particular form of the mature suture-line
has been the criterion of classification.

Beyrich proposed six groups, which he called (i) Nautilini,
(2) Simplices, (3) ^Equales, (4) Irregulares, (5) Primordiales,
(6) Carbonarii.

Sandberger made a more minute analysis, based upon the
form of the lobes and saddles making up the suture. His
nomenclature is: (i) Linguati, (2) Lanceolati (= ^Equates in
part of Beyrich), (3) Genufracti (= Carbonarii Beyr.), (4)

* See " Genera, Foss. Ceph.," p. 313.

35° GEOLOGICAL BIOLOGY.

Serrati (= Irregulares Beyr.), (5) Crenati (= Primordiales
Beyr.), (6) Acutolaterales, (7) Magnosellares (= Simpliees
Beyr.), (8) Nautilini (= Nautilini Beyr.). (See Fig. 113.)

Hyatt distributed the Goniatites into several families,
including in each the several groups based on sutural charac-
ters as follows: (i) Nautilinidae (Nautilini Beyr.), (2) Primor-
dialidae (Primordiales Beyr., and Crenati Sandb.), (3) Magno-
sellaridae (Magnosellares Sandb., Acutolaterales Sandb., Sim-
pliees Beyr., /./., and ^Equales Beyr.), (4) Glyphioceratidae
(Carbonarii Beyr., Simpliees Beyr., /./., Genufracti Sandb.,
Indivisi Bronn), (5) Prolecanitidae (Lanceolati, Linguati,
Serrati Sandb., Irregulares Beyr.). (Hyatt included here the
genera Medlicottia, Sageceras, and Lobites, referred to the
Ammonites by Zittel.)

Differences in the Sutures of the Ammonoidea explained as
Various Degrees of Crimping of the Edge of the Diaphragms. —
The sutures may be considered as simply the edges of the
diaphragm which is built by the animal across the conical
shell in which it lives, to constitute air-chambers of the va-
cated part as the animal grows in size. A simple explanation
is suggested by the mechanical principle that the natural result
of attempting to force a diaphragm into a tube too small for it
would be the crimping of the edges of the diaphragm. With
this clue applied to the interpretation of the sutures, we dis-
cover that all the various sutures may be defined in terms of
difference in degree of complexity of the crimping of the edge
of the septum.

Classification of the Types of Sutures. — Gathering statistics of
all the known forms, and studying their embryological devel-
opment as well as their actual differences, we find the follow-
ing facts to be true regarding the modifications of the suture-
lines which result from the crimping or fluting of the outer
margin of the septum where it is attached to the wall of
the chamber of the shell :

A. The Nautilian Type of Suture. — In the Nautilidae the
suture is simple, either straight or slightly curved, but never
folded, i.e., in its complete circumference not exceeding a
single oscillation of curvature (see Fig. 102). This is the
Nautilian or simple type of suture.

EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 35 1

B. The Goniatitic Type of Suture. — In the Goniatites we
find the suture lobed, forming rounded or bluntly angular
curvatures ; these curvatures in the simplest stage of the pro-
toconch are arched forward at the siphonal side (Fig. 112,
<z, b). In the growth of the individual, as well as in the dif-
ferent genera or subgenera of Goniatitidae, the lobation never
exceeds the repetition of these forward and backward curva-
tures of the suture. The multiplication of the curvatures is
accomplished by the infolding of the node of each curve (Fig.
112, d, e, f, g, and Fig. 113).

This constitutes the Goniatitic type of suture, and consists,
with all its complexity and variation, of a system of curva-
tures forward and backward ; the forward curvatures (upward
in the figure) are called saddles, the backward curves (down-
ward in the figure) are the lobes.

The various modifications of this type of suture are pro-
duced by different degrees of division of the lobes and saddles
in different parts of the circumference of the whorl. This
kind of bending of the suture may be called lobation of the
suture, and may be defined as the type of suture formed by
the primary crimping of its edges.

C. The Ceratitic, Helictitic, and Medlicottian Types of
Suture. — The primary lobes and saddles may be again
crimped so that the lobes are cut by a series of lesser lobes,
the saddles are dentate by secondary slits, or the sides of the
curves connecting the lobes and saddles are secondarily lobed ;
this modification constitutes a secondary system of lobation
of the suture ; and there are three stages of this mode of
crimping of the edge of the septum.

FIG. 119. — Suture of Medlicottia pritnas. (After Zittel.)

I. The Ceratitic type, in which only the lobes (L, /, a/l9
al^ of Fig. 1 14) or the backward curves of the septum edge
are secondarily crimped.

352 GEOLOGICAL BIOLOGY.

2. The Helictitic type, in which the saddles (see £S, LS,
Fig. 1 14) is alone secondarily crimped.

3. The Medlicottian type, in which the sides of the
saddles and lobes, or lines connecting them, are dentate or
secondarily crimped (Fig. 1 19).

To distinguish these three from the former type they may
be classified as the crenulated or secondarily crimped type.

D. The Ammonitic Type of Suture. — There is a still
higher complication of this system of sutures. The secondary
curvatures may be themselves tertiarily crimped or notched,
forming a tertiary system of lobation of the suture ; this gives
us the Ammonitic type of suture, and the suture is called
foliate to various degrees of elaboration in different genera.

E. The Pinacoceran Type of Suture. — A further extreme
of differentiation is attained in the crimping of the edge of
the septum of Pinacoceras of the Trias (Keuper), of which
twenty-seven species are reported, in them the tertiary lobes
are again dentate or crimped, forming the quaternary system
of lobation. This is the highest stage of elaboration recorded
for the suture line of the Ammonoids (Fig. 1 17).

Relation of Order of Succession of Initiation to Order of Ontoge-
netic Development and Evolutional History. — The natural law of
sequence of these various types of lobation of the suture is
that given above: (i) Nautilian, (2) Goniatitic, (3) Ceratitic,
(4) Ammonitic, (5) Pinacoceran, — that is, the order of succes-
sion is (ist) the simple, (2d) the lobed, (3d) the crenulate or
secondarily lobed, (4th) the foliate or tertiarily lobed, (5th)
the quaternarily lobed form of Pinacoceras, — and is so far an
arrangement of a series of related characters in normal pro-
gressive order.

The question naturally forces itself upon us, What rela-
tion has this normal order of sequence of the characters to
ontogenetic development and to phylogenetic evolution?

Order of the- Ontogenetic Growth of these Characters. — i.
First, in ontogenetic growth (as illustrated in Fig. Ii6)we
find this order to be the order of sequence in the develop-
ment of the shell of an individual. The first, or protoconch,
stage has a Nautilian or simple suture, or what is the primi-
tive form of that suture (Fig. 116, G, and Fig. in, A); the

EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 353

second stage (Fig. 116, ff) shows the formation of a siphonal
lobe by the indenting of the primary siphonal saddle. The
Goniatitic modifications are seen in the sutures K, L, M, O
of Fig. 116, and suture O expresses the combination of the
Ceratitic and Medlicottian types of crenate suture ; but it is
the secondary lobation clearly, although in this particular
specimen it has not its simplest expression.

This is the general law of ontogenetic growth as developed
by the authors who have specially examined these facts ; but
in Ammonites, as Zittel says, the Ceratitic stage is wanting or
passed over. This we may interpret to be due to the fact
that the Ceratitic type of suture alone is not expressive of a
stage of evolution ; but the true fact expressed by Ceratites,
so far as its relations to differentiation of suture line are con-
cerned, is its crenate or secondary lobation. This secondary
lobation may take place in the lobes, on the sides, or on the
saddles, and is a stage which, in the individual growth, is
quickly passed over ; the order of sequence is preserved by
the secondary lobation always preceding the tertiary lobation.
The particular part of the curved surface which first suffers
the secondary crimping appears to be the lobe, as is seen in
Trobites.

Chronological Succession of the Characters. — 2. When we
look at the chronological relations of this differentiation, we
find that the time of first appearance or initiation of the
several types of suture lines corresponds with the normal
state of differentiation of the character. That is, the Nau-
tilian suture line is the first to appear, in the Ordovician.
This continues to be the only one until the close of the
Silurian, when the Goniatitic suture line appears. These two
are the only types existing, so far as known, until we reach
a late Carboniferous stage — the Permo-carboniferous, or Per-
mian— when the third, the Ceratitic and Medlicottian types
appear, seen in the genera Sageceras, Medlicottia, and
Xenodiscus. But in this same geological period, in the Salt
range group of India, is found first appearing the form of
suture characteristic of the fourth or Ammonitic stage, in the
two genera Cyclolobus and Arcestes. Thus, before the close
of the Paleozoic faunas, as now defined, there is seen de-

354 GEOLOGICAL BIOLOGY.

veloped each, except the extreme Pinacoceran, stage of this
character. Immediately after, in the Trias, the Ammonitic
and Ceratitic types are both well developed and represented
by many genera. The historical order of initiation of the
several types of sutural modification is thoroughly consistent
with the order which an analysis of the nature of the modifica-
tions themselves suggests to be the natural order of sequence.

When we examine the order of sequence of the stages of
dominance of the several types of suture the former conclu-
sions are also confirmed. The Nautilian, the Goniatitic, the
Ceratitic and its modifications, and the Ammonitic and its
modifications, became dominant in the normal order. And the
appearance of the extreme Pinacoceran type in the Trias, with
its failure ever to become dominant, is in keeping with the
general principle that it is rarely the case that extreme modi-
fications of a type are either longest to live or the best
adapted to struggle with competing types of organization.

Rate of Elaboration of the Various Types of Suture. — 3. When
we look at still another relation of this series of facts, and
ask, What was the relative rate of expansion of this character
in comparison with the life-period of the race expressing the
modification ? we learn that regarding the character as origi-
nating in the straight Orthoceran form, the first stage of
sutural modification was reached when the first Goniatite
appeared ; this was near the base of the Devonian. The Cera-
titic and Ammonitic stages had both appeared before the close
of the Paleozoic, and by the early Trias the Pinacoceran had
appeared ; hence the extreme expansion of this character had
taken place between the base of Devonian and base of Trias,
but the life-period of this particular race of organisms reached
its close rather suddenly at the end of the Cretaceous ; and
we may infer that the extreme limit of modification of this
particular character had been attained before the race ex-
pressing it had half finished its course.

Rapidity of Modification of each Type soon after it was Initiated.
— 4. When we consider the degree and rapidity of develop-
ment in each of these types of suture-lines, we observe that
after the character had once appeared it was expressed in
numerous species and genera, and it expressed a tendency to-

EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 355

expand in a definite direction in all the lines which assumed
it, but its rate of development in the different lines was not
uniform.

The rapidity of development of this character may have
been determined, more or less, by environment, but the facts
seem to preclude the possibility of the determination of the
nature of the differentiation, or of the order of the sequence
of its expansion, by environment. We see here an exhibition
of evolution proceeding in a definite and continuous line of
expansion. It consists in a differential expansion in a defi-
nite direction and in a definite manner, by slow stages of
progress from generation to generation ; and it is as distinctly
a predetermined law of evolution for the race as increase of
size and development of organs is a predetermined law for the
individual living organism at its birth. Environment checks
or accelerates it just the same as temperature or climate affects
the vigor of growth of the tree ; but the law of expansion
from Nautilian to Goniatitic, and then to Ammonitic suture
is the only one which the race can follow out ; and the ex-
pression of this law is as sure to follow in case the genera-
tions succeed each other, as the tree is sure to bear its appro-
priate fruit in case it lives and grows.

Summary of the Laws of Evolution of the Suture-Lines of the
Ammonoidea. — The following may be given as a summary of
these interesting laws recognized in the history of the suture-
lines of the Cephalopod shells. The various suture-lines of
the chambered Cephalopod shells can be distinguished by the
differences in degree of complexity of the crimping of the
edge of the septum, viz. :

(a) In the Orthoceran and Nautilian type the edge of the
septum is straight, or the curving is not enough to produce
more than a single oscillation of the suture-line during its
complete circumference.

(b) The Goniatite septum presents a lobed suture, but the
edges of all the lobes and saddles are simple.

(c) In the third type the lobes and saddles are variously
crenulated. In the Ceratite the crenulation affects the base
of the lobes, in Helictites the top of the saddles is crenulated,
and in Medlicottia the lobes, the saddles, and the connecting
parts of the suture are crenulated.

356 GEOLOGICAL BIOLOGY.

(d) In the typical Ammonite there is a tertiary crimping
of the suture-line, i.e., each of the archings of the line corre-
sponding to the crenulations of Medlicottia is again crenu-
lated, forming a complexly foliate suture.

(e) In the adult forms of Pinacoccras there is a still further
elaboration of the crimping, the tertiary archings of the Am-
monite are again crenulated, forming a quaternary stage of
corrugation.

The series presents a gradual elaboration of the crimping
of the edge of the septum, forming a suture line, 1st, simple
2&, primarily lobed, 3d, secondarily corrugated (the crenulated
type), 4th, tertiarily corrugated (the foliate type), and 5th,
with the quaternary corrugations of Pinacoceras.

In their historical bearings it may be said of this series
that:

1. It is the order in which the various types made their
first appearance in the geological series.

2. It is the order in which the several types became
dominant.

3. It is the order of elaboration in the ontogenetic growth
of the individual.

4. It is the normal order of mechanical relation borne
by the several types to each other; each type is a mechanical
elaboration of the next preceding type.

The convolutions of the suture are crimpings of the edge
of a more or less flat disk, — the septum, — and these convolu-
tions are the simplest mode of adjustment of the edge of
such a disk, whose circumference increases more rapidly than
its radius.

Considering only the differences in the sutures, it would
be correct to state that if we assume that the one is derived
by modification from the other, it would be mechanically im-
possible for the Ammonite's septum and suture to be formed
without passing through the stages represented by the
Nautilus, Goniatites, and Ceratites. In other words, this
exhaustive analysis of this one element of structure of
cephalopod shells shows us that the actual history of these
organisms has been exactly that which a serial classification
on the basis of differences of this part would suggest, and

EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 357

that no other classification or order of succession could take
place by natural descent.

Evolution of the Suture results in the improvement of the
Structure of the Shell. — When we look at the complex foliated
septum of the Ammonite in relation to its use, we are struck
with the economical use of materials for greatest strength
with least weight. The principle of using thin plates of
corrugated material in place of solid supports in engineering
and building is well understood by man, and from this point
of view it appears evident that the result of the evolution of
the cephalopod septum has been the improvement of the
device concerned.

In conclusion, the analysis of the structure of the Cepha-
lopoda, based upon a comparison of the different modifica-
tions of their structure and upon the historical study of the
fossil remains of this class of animals, shows very clearly that
there is an intimate co-ordination between (a) the morpho-
logical differentiation of the characters, and (U) the historical
sequence of initiation and of dominance in numbers of the
individuals exhibiting them. Thus we notice, upon exami-
nation of the characters of the two great divisions Tetrabran-
chiata and Dibranchiata, that the group which appeared
later, and after the first had flourished and the great majority
of its families and genera had become extinct, was the one in
which is found the greater amount of differentiation of each
of the characters by which the two groups are distinguished.

It is also to be observed that, among the characters, in-
cluding all that is known of the group, by which the grand
divisions of the Tetrabranchiata are discriminated those which
were less differentiated morphologically were first to appear.
In the case of the modification of the sutures, about which the
facts have been minutely studied, the types follow each other
in regular successive order from the less differentiated to the
more highly differentiated ; and the same order is observed in
the numerical dominance of the several types. We notice
also that this order of increasing differentiation, which may
be traced in the case of the suture of the Ammonoidea, is
the natural order of evolution when viewed from the points
of view of (a) mechanical differentiation, that is, the greatest

358 GEOLOGICAL BIOLOGY.

amount of effective use for the least expense of energy or
material; (b) from the point of view of ontogenetic growth,
that is, the natural order by which the structure is produced
in the normal growth of an individual organism ; and (c)
from the point of view of historical sequence.

But this is not a case of the survival of the fittest, — it is
the evolution of the fittest, — and, from this point of view, too,
it is not the fittest that survives; for of these ancient forms it
is the Nautilus, and not the Ammonite, that survives; but of
the order of initiation there is no mistake — the Ammonite does
not appear before the Nautiloid; and the sequence Goniatite,
Ceratite, Ammonite is not reversed, but is the order which
the structure would suggest. The general law of survival
of the fittest is exhibited in the general dominance of one
type over another, but a structure once developed may persist
entirely beyond the period of its relative importance or rela-
tive stage of perfection, as is wonderfully exhibited in the
Lingulas of the modern sea, which are traceable back to the
Cambrian period through a line of ancestry that was very
highly modified in many parallel lines, of which only Lingula
survives.

CHAPTER XX.

THE LAWS OF EVOLUTION EMPHASIZED BY THE STUDY
OF THE GEOLOGICAL HISTORY OF ORGANISMS.

Testimony of Vertebrates. — The vertebrates might be used
with great force to illustrate the general laws of evolution.
No better example than the vertebrates could be selected to-
illustrate the fundamental law of the gradual inciease, in
differentiation and in rank, of the great classes of a branch in
the order of their successive appearance and dominance in the
geological formations.

In the lowest system of stratified rocks, the Cambrian, no-
trace of vertebrates has yet been found. In the Ordovician
and Silurian only the lowest type of fishes, and they very
rare, have been seen. Fishes were abundant in the Devonian.
The Lower Carboniferous shows the first amphibians; and
large-sized and extinct types of amphibians prevailed in the
Carboniferous era. In this era also a few traces of true
reptiles have been found. In the Triassic the great Dino-
saurian reptiles were abundant on the land. In the Jurassic
the shallower seas swarmed with the Enaliosaursor sea-lizards,
and in the lower Jurassic (Lias) the flying reptiles infested the
air and culminated the reptilian domination of the Mesozoic
time.

While reptiles were the masters of sea, land, and air, the
lower types of mammals — the marsupials, and probably
monotremes — began to appear in feeble representatives as
early as the Triassic, and in the Cretaceous birds, too, make
their appearance : though true birds in structure, they com-
pete with the flying reptiles in their use of reptilian teeth
for offence and defence.

Remarkable and Extreme Evolution of the Mammals in the
Eocene. — As we examine the earlier beds of the Tertiary rocks
we observe for the first time the dominance of mammals ; and

359

360 GEOLOGICAL BIOLOGY.

perhaps no more remarkable fact is established in the history
of organisms than the sudden expansion of the placental mam-
mals in the Eocene.

Over fifty genera, representing the chief ordinal types of
the placental mammals, are already reported from the lowest
Eocene, none having been discovered in the underlying Creta-
ceous. In Europe alone Zittel reports for the fauna of the
Upper Eocene about 1 10 genera and about 200 species. To
:show the richness of this fauna, in spite of the imperfection
of the records, he cites the facts that " Our present European
land mammalian fauna contains 54 genera with about 150
species, and of these 60 per cent belong to the microfauna,
consisting of the smaller forms of Rodents, Insectivora, Bats,
and Carnivora, for which the conditions of preservation in
earlier epochs were very unfavorable" (" The Geological De-
velopment, Descent, and Distribution of the Mammalia," by
Karl A. von Zittel, Geol. Mag., Dec., III., vol. X., Sept.,
Oct., Nov., 1893).

If we glance at the whole group of mammals, we find the
actually known forms included in three subclasses : the (I)
Prototheria, with the order Monotremata; (II) Metatheria,
represented by the order Marsupialia; and (III) Eutheria, or
the Placentalia.

There can be no doubt as to the higher rank of the Pla-
centalia over the marsupial and monotreme types. No cer-
tain traces of the Placentalia are known to occur below the
Eocene. Stegadon, a genus of the Tillodontia, is thought
to have appeared possibly in earlier beds.

Of these mammals, ten orders, fossil and recent, are recog-
nized. Two of these are marine — Sirenia and Cetacea.
The Edentata is a South American order, and has its repre-
sentatives in the earliest known South American mammalian
fauna (Vera Cruz fauna of Patagonia), which is probably
equivalent to the northern Eocene.

If we omit the above three orders, of the remaining seven
orders of land mammalia five are represented in the older
Eocene of Europe — the Ungulates, with 5 suborders; the
Rodents, the Insectivores, the (Carnivora) Creodonta, the
Prosimiae — the forerunners of, if not true, Primates.

THE LAWS OF EVOLUTION EMPHASIZED. 361

True Carnivores appeared in the newer Eocene, Chei-
roptera in the middle Eocene, and true Primates in the older
Miocene.

In these orders of placental mammals 56 genera appeared
for the first time in the older Eocene, and there were succes-
sively added to them, in the middle Eocene 40 new genera,
in the newer Eocene 105 new genera, Oligocene 5, older Mio-
cene 49, newer Miocene 34, Pliocene 27; or previous to the
opening of the Pleistocene 260 genera, distributed among the
seven land orders of mammals, of which the first traces were
obtained from the older Eocene beds of North America and
Europe. The Australian, South American, and African types
are not here included ; and it must be remembered also that
new discoveries are constantly adding to these statistics, and
in general they augment the earlier more than the later totals.

Again, the fact that (? Prototheria and) Metatheria were
already well developed in genera in the Mesozoic does not
lessen the significance of the remarkable expansion of the
mammals in the older Eocene period ; nor does the imperfec-
tion of knowledge lessen the testimony to the relatively-
sudden expansion which the evidence now in hand indicates.
The approach to recent time, and the increasingly better rep-
resentation of the land faunas among the preserved remains,
does not invalidate the truth of the general proposition, that
all the grand features of structural modification, expressed in
the subclass of placental mammals, made their appearance in
distinct genera with great rapidity at the first stage of ap-
pearance of the Placentalia.

The prominent differences, expressed in the limbs, teeth,
form, and habits, in the hoofed animals, the odd and even
toes, the gnawing rodents, the flesh-eating Carnivores (Creo-
donts), the insect-eaters, the flying bats, and the climbing
monkeys, were all seen among the members of the first fauna
of the new type of placental mammals, in the Eocene period.

Synthetic Types Illustrated by the Vertebrates of the Mesozoic.
— No better illustration of the principle of the " synthetic"
or " comprehensive" character of early types of organization
is to be found than that presented by the Dinosaurian rep-
tiles and the reptilian birds of the Mesozoic. Here we find

362

GEOLOGICAL BIOLOGY.

biped reptiles, three-toed and with avian pelvic structure ;
flying reptiles, with beaks instead of teeth ; birds with teeth,
and birds with long vertebrated tails.

So many points of combination of features have been seen
in the Mesozoic fauna, which are now only found separated
in the two great classes Aves and Reptilia, that zoologists
have been forced to provide an intermediate group to include
these ancient types, or to expand and combine the two classes
into the one superclass Sauropsida of Huxley.

Specialization of Five Fingers in Reptiles and its Relation to
Later Specializations. — The principle of synthesis, or combina-
tion, in an early type, of the characteristics of two or more sepa-

TV

'FiG. 120. — Left forefoot of A, Phenacodus primavus Cope, Eocene ; /?, Hyracotkerium venti-
Colum Cope, Eocene ; C, Paleotherium medium Cuv., Oligocene ; ./?, Anchitherium aureli-
anense Blainv , Miocene ; £, Hippotherium gracile Kaup., Pliocene ; F, Equus caballus L.,
Recent. / = lunar ; m = magnum ; p = cuneiform ; o — scaphoid ; / = trapezoid ; tz —
trapezium ; u = unciform ; I-V = ist to 5th finger or metacarpal bones ; me — metacarpal.
(Steinmann and Doderlein.)

rate types of a later stage, is seen in the case of the Permian
reptile Mesosaurus tumidus Cope, in which five tarsals are
present, rather than four — the normal number of later rep-
tiles. Such a fact shows, according to Cope, that five is the
primitive number of tarsals, and that four is a specialization —
just as we find in general in the evolution of paws, feet, and
hands the full number of parts was provided before the spe-
cialized reduced number was evolved. The fewer number of
fingers or of bones, entering into the mechanism of the foot
or hand, is the result of selection and specialization of parts
rather than the direct production of any new function or part.
The Eocene Phenacodus primcevus Cope illustrates this princi-

THE LAWS OF EVOLUTION EMPHASIZED. 363

pie in the evolution of the forefoot of mammals, as shown in
the figure on the opposite page.

Finger-bones and Teeth as Tests of Degree of Differentiation. — In
tracing the history of mammals we find the principle of five
fingers already developed before mammals began. Hence
the wonderful modifications noted by Owen, Kowalevsky,
Ryder, Marsh, Cope, and others, in the arrangement of the
bones of the mammalian feet, their specialization in form, and
relative size, shape, and position, have constituted the chief
data for both classification and phylogenetic series.

The teeth, as highly specialized organs, and as terminal
parts of the individual organization, coming into most im-
mediate contact with the outside elements of resistance to
the life of the individual, are particularly sensitive expres-
sions of the stages of evolution.

Any device of offence or defence, particularly when hard-
ness and resistance to attrition are characteristics of its struc-
ture, becomes at once a mark of the effects of environment
in inducing modifications, and of the stage of progress
attained by the individuals in their evolution. Their resist-
ance to destruction makes such parts most valuable records
in the rocks of the history of organisms.

Laws Derived from the Study of the Teeth of Mammals by
Osborne. — ProfessorH. F. Osborne, following the investigations
of Riitemeyer and others, has recently written several instruc-
tive papers setting forth the laws to be observed in the
history of the development of the teeth in mammals.

In a memoir (first read as the address of the vice-president
of the section of Zoology, of the American Association for the
Advancement of Science*) he narrates both concisely and
admirably the laws expressed in the modification of the cusps
or surface forms of the teeth of mammals.

Osborne shows how the tricuspid tooth is an evolution
from a simple monocuspid tooth, which is the primitive type
of tooth in all earlier vertebrates. He shows further that the
multiple succession of teeth characteristic of reptiles is the
primitive method of arrangement, and this, as is also the in-

* " The Rise of the Mammalia in North America," Am. Jour. Set.,
III., vol. XLVI., pp. 379-392 and 448-466.

3^4 GEOLOGICAL BIOLOGY.

definite number of teeth of the reptilian jaw, is a natural
preliminary condition to the high specialization of the teeth,
with particular form for each.

The selection and specialization seem to be brought about
by the suppression of part of the multiple series, and the
modification of the teeth retained in different parts of the
jaw for special function.

In the primitive Marsupials and Insectivores, he observes,
the regular reptilian succession was early interrupted, while
in all the higher mammals the reptilian succession of two
series was retained in the anterior part of the jaw. In the
Edentates and whales retrogression takes place in fins as well
as in teeth; it is the first set of teeth that persists, the second
set being represented by a rudimental row of tooth-caps buried
in the jaw.* He concludes that there is strong evidence that
the stem mammals had a uniform number of each kind of
teeth and a uniform dental formula; that homodontism is
secondary, and was actually preceded in time by heterodont-
ism in the mammalian dentition.

The ancestral formula for both Marsupials and Placentals,
according to this author, is: incisors 4, canines and pre-
molars 5, molars 4. By adopting Rose's suggestion that in-
cisor 5 of the marsupials belongs with the second series of
incisors, he supposes that Placentals have lost one incisor and
one molar from the primitive formula. The paper is an im-
portant contribution to the interpretation of the method of
evolution, and must be studied with care to be fully appre-
ciated; the author's conclusions are quoted on page 324.

For the purposes of this treatise a sufficient number of il-
lustrative cases has now been presented to show where the
emphasis is placed by the facts of geological biology as to the
true factors of evolution. A great many examples crowd
themselves upon the attention which must be left for the
student to investigate directly and in detail. The evidence
to be derived from the study of living plants and animals is
so vast, that a special treatise would be necessary to do justice

THE LAWS OF EVOLUTION EMPHASIZED. 365

to either, and the reader may find many admirable treatises
giving account of this aspect of evolution.

Method and Purpose in the Selection of the Evidence here
Set Forth. — The facts which have been selected in these chap-
ters have been chosen for the purpose of ascertaining what
the geological history of organisms has been.

Examples have been taken and analyzed to ascertain what
has been the .particular law of succession in particular cases
where the evidence was full enough to be relied upon. If the
interpretation of these selected cases has been correct, the
principles discovered may be applied to other cases.

The facts have been examined for the purpose of learning
(i) what the fossils indicate has been the order of succession
in the initiation of different forms of organisms; (2) what rela-
tion this succession bears to the relative importance of the
characters in the economy of the individual organism, as
shown by the systematic classification of the Animal Kingdom ;
and (3) what have been the determining causes by which the
multitudinous differences in organic structure have been
brought about. The first consideration in their selection
was that they should be from among those of which the most
perfect record is preserved. The cases already cited in evi-
dence are not selected because they are the most important
examples, nor because they illustrate only the most impor-
tant laws of evolution, but they are selected because they are
the best examples to show what the geological records testify
regarding the history of organisms.

Different Kinds of Evidence Borne by Living and Fossil Organ-
isms.— Living organisms present the best evidence of the laws
of ontogenetic development, because they furnish illustration
of each stage in the development. A continuous series of the
stages of development of a single organism is more satisfac-
tory evidence of the essential nature of that development,
than would be any number of detached exhibitions of sundry
stages of development of different organisms.

So it is believed that the evidence borne by a series of
fossils preserved in each stage of the geological record, of
which specimens are well preserved and described from the
first to the last, and which show the beginning, dominance,

366 GEOLOGICAL BIOLOGY.

decrease, and extinction of the type they represent, is of the
highest value as evidence of the actual order of evolution
and of the general laws by which differentiation of form has
taken place. And a few such cases far outweigh any num-
ber of detached specimens tied together by theoretical links.

Natural Selection seems Eeasonable when Based alone upon
the Study of Living Organisms. — When we observe living
animals in competition — the vigorous ones living and the
weaker dying, the strong overcoming and devouring the
weak, the large and fewer in number making their daily
food of the smaller and more abundantly produced ; when we
note how the places for the greatest abundance of individuals
are determined by the presence of favorable conditions for
obtaining food ; and thus, in general, when we observe that
animals as they are are actually adjusted, each to its own
most favorable conditions of environment — it seems reason-
able to draw the conclusion that the differences distinguishing
one animal from another may have arisen as the result of
better fitness for the struggle for existence on the part of
those which survived and carried on the race.

Having once assumed that the law of evolution is a proc-
ess in which the chief active determining force has been nat-
ural selection by the survival of the fittest, it was easy to find
illustrations of adjustment of structure and function to the
conditions of environment among fossil, as has been done
among living, organisms.

Every Species of Organism that has Flourished in the Past the
Fittest for its Place and Generation. — When, however, we stop
one moment to consider the relations of organisms in the past
to their own environment, it becomes evident that, at every
particular stage in the geological history of organisms, the in-
dividuals then existing must have been as thoroughly well
adapted to live under the conditions of their environment as
the present inhabitants are adapted to live in their environ-
ment. Every organism that has lived on the earth has in
some sense been the fittest to live in the particular time and
conditions it occupied.

If environmental conditions (outside of organic environ-
ment) have determined the evolution of organisms, then we

THE LAWS OF EVOLUTION EMPHASIZED. 367

are obliged to assume a degree and amount of change in them
of which the facts of geology give no evidence.

If the conditions which have changed with the geological
ages have been the organisms themselves, and they have con-
stituted the environment, then it becomes necessary to ex-
plain the more powerful contestants before their selecting
agency can result in the survival of fitter races.

But leaving aside for the present the philosophical argu-
ment, the burden of these pages is to show what is in fact
the testimony on these questions furnished by the organic
history as found in the best-preserved parts of the record.

As previously explained, the records which are made at
the place and time of the formation of the rocks are those
which must on that account be the most perfect we can con-
sult. The rocks bearing fossils are not wholly, but are in the
large majority of cases, of marine origin. This determined
the selection of the evidence from among marine animals.
The animals of which the best records could be preserved in
the rocks are those secreting hard parts — shells, or corals, or
similar parts; hence the examples have been taken chiefly
from the corals, the Mollusca, and Brachiopods.

The Geological Evidence does not Emphasize the Importance of
Natural Selection as a Factor of Evolution. — What has already
been said is sufficient to show that the emphasis of the
testimony brought forward differs from the emphasis drawn
by the embryologist, or by the student of living organisms, as
to the relative prominence of the several factors in the evolu-
tional history of organisms.

That which has seemed most conspicuous to the latter
class of observers has been the intimate relationship existing
between morphological difference and environmental condi-
tions ; paleontological facts point to the greater importance
of the continuous and progressive process of differentiation
and specialization of structure and function with the passage
of geological time.

The law of natural selection, suggested to explain the evo-
lution from the first point of view, calls for an extremely slow
rate of modification, but uniform and continuous. The facts
of the history itself point to the reality of rapid strides at

368 GEOLOGICAL BIOLOGY.

critical points, with long periods of almost absolute cessation
of progress ; and suggest that the part played by what is called
natural selection has determined rather the particular indi-
viduals and the place and time for advance steps, than, either
the direction of the steps themselves, or the relative value of
the particular modifications in relation to continuation of the
race, which have taken place.

The study of the actual facts of the geological history of
organisms points unmistakably to a course of evolution by
descent, in which the progress attained by each succeeding
form was a paramount condition of the origin of the next
member of the race.

Objection may be taken to an argument based on so few
examples. I think the force of this objection will be lessened
when we bear in mind that the examples were selected pri-
marily because of their fitness to testify upon the points in
question, viz., the law of the history of organisms, the nature,
the rate, and the order of modification of form, which organ-
isms actually undergo in producing that divergence of specific
forms observed at any particular stage of the history.

It may be said that the particular kinds of animals select-
ed do not fairly represent the total life of the world. To this
objection the reply may be made that a full quota of diversity
of specific forms has been attained by the races examined,
and the chief question before us is, How has that diversity
arisen ?

If the facts we have examined do not support the hypoth-
esis that the chief factor in organic evolution is either external
environment or natural selection, it is not on account of any
lack of fitness to testify on this point, if it were true.

The facts examined — and we believe that fuller examina-
tion of other statistics, both fossil and recent, will support
the same conclusion — show that evolution is rather an intrinsic
law of all organisms, and is to be discovered in the phenomena
of variation, which appear to be constantly active, rather than
in -any accidental operations dependent upon the conditions
of external environment.

The emphasis is placed upon the intrinsic rather than the
extrinsic factors of evolution, as the actual determinants of

THE LAWS OF EVOLUTION EMPHASIZED. 369

the results attained by evolution in specific, generic, and the
higher orders of differentiation.

A Statement of the Laws of Evolution Emphasized by Fossils. —
The analysis of the facts regarding the order of succession and
modification of organisms derived from this critical study of
fossils suggests the following to have been some of the chief
laws of the evolution by which the present conditions of the
organic world have arisen :

(ist) An orderly succession in the geological history of
organisms, which in the main has resulted in an increasing
differentiation of structure and specialization of function with
the progress of geological time. The general name for this
process is evolution.

(2d) While the whole organism is concerned in this evo-
tion, certain parts of an organism (or certain of the morpho-
logical characters) exhibit the evolution more rapidly than do
other parts or characters.

(3d) When these characters are arranged in the order of
relative rank of importance in the economy of the organism,
the characters of least importance (the varietal and specific
characters) exhibit the evolution most constantly and persist-
ently, but at a very slow rate, chronologically considered.

(4th) The characters of higher rank (the branch, class,
ordinal, and family characters) were relatively more rapid in
the expression of their initial evolution and thereafter were
very constant in each successive race.

(5th) These two tendencies are expressive of the two
fundamental laws of evolution — variability and heredity.
Variability is recognized as a common law of organism, ac-
cording to which, in the ordinary process of generation,
slight changes are continually taking place in the morphologi-
cal features of the offspring as compared with the parent form.

Heredity is a common law of the organism, according to
which a character once acquired in the parent tends, in the
process of ordinary generation, to be repeated with increasing
precision, and to result in the transmission of characters with-
out change from generation to generation. The process of
evolution is the combined result of the interaction of these two
antagonistic laws of the organism.

37° GEOLOGICAL BIOLOGY.

(6th) The mode of the evolution consists in the acquire-
ment of new characters by variation, and in the acceleration
or the retardation of the development of characters already
acquired.

(/th) The cause of the evolution is of a twofold nature —
extrinsic and intrinsic.

In the first case, extrinsic evolution, the direction and
specific character of the modifications appear to be determined
by the conditions of environment — using that term in its broad-
est sense for all the outward conditions of life in which the
individual organism finds itself after birth. Adjustment of
the organism to the environment, struggle for existence, and
natural selection are the terms under which extrinsic evolu-
tion is commonly defined.

The intrinsic cause of evolution acts previous to the indi-
vidual birth, and it seems to be at the foundation of varia-
bility. The mode and manner of expression of this kind of
evolution are more difficult to define than in the case of ex-
trinsic evolution ; but the facts of Paleontology clearly indicate
that such a cause exists, prior to the morphological appear-
ance of each individual and species.

(8th) In this discussion classification is recognized as an
orderly and epitomized formulation of the facts already known
regarding the extent and kind of differentiation actually at-
tained in the evolution of the characters of organisms. The
statistics of classification are therefore available for expressing,
numerically, the relations existing between organic characters
and time and place ; and it is observed that the numerical re-
lations of the different kinds of organisms to the time and the
place of their appearance point with overwhelming force to
the conclusion, that acquirement of morphological difference
is co-ordinate with both the passage of geological time and the
divergence of the conditions of external environment in which
the organisms have lived.

CHAPTER XXI.

PHILOSOPHICAL CONCLUSIONS REGARDING THE CAUSES
DETERMINING THE COURSE OF EVOLUTION.

What is the Philosophy of Evolution ? — Statement of the Case.
— In the foregoing chapters a few of the prominent facts re-
garding the history of organisms have been examined,
and the primary conclusion from their study is that
the method of acquirement of all that is characteristic of
organisms has been evolutional. Evolution is a matter of fact
in the description of the history of organisms; but there re-
main for consideration the questions, Why should organisms
express a law of evolution ? and, What are the immediate con-
ditions determining the particular steps of evolutional his-
tory ? and, finally, What is the rational philosophy of evolu-
tion ?

The theory of natural selection may be so applied as to
lead to the philosophical belief that difference in the condi-
tions of environment is the primary cause of the differences
expressed in the form and functions of organisms; and sec-
ondly, the theory of the unchangeableness of matter and the
universal conservation of energy may be carried so far as to
lead to the belief that in the matter of organism, under the
names germ plasm, biophors, pangenes, gemmules, physiological
units, or some other names resides the power and potency
of all that is evolved in the course of the total history of
organisms.

Are these beliefs incident to the proposition that evolu-
tion is a fact in nature, or is there a philosophy of evolution
which more completely recognizes the whole body of facts in
the case?

The Point of View. — If we were to discuss such a common
topic as the weather, we would find that, although every-

37i

3/2 GEOLOGICAL BIOLOGY.

body has his notions as to the cause of the various changes in
that very variable phenomenon, the likeness and differences
in the theories advanced are determined primarily by the
point of view in relation to land and water of their advocate.
Land and water are sharply contrasted, natural and familiar
phenomena; but the Bostonian is accustomed to look to
the eastward for his ideal expanse ol water, and for him the
land extends from the solid terra firma upon which he walks
for unmeasured miles to the westward. The man of
St. Louis is familiar enough with land, but the ocean is a
foreign thing to him; it does not come into his every-
day reckoning. At San Francisco the Bostonian's notions
are simply reversed; the point of view is totally different.

Residents of these three cities, unless they were to ad-
just their definitions to the points of view of their compan-
ions, could not talk about even the weather without constant
misunderstanding.

The Act of Evolving as well as the Order of Events Included in
the Discussion. — In the same way it may be said that some of
the chief misunderstandings and differences of opinion regard-
ing the problems of evolution are due to a failure to appreci-
ate the differences of philosophical attitude from which the
matter is viewed.

Evolution is concerned with two very distinct fields of
human inquiry. On the one hand, evolution is the name for
the natural order of unfolding of the characters of organic be-
ings that have lived on the earth ; on the other hand evolution
is the name for our conception of the mode of operation of the
fundamental energy of the universe. Thus it will be seen that
the notion of God is as intimately involved in a discussion of
evolution as is the notion of organism ; in elaborating the
definition of the one we consciously or unconsciously elabo-
rate* our definition of the other. We are obliged to consider
the act of evolving as well as the results of the evolution.

The Course of the Discussion. — In the present discussion
the reader has been led step by step from the detailed, sta-
tistical description of actually existing objects of nature, in
their relations to time and space and to each other, through
the consideration of their classification on the basis of order

PHILOSOPHICAL CONCLUSIONS. 373

of arrangement, proportion, and intricate relationships of struc-
ture and function, up to a consideration of the scientific ex-
planations proposed to account for them. We have passed
from the promiscuous array of facts, through analysis and
systematic classification, to the reasons for the classification
and the interpretation of the meaning of it all in terms of
force and cause.

So long as we deal only with sequence of forms, and con-
sider only the relation of particular forms to particular places
in the series, evolution is simply an analysis of the order of
events. When we step one side or the other of this simple
process of the narration and classification of facts and events,
we leave the field of scientific observation and are dealing
with the principles of causation. It is useless to disregard
the philosophical side of the study of nature, and it is a mis-
taken notion to think that those who spend their time in
measuring and recording phenomena have no need to con-
sider the meaning of such terms as cause and effect which
elude actual observation.

Darwin's Origin of Species Centres its Interest in the Search for
Causes. — Darwinism is an attempt to find a cause for the dif-
ferences in form and function observed in the organic world,
and the search for this cause has aroused a world-wide interest
in observing and recording the phenomena of nature ; but the
real stimulus inspiring all the investigations has been the ex-
pectation of discovering somehow the true cause of these
things in some visible, tangible and describable form. Origins
and creations have been said to be discovered in the search,
but the calm philosopher knows full well that the origins
described have been only apparent origins; they have not
reached to the essence, or to a fundamental explanation of
nature.

The Evolutional Idea of Creation. — It has been supposed by
many that evolution is intrinsically antagonistic to, and has, in
fact, replaced the creational conception of the origin of things
in the world. In one respect this is partly true ; the new
view has fundamentally changed the conception of creation.
Evolution has given us another notion of God. In the
old conception God was an artificer making organisms out of

374 GEOLOGICAL BIOLOGY.

inorganic matter directly, as one might build up a vessel of
clay and then vivify it. The new conception of God as creator
finds its concrete, empirical representation in the act of ex-
pressing a thought or purpose into the spoken word. Creation
is the phenomenalizing of will, so sublimely described in that
ancient formula, In the beginning God spoke and it (the whole
phenomenal universe) became.

The origin of the universe is thus the becoming phe-
nomenal of an eternal purpose; the only alternative is to
deny all origin, and to assume that the phenomenal universe
itself is eternal.

Evolution the Mode of Creation of Organic Beings. — And, as
we have seen, the great distinction between organic and in-
organic matter consists in the evolving of the organic characters
in an appreciable and often very slow course of time ; whereas
the qualities of inorganic matter were originally committed to
the particular matter, which has continued to exist from the
beginning without change.

The slowness and continuity of the process of organic evo-
lution is thus an evidence of the continual presence of crea-
tive energy in the world, and the permanence of qualities of
inorganic matter is evidence of the ultimate distinctness be-
tween the created and the Creator. The human mind is
utterly incapable of accounting for intrinsic differences in the
universe except by conceiving of some mode of their origina-
tion, and we have not explained their origin by simply saying
that they have evolved.

The change which the speculations of the last fifty years
have wrought in the notion of creation has been a most im-
portant and radical one. There has been substituted for the
old idea of an artificer constructing a machine out of materials,
with the addition of his making his own materials out of
nothing, the higher conception of the transformation of a
conscious purpose into physical action — the visible expression
of invisible will.

The Properties of Matter Coexistent with it, and either Eternal
or Created. — The new notion of creation does not include the
idea of the making of something out of nothing, but it does
mean that what has existed already in one state of being

PHILOSOPHICAL CONCLUSIONS. 375

(which we describe under the simile of purpose of the eternal
mind) becomes expressed in another realm of existence (which
we describe in terms of form and function of living matter).

When we define matter as being of various elemental
kinds, their differences being expressed by their behavior
under sundry conditions, and called properties or qualities,
we proceed on the assumption that these properties are char-
acteristic of the particular kind of matter, and have been from
its first existence, so that there is no evolution : the properties
are either eternal or were immediately created as they are.

In the case of organisms it does not free us from the same
conclusion, if we liken their characters to the properties of
matter, and imagine that there is some original endowment of
differences which gradually finds expression by evolution.

If we attempt to treat the characters of organisms as if
they were properties of matter, we are forced to imagine
infinite and inconceivable ultimate units, like atoms, of which
the original organic matter of ancestral organisms was com-
posed, and it has been found necessary to endow these units
with qualities of persistence and definition, of will and deter-
mination, of power over the environment in which they reside,
and of judgment of the value of the to-be-attained morpho-
logical structure and functional activities of the organisms,
which in the creational idea are ascribed to the will and mind
of the Creator.

Any one who is not already prejudiced against the notion
of God cannot fail to see in the theistic view of the Creator, in
which eternal will and purpose constitute the powers and poten-
cies back of phenomena, a more rational and satisfactory
theory of the universe than the materialistic view in which
the same powers and potencies, invisible and infinitesimal, are
made to be the endowments of an infinite number of undying,
determinant, organic units.

Evolution does not apply to the Mode of Becoming of Chemical
or Physical Properties of Matter, but is the Distinctive Characteristic
of Organisms. — In the case of chemical and physical properties,
as related to particular material things and on the assumption
that matter is not eternal, their creation can be considered only
as having been immediate, since our whole science of physics

GEOLOGICAL BIOLOGY.

and chemistry is based on the assumption that these properties
persist without change.

But in the case of organisms their characters are constantly
changing, and evolution as a theory is based upon the assump-
tion of not only constant but progressive change. The origi-
nation of the organic characters was not done all at once, but
evolution as the mode of creation of organisms has been more
or less continuous throughout the geological ages. It is this
continuation of the process of phenomenalizing that distin-
guishes the mode of creation in the organic realm from that in
the lower realm of inorganic matter. Whatever is character-
istic of organisms was not created at once in any remote be-
ginning, but has been unfolded by degrees, and there is no
reason for supposing that the process is not still going on.
Such expressions as "effort," "growth force," "conscious
endeavor, "reactions, "producing modification." "deter-
mination," " memory, " etc., used in describing the phenomena
of evolution, all express the notion of the pre-existence of
some unphenomenal property, or power, or potency, which
constitutes the cause of the particular characters which are
acquired by organisms in the process of their evolution.

The Evolutional Idea an Enlargement of the Conception of God
as Creator. — On the assumption that the ideas of creation ind
Creator are fundamental to a rational explanation of the
universe — and such an assumption seems to be a logical neces-
sity to account for any intrinsic heterogeneity — we observe
that the effect of adding the idea of evolution to creation en-
larges the conception of creation by making it a continuing
process instead of an ancient act, and brings God into the
midst of the present universe.

The purpose of the living God then becomes immanent
by continuously phenomenalizing itself into living form. God
thus becomes a living, present, active reality in the existing
universe, and the course of the evolution of organisms be-
comes in a true sense the history of creation. This term
" Schopfungsgeschichte " was chosen by Haeckel for the title
of his treatise on the laws of evolution, and in one of its
closing chapters he acknowledged that there are only two ways
of accounting for the original organisms — spontaneous genera-

PHILOSOPHICAL CONCLUSIONS. 377

tion or creation.* Both of these hypotheses are alike in recog-
nizing that nothing in the visible universe is capable of account-
ing for the properties of living matter.

Evolution as an Account of the Course of the History of Creation,
a Gain upon the Older Idea of Arbitrary Creation, but not a Satisfac-
tory Substitute for Creation. — Evolution as a theory of the mode
of the orderly appearance of heterogeneity among organisms
is a great gain upon the older theory of creation, which found
no natural or regular method in the history, but only an
arbitrary and unfathomable complexity and heterogeneity.

That this order of sequence is correlated with genetic
succession, and is thus bound up with the organic nature of
the evolving beings, is a most rational inference from the
facts observed.

But evolution as a theory of origins, as an attempt to ex-
plain why things are as they are, as a philosophy of the cause
of organic diversity, is an utterly inadequate substitute for
creation. And we find the most zealous advocates of pure
scientific observation unable entirely to avoid the inquiry Why
are things as they are?

Consideration of Causation Indispensable to a Thoughtful Study
of Nature. — In our studies we may for a time confine our
attention to the " course of nature," entirely excluding all
consideration of matters not pertaining strictly to definition
and classification of the facts actually observed and measured ;
but sooner or later we must think, and when we think the
question of cause, and the nature of the relation of cause and
effect, inevitably arise.

A scientist, so ardent for the elimination of everything un-
scientific from science as Mr. Huxley, was not unconscious of
something beyond, as is illustrated by the following quota-
tions.

In the admirable study of the " crayfish" as a typical
organism we find the following definition: " The course of
nature as it is, as it has been, and as it will be is the object of
scientific, inquiry ; whatever lies beyond, above, or below this is
outside science ; " but such a definition only follows the state-

* Vol. i. p. 348.

378 GEOLOGICAL BIOLOGY.

ment that ' * the phenomena of nature are regarded as one con-
tinuous series of causes and effects, and the ultimate object of
science is to trace out that series."' . . .*

And in the same essay the remark is made that " Under one
aspect the result of the search after the rationale of animal
structure thus set afoot is Teleology, or the doctrine of adapta-
tion to purpose ; under another aspect it is Physiology."

If we admit into the discussion of science the question as
to the causal relation of one thing or event to another, the
consideration of a supreme cause necessarily comes into
the case. As is tersely phrased by Whewell : "In contemplating
the series of causes which are themselves the effects of other
causes, we are necessarily led to assume a supreme cause in the
order of causation, as we assume a first cause in the order of
succession. ,"f

Causes not Discovered by Observation, but Discerned by the
Heasoning Mind. — In the scientific study of organisms it is
possible to separate in our minds the act of observation from
the act of the associating one observed fact with another as
cause and effect. It is one thing, however, to observe, note,
measure, define, and classify organisms and their structures and
functions, and quite another thing to state that a particular
structure and function is caused by a particular preceding
structure and function or by any other preceding conditions
of the world.

For instance, there can be no dispute that the heat of the
sun, the various conditions of moisture, of air and soil, inci-
dent to the spring season, are the direct causes of the leafing
out of the elm-trees on the street side; but it is far from
the truth to say that these conditions of environment have had
any causative agency whatever in producing the elm leaves,
when the elm leaf is considered as differing from a maple
leaf. The mere association of two phenomena together does
not determine the one to be the cause of the other.

The fact that we are familiar with and understand the
effects of heat and moisture, and do not understand the oper-
ation of the more hidden biological forces, does not influence

*"The Crayfish," p. 3. f Nov. Org., III., x. § 7-

PHILOSOPHICAL CONCLUSIONS. 379

at all the decision that the sun, while it is the cause when we
speak of the development of the leaf, is not the cause when
we speak of the particular course of that development.

When we seek the cause of the changing of the characters
of organisms in the course of geological history the same
reasoning applies.

The fact that an infinitesimal part of the differences in the
characters of organisms is an expression of adaptation to the
immediate conditions of its local and temporal environment
does not suffice to prove that the environment is the cause of
the adjustment.

The determination of the true relations of cause and
effect in nature is therefore not a matter of observation, but
interpretation of cause is founded upon the philosophy we
apply in the interpretation of the course of nature.

Ability to Adjust the Organization to Conditions of Environ-
ment a Chief Element in the Fitness for Survival. — It is un-
doubtedly true that the fittest do survive, but too much is
made of the theory that fitness consists in precision of adjust-
ment of organic structure to conditions of environment. If
this were true the less variable would be more fit than the
more variable, and the result of survival would be the cessa-
tion of variation; whereas it is probably much nearer the
truth to say that fitness to survive is in almost direct propor-
tion to the ability to vary.

Darwin did not find it essential to inquire why variation
takes place: variation was assumed to be a common fact in
the life of organisms, and it is one of the chief factors of
evolution. But when we push the question, why has a par-
ticular variation arisen, become abundant, and been trans-
mitted from generation to generation? we are forced to
the conviction that the primal characteristic which distin-
guishes it from its unsurviving fellows is its greater capacity
to modify its structure, function, and habits into fitness for
the particular conditions of environment. It is the greater
ability to adjust, not the closer adjustment of structure to
environment, which constitutes the higher fitness to survive.
An organism is the fittest to survive, not because it has less
to oppose it, or less to overcome, not because the condi-

380 GEOLOGICAL BIOLOGY.

tions of life are easier or more congenial to its particular con-
dition, but because it has more of the essence of evolution ia
it.

The Philosophy of Evolution: a Summary. — It is this view of
evolution which the geological history of organisms emphasizes.
When we look back historically to the early geological ages, and
not assuming that we have reached the beginning, but allow-
ing that there may have been as long a stretch of time before
the Cambrian as since, for organisms to evolve in, — when we
compare the rate of initiation of characters of higher rank
with the rate of initiation of varietal or specific rank, — we
find it to be a striking fact that relatively the initiation of
higher characters predominated in early times, and as time
went on differentiation in each line was confined to characters
of less and less taxonomic value ; to use the oft-cited figure of
a phylogenetic tree, all the main branches dichotomized near
the roots of the tree, and as we advance chronologically
toward the present the branching has been confined to
secondary and tertiary limbs and terminal twigs. Although
such a tree is used as a figure of the way in which differentia-
tion has arisen, it seems never to have occurred to those
adopting this analogy that all the branching of a tree is
peripheral, at the very terminal twigs. The bifurcation of
two contiguous twigs becomes the main crotch of the trunk
only by the circumferential growth of the twig into a great
limb ; but does any one imagine that the difference between
a Crustacean and a Pteropod was in any particular of less
taxonomic value in the Cambrian time than it is now? or has
the difference between two species of Silurian Rhynchonellas
become of any greater significance by the continuous evolu-
tion of the Brachiopods up to recent time? No; natural
selection only works at the adjustment of varietal modifications
in making them permanent, or in dropping them out of the
race; and the mere transmission of an insignificant character
from parent to offspring for a million generations cannot in
itself have the least effect in raising the economic impor-
tance of that character among the functions of its possessor.
It is this view of the case which shows natural selection to be
but one of the phenomena incident to evolution, and not the

PHILOSOPHICAL CONCLUSIONS. 381

main factor in the case. The same force which is expressed
in the appearance of the new variation in the first place is
required to account for the appearance of the new generic,
the new ordinal, or the new class character. This force has
been distinguished as intrinsic evolution ; it is expressed in
variation itself, which is the chief factor assumed in the
theory of natural selection. The nature of the force is ex-
pressed in the term blastogenic of Weismann, and in the term
centrifugal, as used by Poulton ; but whatever it is called the
importance of the distinction lies in the fact that the selection,
the preservation, or the transmission of a character does not
account for its origin.

Evolution is thus seen to be a process that is primarily
organic : it is expressed in the acquirement of new characters
in the course of growth by living organisms; and we may as
reasonably speak of evolution force, as of the growth force of
the individual, or the force of gravitation. As the normal
laws of growth of the individual are thwarted and diverted by
external conditions, so undoubtedly a greater or less modifica-
tion of the course of evolution has been produced by the
conditions of environment.

When we attempt to explain the course of evolution by
tracing it backward from the differentiated, adjusted organ-
isms to their ancestors, it is natural to place great importance
upon the fact of the accomplished adjustment of the indi-
vidual to its particular environment; but when the point of
view is reversed and the organism is traced from the earlier
geological periods through the ages down to the present
time, the conviction becomes impressed upon the student that
environmental conditions are but the medium through which
the organic evolution has been determinately ploughing its
way.

Differentiation of form and function has been the expres-
sion of vitality, and environment is never exhausted. With
the occupying of unexplored fields has come divergence and
the appearance of new form and structure ; progress has not
been made in overcrowded fields by the survival of the fittest.
The crowding of the field has led to division and co-ordina-
tion of labor. All die in due time, and thus end the struggle;

382 GEOLOGICAL BIOLOGY.

but they who could best adjust themselves or their actions to
adverse conditions were the fittest while they lived, and it was
they who diverged. Those expressing more strongly than
their fellows the originative energy of life itself are the ones
to push forward and furnish the surviving and persisting mem-
bers of the race. The pioneers, the skirmishers in the front
line, are those among whom appear the founders of new spe-
cies and new races, as with men they are the makers of new
nations and of higher civilization.

Thus evolution has been working in the midst of the races
from the earliest recorded times; in each line it has been
regularly progressive in its order, everywhere advancing as
rapidly as the conditions already attained have rendered it
possible.

The great facts attested by geology are that the grander
and more radical divergences of structure were earliest at-
tained; that, as time has advanced, in each line intrinsic
evolution has been confined to the acquirement of less and
less important characters: such facts emphasize with over-
whelming force the conclusion that the march of the evolu-
tion has been the expression of a general law of organic
nature, in which events have occurred in regular order, with
a beginning, a normal order of succession, a limit to each
stage, and in which the whole organic kingdom has been
mutually correlated.

In closing, an illustration may be used to emphasize the
real points at issue.

Suppose a handful of lead shot were placed in a blunder-
buss, and the whole load discharged at a burglar climbing
into my chamber window.

The individual shot, originally of globular form, would be
found at the end of their journey of various shapes and in
various positions. Some of them would have travelled till
they expended their force and dropped to the ground in the
distance comparatively unchanged ; others would be slightly
distorted by impact upon the soft clothing or flesh of the
intruder; others would be flattened by meeting the resist-
ance of bone; a few would be stamped with the shape of
some brass button, surface of nail-head, or some other im-

PHILOSOPHICAL CONCLUSIONS. 383

penetrable substance against which they had struck. All the
modifications of the separate shot, their particular stopping-
places of rest, in fact every particular of the shape, condition,
and position finally assumed by the shot, would in greater or
less measure be the result of the influence upon them of the
conditions of the environment.

The immediate conditions of environment in which each
shot was found would appear to be a sufficient cause to ex-
plain the particular modification of that shot from its original
simple globular condition. The exact repetition on the shot
striking the brass button of its particular form would seem to
be sufficient evidence to prove that the one cause of the form
assumed was the adjustment of the shot to the conditions of
its environment. The fact observed is the actual perfect ad-
justment of the lead pellicle to its conditions of environment;
this adjustment is interpreted as an expression of equilibrium
between the moving pellicle and the resisting environment,
and the interpretation leads to the theory that the modifica-
tion is the resultant of natural selection among the numerous
forces expressed in the resisting obstructions to the once
started shot. When we use results in this sense it is evident
that the causes are various and of various values. There is
the initial energy expressed in the properties of the explosive
powder, the directive force expressed in the barrel of the gun
which guides the explosion in one direction; and there is the
aim of the gun made by the man shooting it, and even be-
hind this is the mental direction of the muscular action.
Each of these was a determining cause in bringing about the
shape of the pellet, and in accounting for the distribution and
shaping of the shot each was a cause of greater importance
than the particular conditions of its place of final rest.

Although it is scientifically true and accurate to define the
particular flesh, bone, button, or nail-head as determinants in
bringing about the final result of the motion of the lead pel-
lets through space, their actual and relative positions, and the
shape they finally assume, these conditions of environment
are but causes of diversion from the direction, position, and
relative distribution which were determined before the en-
vironment was met with.

384 GEOLOGICAL BIOLOGY.

The reason why each individual pellet stopped exactly
where it did is correctly defined as the result of its particular
environment, but the reason why it got there is not so
explained. So it is not difficult to understand that as long-
as we only microscopically examine the perfect adaptation
of organic structure to the particular place it occupies in
nature, the theory that species were originated by the ac-
tion of the conditions of environment through natural selec-
tion and the survival of the fittest seems sufficient and apt.
But when we consider what an immensely greater demand
is made upon causative energy to account for variability, com-
pared with that required to adjust to its environment an al-
ready living and varying organism, it becomes evident that
evolution is a far greater matter than the result of natural
selection.

To use the same illustration, we note that the fact, that
the lead pellets are observed in the act of travelling through
space, and finally stopping as they strike the resisting
bodies, does not remove the necessity of assuming the initial
explosion of the powder and the aim of the gun to account
for their motions.

So were we to lengthen out the gyration of organic
plastidules, or biophores, a million million years, continuously
holding on to their original powers and potencies for all that
time, we are not relieved in the least from the logical neces-
sity of endowing them at the outset with the real directive
energy which phenomenally expresses itself for the first time
when the finally adjusted organism appears. And the incre-
ment to organic structure expressed by their final bursting
into morphological reality, after travelling unobserved but
potential through the organic matter of countless generations,
is as much a result of creative energy as if a new species were
to arise out of the dust of the earth.

INDEX.

Ability of adjustment, 379.

Abysmal zone, and ctenobranchina,
138.

Acadian revolution, 42.

Acceleration of development, 319;
and retardation, 197.

Accounting for variability, 198.

Acme of life-period of genus, 293.

Acquirement of characters, 252; of
differences by modification, 344; of
permanency, 198, 296, 299; of va-
riation, 158.

Act of evolving and order of events,
372.

Actinimeres, 222.

Adaptation to environment, 114.

Adaptation of Cyclobranchina, 141;
Aspidobranchina, 141 ; Pteno-
glossa, 141 ; Rachioglossa, 141 ;
Toxiglossa, 141 ; Rhipidoglossa,
141 ; Taenioglossa, 141; Siphonos-
tomata, 141 ; Holostomata, 141 ;
families of Gastropoda, 142 ; gen-
era of Gastropoda, 142 ; of gen-
era with restricted specific ad-
justment. 142 ; and taxonomic
rank, 148.

Adjustment to environment and
time, 117; to changed habitats, 139;
closeness of, and rank, 142; con-
cerns varietal characters, 143; and
structure, 147.

Adolescent, 94.

Adult, 94.

Agamogenesis, 169.

Ages, geological, 26, 69.

Age of earth, Dana, 58; Houghton,
59; Kelvin, 58; Clarence King, 58;
Upham, 59; Wallace, 60; of Fishes,
26; Invertebrates, 26; Mammals,
26; Man, 26; Reptiles, 26, 68.

Algonkian, 30.

Alluvial formation, 13, 19.

American continent and revolutions,
46; geological history, 25; school
of evolutionists, 197 ; spirifers,
range of, 313.

Amoeba, 221, 165.

Ammonites, and formations, 28.

Ammonoidea, a description of, 345.

Anabolism, 177.

Analogy and analogous parts, 227.

Analytic and synthetic methods of
classification, 238.

Ancestry, definition of, 120; and en-
vironment, 98; and environment,
as causes of evolution, 119; and
the beginning of the individual,
120; and hard parts, 98; and ori-
gin of species, 127.

Ancient notions of geology, n.

Ancylobrachia, evolution of, 256,
263.

Angeschivempt Gebirge, 13, 16.

Am^ulatus zone, 68.

Animal kingdom, classification of,
201.

Antimeres, 222.

Antiquity and distribution, 144; of
individual characters, 190.

Appalachian revolution, 34, 40, 42.

Appearance, first, of new characters,
267.

Archaean, 14.

Archetypal structure, 233.

Area, 70.

Aristotelian species and genus, 200.

Arthromeres, 224.

Arthropoda, definition of, 204.

Arthropomata, evolution of, 256.

Astacus ftuviatilis, development of,
1 80.

Astrceidce, rate of differentiation, 85.

Astronomical time estimates, 56.

Atavic, 94.

Athyridce, 279.

Athyris, 286.

Atrypa reticularis, life-history of,
315, 320.

Atrypidce, 279.

Attainment of diversity by cell, 165.

Auxology of Bather, 94.

Avicenus, u.

Axes of spiral cones, in Helicopeg-
mata, 287.

Azoic, 22.

Bather, and the term Auxology, 94.
Bathmism, or growth force, 197.
Bathmology, Hyatt, 94; Cope, 94.
Bathybic Plankton, 116.
Bathymetric zones, 117; and Cteno-
branchina, 137.
Beginning of individual life, 220.

385

386

INDEX.

Benthos, abyssal, 116; littoral, 116;
sessile, 116; vagile, 116.

Bilateral symmetry, 222.

Biological, classification, 28; nomen-
clature, 24.

Biology, zoological and geological,
98.

Bionomy of the sea, Walther, 116.

Blastula, 172.

Bonnet, and theory of mutability,

151-

Botanist, method of, 5.

Brachidium and loop, 282; structure
of, 280.

Brachiopods, and acquirement of
characters, 252; and evolutional
history, 239; described, 244; life-
history of, 277; zone, and Cteno-
branchina, 137.

Brephic, 94.

Brongniart, Cuvier and, 12.

Bryozoa, described, 244.

Buckman, hemera of, 68.

Budding, agamogenesis by, 169.

Cainozoic, 22, 23.

Calcified loops of Brachiopods, 267.

Cambrian, 30; ancestors and char-
acters, 258; characters of living
Brachiopods, 258; differentiation
in, 209.

Carboniferous, 30; age, 26, 30; group,
18.

Catabatic, 94.

Catastrophe, n.

Causation and evolution, 119; legit-
imately discussed, 377.

Cause of varying order of sedi-
ments, 73.

Causes, discerned not observed, 378;
search for, 373.

Cell and molecule, 166; an organ-
ism, 174; division, 165; modifica-
tion, three modes of, 165; move-
ment, 165; multiplication, 165; nu-
cleus, 165.

Cells, organism an aggregate of,
164.

Cell, the undifferentiated, 221; wall,
165.

" Centres of Creation," Forbes, 128;
of distribution, specific, 114.

Cephalization, principles of, 226.

Cephalopoda, described, 245.

Cephalopods, evolution of, 325, 342;
structure of, 329.

Change, incessant in living organ-
isms, 164; in fossils, with time,
83; of functions in ontogenesis,
95; of function, none in phylogen-
esis, 95.

Characteristic form of fossils, 83;
fossils, 68, 75.

Characteristics of primitive mol-
lusk, 327; of the genus, 293.

Characters adjusted to environment,
138; evolved since Cambrian, in-
significance of, 218; of new species,
not all new, 191; traceable to Cam-
brian ancestors, 258; whose origin
is traced back to Cambrian, 212.

Checks to increase, in Darwin's
theory, 194.

Chemical and physical properties
not evolved, 375.

Chemical element, 166.

Chemung group, 67.

Chronological periods, 29; scale, 7;
succession, 24; value of groups of
genera, 88.

Chronology of rocks, laws of, 76,

Ciliary motion and cilia, 228, 229.

Class characters, evolution of, 266,

Classes, geological range of, 206;
importance of, in Paleontology,
205.

Classification, meaning of, 130; of
functions, 177; of Mollusca, Lan-
kester, 246; principles of , 200.

Classification, terms of, Aristotle,
200; Cuvier, 201; Linne, 201; Sca-
liger, 201.

Classifying stratified rocks, 65.

Classis of Linne, 201.

Claus and Sedgwick, definitions of,
203.

Climax of generic evolution, 255.

Closeness of adjustment and rankr
142.

Closing part of life-period, 319.

Coelenterata, definition of, 203.

Coelomata, 246.

Columbia River lava outflow, 44.

Community of descent and species,.
123; of form of individuals, 162.

Comparative study, scale for, 54;
time-scale, 53.

Conditions of environment, 113; and
rock formation, 73.

Conditions of evolution, organic,
119; physical, 119.

Constancy during life-period, 292;
in transmission, and species, 297.

Continental value of revolutions, 45.

Continuous plasticity of species, 316.

Conybeare and Phillips' system, 18.

Cope and bathmology, 94.

Corallites, 90.

Corallum, 91.

Corals — the zoantheria, 84.

Coral structure, 91.

Correlation, 177.

INDEX.

387

Creation, evolutional idea of, 373.

Creationism, 121.

Creator, and originations, 374, 375.

Cretaceous group, 18; period, esti-
mate of length, 55; tertiary divi-
sion line, 44.

Criterion of age of rocks, 28.

Croll, time estimate, 57.

Crura and primary lamellae, 285.

Cryptogenesis, 166.

Curve of differentiation, 88.

Cutting of the Columbia gorge, 56.

Cuvier and Brongniart, 12, 20, 21;
and Lamarck, 154; terms of clas-
sification, 201.

Cuvier's classification, 233.

Cycles of repetition in a species, 95.

Cyrtina, 285.

Dana, J. D., Archaean, 14; and ceph-
alization, 226 ; geological time-
scale, 24; nomenclature of geol-
ogy »25 1 and thickness of deposits,
57; time-ratios, 47, 48.

Darwin and specific centres, 123 ;
and the origin of species, 155, 156.

Darwin's "Origin of Species," 126,
128, 156.

Darwin's theory, factors of, 193.

Darwinism, 156, 158.

Darwin (G. H.), time estimate, 56.

Data of time estimates, 56.

De la Beche, 18.

Deluge, Noachian, n.

Depression and elevation, and order
of deposits, 73.

Descent, 161 ; and recurrence of
characters, 211; with modification,
179; without modification, Forbes,
124, 125.

Development, 168 ; and evolution,
70; and Lamarck, 152; main feat-
ures predetermined, 180; of indi-
vidual, 176, 219 ; of individual
characters, 185 ; of systems of clas-
sification, 27; purposeful, 97.

Devonian age, 25, 30.

Devonian system, 71.

Diagram of evolution curves, 86.

Diarthromeres, 224.

Dicellocephalus fauna, 52.

Difference in structure and environ-
ment, 147; of form and environ-
ment, 139.

Differentiation along digestive tract,
232; attained in Cambrian, 209; of
cell, 174; of cephalopods, 336; of
characters of brachiopods, 254 ;
of foot-organs in mollusks, 327 ;
of generic form, 87; illustrated,
228 ; mark of organism, 174 ; of

Nautiloidea, 337 ; of nervous sys-
tem, 231; of a race into species,
318; and specialization, 175.

Digestive tract, differentiation along,
232.

Dimeric and monomeric types, 235.

Direct evidence of evolution, 96.

JDisjuncta epoch, 67.

Distribution and adaptation, 140 ;
centres of, 114; geographical, 70;
and structure, 147; and temperat-
ure, 145; varieties, 114.

Distinctive features of Lankester's
classification, 251.

Divergence, accounted for by evo-
lution, 260; of characters, in Dar-
win's theory, 195 ; of form and
lapse of time, 89.

Division of eras into periods, 51.

Division-planes, local, 29.

Divisions of classification, early dis-
cerned, 234.

Early plasticity succeeded by per-
manency, 297.

Eaton, Amos, classification of, 19;
New York rocks, 19.

Echinodermata, definition of, 204.

Ectoderm, 172.

Effect of Darwin's "Origin of Spe-
cies," 156; of environment, 98;
slight, 181.

Elements, chemical, and the cell,
166.

Elevation of land and order of de-
posits, 74.

Embryological likeness and mature
diversity, 241.

Embryologist and Morphologist,24O.

Embryology, 168.

Embryonic, 94; development and
succession, 230.

Embryos or fossils, 208.

Embryo stage, no struggle for ex-
istence, 173.

Emphasized, laws of evolution, 369.

Endoderm, 173.

English usage, 30.

Environment, adaptation to, 114;
and ancestry, 98; conditions of,
113, 120; and the divergence of
characters, 140; and hard parts,
98; and organism, 6; and origin
of species, 127; slight effects of,
181; and structure, 147.

Eocambrian, 52.

Eocene, 21, 30.

Ephebic, 94.

Epochs in geology, 25, 69; of ex-
pansion in spirifers, 314; use of, in
time-scale, 53.

388

INDEX.

Eras in geology, 69; relative lengths
of, 54; and systems, 71.

Errors in estimates of age, 58, 59.

Estimate of rate of limestone for-
mation, 60.

Etienne Geoffrey St. Hilaire and
species, 152.

Evidence, selection of, 365; fossils
and living organisms as, 365;

Evolution, 168; the acquirement of
characters, 219; acquirement of
variations, " 158; and adaptation,
118; and Anaximander, 152; an-
tiquity of, 153; definition of, 369;
relative rapidity of, 369; variabil-
ity, 369; heredity, 369; mode of,
370; cause of, 370; conditions of
environment and, 370; adjustment
and, 370; struggle for existence
and, 370; natural selection and,
370; intrinsic, 370; classification
and, 370; the philosophy of, 371;
of calcified loops of Brachiopods,
267; characteristic of organisms,
375; of class characters, 266;
curves of Brachiopods, 256, 263;
curves, meaning of, 87; curve of
organisms, 85; descent, 124; or
development, Huxley, 124; and
development, 70, 152! 157; of ex-
trinsic characters slow, 311; ex-
pressed in specific characters, 261;
fact of, established, 160; of funda-
mental characters, 268; of genera,
Cope, 196; in geological history,
89; idea of, and creation, 376; an
intrinsic law of organism, 127;
laws of, 261, 265, 269; of mammals
in Eocene, 359; the mode of cre-
ation, 374; modifies and not re-
places creation, 377; nature of,
160; not an inorganic process, 96;
of ordinal characters, 266; an or-
ganic process, 96; records chiefly
in generic and specific characters,
219; of shell curvature in Nauti-
loidea, 340; of spiral appendages,
302; shell proportions, of spirifers,
302; of suture lines, laws of 355.

Evolution theory, definitions, 158.
Lamarckian, 158; Darwinian, 156,
159; phylogenetic, 159; and uni-
formity theories, 157.

Evolutionism, 121.

Excretion, 177.

Explanation of succession required,
118.

Extinction of Brachiopod genera
254.

Extremes of acceleration and re-
tardation, 319.

Extrinsic character, example of, 271.

Facies, 69.

Factors of evolution, 121, 197, 364,
367-

Factors of origin of species, 193.

Family groups of genera, chronolo-
gical value, 38.

Fauna, 113.

Fauna of the Cambrian, 212.

Fauna and flora, 69.

Faunas and floras, classification of,
116.

Faunas of New England coast, 117.

Faunas and Provinces, 115.

Faunal distinctness, 115.

Favo sites niagarensis, 90.

Favosites in the Niagara formation,
92.

Favositid(Z, rate of differentiation of,
85-

Fertilization of ovum, 172.

Finger-bones and teeth of verte-
brates, 363.

First appearance of genera, 85, 86.

First cause essential to evolution,
121 ; in nature, 378.

Fission, agamogenesis by, 169.

Fittest organism, the, 81, 366.

Fixation of plastic characters of
Spirifers, 301.

Fixed characters, acquired by trans-
mission, 192.

Flora, 113.

Flora and fauna, 69.

Floral distinctness, 115.

Flcetzgebirge, 13, 16, 19.

Food, as environment, 113.

Foot-organs in mollusks, differen-
tiation, 327.

Forbes, Edward, on centres of cre-
ation, 121 ; and classification, 22;
and Lamarck, 127; on origin of
species, 121, 123.

Form of loop in jugum, 288 ; and
matter of individual, 160.

Formation in geology, 7; definition
of, 30.

Formation and period, 66; scale, 66;
scale, relative antiquity, 73 ; of
individual characters, 125; of pe-
riod names, 52; of stratified rocks,

71-
Fossil coral, favosites, 90; fauna and

flora, and periods, 52; records, 81.
Fossils as basis of classification, 21,

25: the basis of the time-scale, 66;

characteristic, 75 ; characteristic

of period, 83; to determine age of

systems, 37; form of and time, 83;

interpretation of, 78; kinds of, 80;

INDEX.

389

of marine origin, 80; materials of,
78; nature of, 78, 80; preservation
of, 79; mark relative age, 77; the
marks of geological period, 67 ;
their nature and interpretation,
109; occurrence of, 81; represent
hard parts, Si ; substituted for
minerals, 20; and zoological speci-
mens, 163.

Fragmental materials, and strata,
72.

Fresh-water families of gastropoda,

143-

Function of assimilation, 177 ; of
correlation, 177; generation, 177;
meaning of, 178; of metazoal or-
ganism, 169; of sustentation, 177;
and property, 178,

Functions of vertebrate, 177.

Fundamental law of evolution, 89.

Gamogenesis monoecious, 170; dioe-
cious, 171; hermaphrodite, 171.

Gastropoda described, 245.

Gastropods, characters of, 131; clas-
sification of, 133 ; selected for
study, 133.

Gastrula, 172; stage, 222.

Gebirge and formation, 15.

Genera and the time-scale, 85; of
ctenobranchina, and zones, 137;
of madreporaria, and eras, 86.

Generation, a function of organism,
167, 169, 177.

Generic evolution, 253; climax of,
255; expansion, 262; form and dis-
tribution, 130; initiation, in helico-
pegmata, 290 ; life-history, 276 ;
life -period, 88; life -period of
brachiopods, 254; series, fixation
of characters, 301.

Genetic affinities, 98.

Genus, proximum, medium, and sum-
mum, 201; and species, 89; species
and, of Aristotle, 200.

Geobios, 116.

Geoffroy St. Hilaire, Etienne, and
species, 152.

Geographical conditions, and strata,
71; distribution, 70, ill, 112.

Geological aspect of organisms, 3;
eras and times, 51; formations,
systems, 27.

Geological range, 70; and adjust-
ment, 144; of characters, Favosites,
93; of Atrypidce, Spiriferidae, 280;
Athyrida, 280; and Taxonomic
rank, 92.

Geological revolutions, 39; survey,
nomenclature, 30; systems and
revolutions, 41; Terranes, 28;

time-scale, 10, 54; geological time,
length of, 8; McGee, 61; A. Gei-
kie, 62; Kelvin, 62; Clarence
King, 62; G. H. Darwin, 62; Tait,
62; Dana, 62; Upham, 62; Prest-
wich, 63; Walcott, 63; and zool-
ogical biology, 98.

Germ and embryo stage, 173.

Gerontic, 94.

Glacial and post-glacial time, 62, 63;
revolution, 45.

Glossophora, 249; mode of exist-
ence of, 135.

God in evolution, 372, 373, 376.

Goniatites, classification of, 349.

Grauwacke group, 18.

Group in geology, 69; of strata or
stratum, 69.

Growth, 168.

Growth force or Bathmism, 197;
normal, 179.

Habitat, 113; normal, 115.

Haeckel and Bionomy, 116; and im-
portance of species, 149.

Hall, James, on variability of
Atrypa, 317.

Halobios, 116.

Hard parts in animal kingdom, 99;
and evolution, 98; of organisms,
importance of, 81; and relation to
ancestry, 98; and relation to en-
vironment, 98; of Anthropoda,
101; Ccelenterata, 100; Echinoder-
mata, 101; Mollusca, 105; Mollus-
coidea, 104; Protozoa, 99; Vermes,
101; Vertebrata, 107.

Helicopegmata, evolution of, 256,
263; life-history of, 377; rate of
expansion of, 290; three families
of, 229.

Hemera of Buckman, 68.

Heredity, 193; law of, 219.

Heterogeneity, attainment of, 176.

Hexacoralla, rate of differentiation
of, 85.

Himalayas, elevation of, 55.

Histogenic development, 173.

Histogenesis, of metazoa, 165.

Historical classification, 25.

"History of Creation," Haeckel,
128.

History of the individual, 5; law of,
89.

History of organisms, law of, 89;
methods, 207 ; (Ontogeny), 76 ;
(Phylogeny), 76 ; scope of, i ;
time-scale for, 54; of species, 5;
Spirifers, 300.

Homology and homologous parts,
227.

390

INDEX.

Horizon, 69.

Houghton, relative time-duration,

48.

How does evolution proceed, 269.
Haeckel and phylogenesis, 95.
Humphreys and Abbott, report, 57.
Hyatt and Bathmology, 94.
Hyatt's law of rapid expansion at

point of origin, 341.
Hypostrophic, 94.

Idea of creation, evolutional, 373.

Idea of mutability and origin of spe-
cies, 187.

" Ideal plan " in classification, 236.

Immutability and mutability, 125.

Immutability of species, 125, 155;
idea of, 153.

Imperfection of evidence, 208.

Importance of fossils, 20.

Improvement resulting from evolu-
tion, 357.

Increase in Darwin's theory, 194.

Increment, in evolution, 296.

Individual characters, 5; formation
of, 125; development, 219; nature
of, 160, 162; of Scaliger, 201.

Individuality of an organism, 166.

Infantine stage of growth, 94, 174.

Inferior stratified rocks, 18.

Initial stage of evolution, 282.

Initiation, development, and evolu-
tion, 352; of generic characters,
291; new characters, 267; new gen-
era, 256; a new genus, 291; new
species of cephalopods, 340; and
origin, 70; of Cyrtoceratidffi. 339;
of Nautilidae, 340; of Orthocera-
tidse, 339; of species of Ptychop-
teria, 322.

Inorganic and organic matter, 166;
properties and organic characters,
186; things not evolved, 96; things,
unchangeable, 83.

International Congress, nomencla-
ture of, 69.

Interpretation of facts of evolution,
119, 121.

Interruption of record, 46.

Intrinsic character, example of, 271;
and extrinsic characters, 265, 270;
and extrinsic evolution, laws of,
311, 312; and extrinsic in machin-
ery, 272; marks of organism, 175;
tendency of organism, 176.

lonians and transmutation, 152.

Jugum of Brachiopods, 283, 288.
Jurassic formations, 28.
Jura-trias, 30.
Juvenescent stage, 174.

Katabolism, 177.
Kelvin — time estimates, 56.
Kinds of hard parts, 99.
Kirwan, Richard, 14.

Lamarckians and Neolamarckians,

198.
Lamarck and mutability of species,

152.

Lamellibranchs, described, 245.
Laminarian zone, Ctenobranchina

of, 137-

Land, as environment, 113.
Land surfaces, lowering of, 60.
Lankester's schematic mollusk, 325;

classification of mollusca, 246.
Larval stage, 94, 174.
Law of adjustment to environment,

138; of chronology of rocks, 76;

of development, 185; of mutabil-
ity, 158.
Laws of adaptation of Gastropoda,

147; of evolution, 129, 140, 197,

322; emphasized, 359.
Le Conte, Joseph, 24.
Lehmann's classification, 12, 17.
Length of geological time, 55, 61.
Leonardo da Vinci, n.
Life-history, generic, 276; of Heli-

copegmata, 277; time-scale for

study of, 57.

Life-period of a genus, 88, 291.
" Like produces like, with an incre-
ment," 296.
Limnobios, 116.
Linne and number of species, 149;

Ordo, Class is of, 201.
Lipocephala, 249.
Littoral zone, and Ctenobranchinar

137-

Living, characteristic of organism,
163; implies change, 164; organ-
isms and purposeful development,

97-

Locomotion, 230; and nervous sys-
tem, 231.

Loop of Ancylobrachia, 282 ; and
Brachidium, 282.

Lyell, Sir Charles, n.

Lyell, and time-value of fossils, 66.

Lyell's classification, 21, 22, 24.

Madreporaria, evolution curve of,
86; rate of differentiation of, 85.

Maclure and American rocks, 18.

Magellania flavescens, 265.

Mammals, evolution of, 323.

Man, an organism, i.

Marine conditions of life, 116 ; or-
ganisms and paleontology, 116.

Marine province, 113.

INDEX.

Mark of age, fossils, 83.

"Mark or seal" of living types,

Forbes, 124.

Marks of an organism, 175.
Matter and form of individual, 160;

properties of, not evolved, 374.
Mature individuals used, 209.
Maximum thickness of rocks, 59.
Medial order, 18.
Medium, as environment, 113.
Mesocambrian, 52.
Mesoderm, 173.
Mesosaurus tumidus, 362.
Mesozoic, 22, 23, 26.
Metabolic changes, 117.
Metameric type in classification, 236.
Metameres, 222, 224.
Metazoa, characters of, 225.
Metazoan, a tissue-bearing animal,

173-
Migration and modification, 140; of

species, 123.
Mineral character not sign of age,

77-

Miocene, 21.

Mississippi River, and time, 57.
Mode of curvature of nautiloid shell,

339-

Modifications of brachidium, 277; of
specific characters, 125; of sut-
ures, 354.

Molecule, and cell, 166.

Mollusca, according to Lankester,
246; branch, and classes of, 246;
definition of, 204; described, 244;
245 ; digestive system of, 247 ;
and evolutional history, 239; gen-
eral character of, 242 ; muscular,
nervous, and motory systems of,
247; Zittel's classification, 239.

Molluscan type of structure, 225.

Molluscoidea, definitions of, 204 ;
described, 244.

Monomeric and dimeric types, 235;

Morphephebic stage, 94.

Morphological characters and an-
cestry, in; and time, in; differ-
entiation (evolution), 89 ; and
physiological characters, 176; unit,
the cell, 164.

Morula, 172.

Motor organs, differentiation of, 228,
232.

Multiplication of parts before spec-
ialization, 229.

Murchison, 22, 23, 28.

Murchison's term system, 71.

Muscular motion, meaning of, 228,
230; and skeletal organs, 232.

Mutability, example of, 187; a fun-
damental law, 296; a law of evo-

lution, 162 ; law of, expressed by
symbols, 187 ; and immutability,
125 ; and origin of species, 126,
154, 187; and phylogeny, 294;
of species, 125, 126, 151, 155; of
species and evolution, 151; tenet
of, 155-

Mutable species, temporary, 154.

Mutable, what is? 187.

Mutations, 207; and variations, 70.

Narrowing the limit of variability,.
322.

Natural-history classification, 115,
200.

Natural-history provinces, 113, 122;
of Woodward, 115; of Sclater, 115;
of Wallace, 115; Fischer, 115.

Natural law of succession, in.

Natural selection, 156, 179; in Dar-
win's theory, 194; and geological
evidence, 367; and living organ-
isms, 366.

Natural variation, 296.

Nature of species, new conception
of, 156.

Neanic, 94.

Nekton, 116.

Neocambrian, 52.

Neocene, 30.

Neozoic, 22.

Neritic plankton, 116.

Nervous system and locomotion, 231.

Neues Floetzgebirge, 13, 1 6.

New species, characters of, not all
new, 191; idea of, 189; New York
geologists, 25; rocks, Amos Eaton,
19.

Niagara gorge, cutting of, 56.

No evidence of evolution of classes,
241.

Nomenclature of geological con-
gress, 69; of provinces, 115.

Normal growth, 179; habitat, 115.

Nullipore zone, and Ctenobranchina,
137-

Numbers of genera, and systems, 86,

Numbers of genera of Zoantheria,
84.

Old and new schools of opinion, 121.

Olenellus fauna, 52.

Ontogenesis and change of function,
95; and phylogenesis contrasted,
95, 178; a repetition of phenomena,
95; results of, 176; and stages of
growth, 94.

Ontogenetic growth of sutures, 352.

Ontogeny and Ontogenesis, 180; and
Phylogeny, 158.

Oolitic group, 18.

392

INDEX.

Oppel's classification, 28.

Order of deposits with elevation, 74;
with sinking land, 74; of for-
mations, 16, 17; of original for-
mation, 76; of stratification, 73;
of superposition, 24, 77.

Ordinal characters, evolution of, 266.

Ordo of Lirine, 201.

Organic cell and atom of matter,
166; and inorganic element, 166;
the morphological unit, 164; con-
ditions of evolution, 119; growth
(development), 89; individual,
160, 162; primitive form of, 165;
and inorganic action, 175; process,
evolution an, 96.

Organism, an aggregate of cells,
164; definition of, 163; and en-
vironment, 130; Huxley's defini-
tion, 163; incessantly changing,
164; individuality of, 166; intrinsic
marks of, 175; Kant's definition,
174; Man an, i; old and new view,
4; purposeful development of, 97;
related genetically to ancestor, 129.

Organisms affected by environment,
130; and environment, 6; as en-
vironment, 113; express evolu-
tion, 89; time-scale for study of
history of, 54; scope of history of ,
i.

Organs, 97; and taxonomic rank,
225.

Origin of form, not of matter, 184;
and initiation, 7°! °f provinces,
123.

Origin of species, 183; by evolution,
126; illustrated, 188; meaning of,
185; unsettled problems of, 197

Origins, unknown cause of, 127.

Osborne, H. F., evolution of mam-
mals, 323, 363.

Ovum, segmentation of, 171.

Palaeo-biology, 68.
Paleontologist and marine organ-
ism, 116; method of, 5; work of,

4-

Paleontology, foundation of, 20;
species in, 207.

Paleozoic time, 22, 23, 26.

Paleozoic brachiopods, 254.

Palisade revolution, 42.

Pangenes, 166.

Paradoxides fauna, 52.

Paris basin rocks, 21.

Permanency of characters, 193, 299;
following plasticity, 297; and limi-
tation in breeding and distribu-
tion, 299.

Permanent characters, rank of, 300.

Permian system, 71.
Periods of climax in evolution, 255;
and formations, 66; of time and
fossils, 83.

Periods in geology, 25, 69; defini-
tion of, 30; divisions of eras, 52;
and groups, 24; relative lengths
of, 54.

Perpetuation of characters, 259.
Phenacodus primccvus, 362.
Phillips, John, classification, 23.
Philosophy of evolution, 371; a

summary, 380.
Phylephebic, 95.

Phylogenesis, 166; of Haeckel, 95;
and change of function, 95 ; a con-
tinuous series, 95; in classifica-
tion, 237; and ontogenesis con-
trasted, 95.
Phylogenetic evolution of races,

220; theory, 295.
Phylogeny, or Phylogenesis, 180; of

race, 294.
Physical conditions of evolution,

119.

Physical time estimates, 56.
Physiological function, 178; signifi-
cance of origin of species, 193.
Physiology and the organism, 163.
Pictet's rules about fossils, 82.
Plankton, 116.
Planar bis zone, 68.
Plastic characters at early stage,

301.

Plasticity of characters, 193, 289.
Pliocene, 21.

Point of view in discussing evolu-
tion, 371.
Polarity, 222.
Polymeric type, in classification,

235-

Post-pliocene, 21, 24.
Predetermined features of develop-
ment, 180.
Prefix inorpho, in morphephebic, 94;

phyl, in phylephebic, 95.
Prestwich, length of glacial and

post-glacial time, 63.
Primary in geology, 14; Fossilifer-

ous Period, 24.
Primitiv Gebirge, 13, 1 6, 19.
Primitive formation, 12, 13, 19; tis-
sues of development, 172.
Production of differences in repro-
duction, 129.
Progenitors, number of, Darwin,

195-

Progress of life, 25.
Progressive change, 3; evolution in

mammals, 323.
Protoplasm, 166, 169.

INDEX.

393

Protoplasm, defined by Huxley, 164.

Protozoa, definition of, 203; develop-
ment of, 165.

Protozoa and metazoa, growth of,
167.

Protozoan, a cellular animal, 173,

Protozoic, 22, 23.

Protremata, evolution of, 256.

Provinces in natural history, 70, 113;
classification of, 115; and faunas,

US-

Provisional units of time-scale, 63.

Psychozoic time, 24.

Ptychopteria, initiation of species of,
322.

Purposeful development of organ-
isms, 97.

Quaternary of Reboul, 12.

Quick evolution of Clymenidae, 349.

Races in paleontology, 294.

Radiate structure, 222.

Rank and adaptation, 148; of ad-
justed characters, 139.

Rank of characters, 192, 203 ; and
precision, 192; and antiquity, 192;
is it modified with descent? 196;
and their time values, 93 ; taxo-
nomic, and adaptation, 142 ; and
geological range, 92.

Range and distribution of Strom-
bidae,i44; Chenopodidae, 144; Ceri-
thiidse, 145 ; Rissoidre, 145 ; geo-
logical, 70; geological and taxo-
nomic rank, 92.

Rapid evolution of brachidium, 289;
of characters, 268 ; and natural
selection, 269; expansion at point
of origin, Hyatt's law, 341.

Rate of accumulation of sediments,
5o,57; of denudation, 60, of differ-
entiation and new genera, 336 ; of
elaboration of characters of sut-
ure, 354; of erosion and geological
time, 57; of evolution, 262; of ex-
pansion of generic characters, 290;
of lowering of land surfaces, 60;
of removal of minerals from con-
tinent 59.

Reality and mutability of species,
153; of specific centres, 122.

Reboul, quarternary system, 12.

Recapitulation theory, 158.

Recurrence of characters, how ac-
counted for, 211.

Red-sandstone group, 18.

Region, 70.

"Relationship of descent" of
Forbes, 124.

Relative age, marked by fossils, 77.

Relative order of deposits and de-
pression, 73 ; and elevation, 73 ;
thickness of deposits, 50.

Removal of soluble minerals, Reade,

59-

Repetition of ancestral characters,
219 ; of characters, 259 ; of parts
and rank, 223.

Representative species of Forbes,
115, 123; varieties, 207.

Reproduction, 177, 193; of cell, 165.

Restricted adaptation to zones, 140..

Restriction of variability, 299.

Retardation, acceleration and, 197;
of development, 319.

Retrosiphonata, 348.

Revolution, Acadian, 42; Appalach-
ian, 34 ; and interruption of record,
46; palisade, 42; post-paleozoic, 34;.
Rocky Mountain, 43; Taconic, 41.

Revolutions, geological, 39; as time-
breaks in history, 45.

Rocks, chronology of, laws of, 76.

Rocky Mountains, elevation of, 55;.
revolution, 43.

Rostracea, evolution of, 256, 263.

Scaliger's expansion of the genusr

201.

Schematic mollusk, Lankester, 325.
Schurman on antiquity of evolution,.

153-

Search for causes, 373.
Secondary, the term in geology, 12;

period, 24.
Secretion, 177.

Sediments, and Mississippi river, 57..
Segmentation of ovum, 171
Selection of evidence, 365.
Senile, 94.
Septum, 91.

Sequence of mineral deposits, 17.
Series, 69.

Sex differentiation, 171.
Sexual selection, in Darwin's theory,.

195-

Silurian age, 25, 36; system, 71.
Similarity of form, and species, 162,
Sinking land and order of deposits,

74-

Skeletal and muscular organs, 232;
parts, 229.

Smith, William, 21.

Soft parts and hard parts, 98; of or-
ganism and ontogenesis, 98.

Somites, 224.

Sorting of materials, in sedimenta-
tion, 72.

Source of sediments, 72.

Specialization and differentiation,,
175; of fingers in reptiles, 362.

394

INDEX.

Species, successive and develop-
mental stages of, 95 ; closely ad-
justed to environment, 143; defi-
nitions of, Buffon, 150; Cuvier, 150;
De Candolle, 150; Haeckel, 162;
Huxley, 154, 184 ; Lamarck, 152;
Linne, 150 ; Pritchard's, 297; Os-
car Schmidt, 162; Tournefort, 150;
Zittel, 150.

Species, Forbes' ideas about, 123.

Species and genera, importance of,
205; and genus of Aristotle. 200;
as immutable, 153; importance of,
149, 162; initiation of, 322; number
of, 149; of the paleontologist, 207;
temporary continuance of, 28.

.Specific centres of Forbes, 114, 122;
Darwin's view of, 123; of distri-
bution, 114; characters, modifica-
tion of, 125; evolution, 253; and
generic names, uniform usage
202; variability, 299.

Spirifer, 285; the genus analyzed
by Hall, 312.

Spiriferidte, 279.

Spirifer striatus and mutability, 188.

Spirifers, evolution of appendages,
302; of delthyrium, 304; deltid-
ium, 304; hinge area, 305; median
fold and sinus, 308 ; median septum,
310; shell proportions, 302; pli-
cation surface, 308; structure of
shell, 310; surface-markings, 307;
surface spines, 310.

Spontaneous generation, 154.

Stage, the term in Geology, 69.

Stages of development, 171; of
growth in ontogenesis, 94; of in-
dividual growth, 94.

Standard classification, 203; periods,
52; time-scale, 54; units of time-
scale, 31.

Steps of progress in development,
167.

Stony corals, 84.

Strata in classifying rocks, 65; as
data of the formation scale, 66;
parts of a formation, 67.

Stratified rocks, classification of, 65;
and geographical conditions, 71;
and geological time, 65,

Stratigraphical division planes, 29;
order and locality, 73.

Stratum, or groups of strata, 69.

Strombidtz and Chenopodidce, 143.

Structure and environment, 147.

Struggle for existence, in Darwin's
theory, 194; wanting in embryo
stage, 173.

Succession and adjustment of spe-
cies, 117; of species and stages of

development, 95; of suture char-
acters, 353.

Supercretaceous group, 18.

Superior order in geology, 18.

Supermedial order in geology, 18.

Superposition of rocks, 77.

Survey, U. S. Geological, 30.

Sustentation, the functions of, 177.

Sutures of Ammonoidea explained,
350; Ammonitic type, 352; Cera-
titic, Helictitic, and Medlicottian,
351; classification, 350; Goniatitic
type, 351; Nautilian type, 350;
Pinacoceran type, 352.

Synthetic method in classification,
238; types, mesozoic vertebrates,
361.

System in Geology, 69; Cambrian,
31; Carboniferous, 34: Cretaceous,
36; determining age of, 37; De-
vonian, 33; and era, 71; Juras-
sic, 36; Murchison's term, 71; Or-
dovician, 32; Quaternary, 37; Si-
lurian, 33; Tertiary, 37; Triassic,
35-

Systematic classification, 10.

Systems of classification, 27, 206;
geological, 31; not world-wide,
29; and revolutions, 39.

Taconic revolution, 41.

Tait, time estimate, 57.

Taxonomic rank and adaptation.
142; and environment, 148; and
geological range, 92.

Teeth of mammals, Osborne, 363.

Teleology, in Biology, 378; and the
organism, 163.

Telotremata, evolution of, 256.

Temperature as environment, 113.

Temporary continuance of species,
28; nature of species, 154.

Tentacles, meaning of, 228.

Terebratulina septentrionalis, char-
acters of, 258.

Terms of classification, genus. 201;
species, 201 \ genus ,proximum, 201;
medium, 201; summum, 201; ordo,
order, 201; classis, class, 201; indi-
vidual. 201 \ embranchment, branch,
201; subkingdom, 201; phyllum,
201; type, 201.

Terrestrial province, 113.

Tertiary of Cuvier and Brongniart,
12.

Tertiary period in geology, 24; sub-
divisions of, 26.

Tetracoralla, rate of differentiation
of, 85.

Tidal friction, time estimates, 56.

Time in cutting Columbia gorge, 56;

INDEX.

395

Niagara gorge, 56; breaks and
revolutions, 45.

Time estimates, data of, 56; geolog-
ical, 57; by geological deposits,
57; hypothetical, 49; Kelvin, 56;
tidal friction, 56, uncertainties in,
49; Ward, 48.

Time-periods and terranes, 28.

Time-ratios of Dana, 47, 54, 61 ; Wal-
cott, 61; Williams, 6l.

Time since glacial age, 57.

Times, geological, 26; relative
lengths of, 54.

Time-scale, and fossils, 66; standard
units of, 31.

Time-values of characters and rank,

93-

Thales and Anaximander, 152.
Theca, 91.

Thecacea, evolution of, 256, 262.
Thickness of deposits, 60; of rocks,

57. 58.

Transition formation, 13, 19.
Transmitted characters, 192.
Transmission and acquirement of

variation, 158.
Transmutation theory of lonians,

152.

Trullacea, evolution of, 256, 262.
Tunicata, definition of, 204; de-
scribed, 244.
Turbinolidce, rate of differentiation,

85.

Two scales, necessity of, 66.
Typical specific characters, 115.
Types of Spirifer, in continuous

series, 313; at initial period, 314.
Typical structure and types, 236.

Uebergangs Gebirge, 13, 16.

Unconformity and revolution, 40.

Undifferentiated cell, 221.

United States Survey nomenclature,
30.

Units of chronology, 51 ; of the time-
scale. 77.

Unstratified rocks, 18.

Use and disuse, in origin of species,
195-

Values of units of time-scale, 64.

Variability of Atrypa, Hall, 317; in
Darwin's theory, 193; an inherent
characteristic, 184; and perm-
anency of characters, 311.

Variation, acquirement of, 158; dis-
continuity of, 199; and evolution,
158; and mutability assumed in
discussion of origin of species,
183; in thickness, 58; the unsolved
problems of, 198.

Variations of Atrypa reticularis^ib;
and mutations, 70.

Varietal characters, 115; alone ad-
justed, 143.

Varieties, 207; in Darwin's theory,
194.

Varying antiquity of characters of
Spirifer logani, 190; conditions and
strata, 73.

Vermes, definition of, 204.

Vertebrata, definition of, 204.

Volutions of spires in Helicopeg-
mata, 286.

Von Baer's classification, 234.

Wallace, Alfred R., and distribution,
112; on species, 298.

Walther, Bionomy, 116; conditions
of environment, 116.

Ward, time estimate, 48.

Water, as environment, 113.

Water-vascular system, 229.

Wernerian theory of formations, 16.

Werner and the Lehmann classi-
fication, 13; and mineral char-
acters, 17; classification of rocks,
17, 19.

What are species? 149; is evolved ?
265, 269; is evolved, summary,
272.

Zittel's classification of Mollusca,
239; on fossils, 82; translation
from, 135, 329 343.

Zoantheria, 84; of each era, 84; and
the time-scale, 85.

Zonal adaptation, of Gastropoda,
140; distribution of Ctenobran-
china, 136.

Zonaric plankton, 116.

Zone, 69, 113.

Zones of Ammonites, 28; of environ-
ment, of Gastropods, 132; and
hemera, 68; of ocean, Forbes, 117;
Verrill and Smith, 117.

Zoological and geological biology,
98; specimen like a fossil, 163.

Zoologist, method of, 5.

Zygospira, jugum in, 284.

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