BIOLOGY

LIBRARY

6

BY THE SAME AUTHOR.

Evolution of To-Day. — A Summary of the Theory of Evo-
lution as held by modern scientists, and an account of the
progress made through the investigations and discussions
of a quarter of a century. Octavo . . . $1 75

CONTENTS : Introduction — What is Evolution? — Are
Species Mutable ? — Classification of the Organic World —
Life During the Geological Ages — Embryology — Geo-
graphical Distribution — Darwin's Explanation of Evolu-
tion— More Recent Attempts to Explain Evolution — The
Evolution of Man.

*' There have been so many volumes upon evolution that an ordi-
nary reader may be inclined to overlook this of Professor Conn. We
warn him, however, that in so doing he is sure to miss a rare contribution.
It is just the thing to set a layman right and is thoroughly judicial. It
sets down the general trend of thinkers as to evolution and Darwinism,
finding limits to both and marking their usefulness when properly
employed." — Hartford Post.

" Dr. Conn evidently favors the theory, but he does not write as a
partisan or to carry a point, but simply to show what has been the result
of the fruitful labors of the last twenty-five years. As a devout theist, he
considers evolution simply a method of creation, and does not believe that
this derogates from the glory of the Divine Architect." — N. Y, Observer.

G. P. PUTNAM'S SONS, NEW YORK AND LONDON.

THE LIVING WORLD

WHENCE IT CAME AND WHITHER
IT IS DRIFTING

A REVIEW OF THE SPECULATIONS CONCERNING THE ORIGIN AND
SIGNIFICANCE OF LIFE AND OF THE FACTS KNOWN IN RE-
GARD TO ITS DEVELOPMENT, WITH SUGGESTIONS
AS TO THE DIRECTION IN WHICH THE
DEVELOPMENT IS NOW TENDING

BY

H. W. CONN

PROFESSOR OF BIOLOGY IN WESLEYAN UNIVERSITY
AUTHOR OF "EVOLUTION OF TO-DAY"

G. P. PUTNAM'S SONS

NEW YORK LONDON

27 WEST TWENTY -THIRD ST. 27 KING WILLIAM ST., STRAND

(£l)e ^nithcrbocker IJpress
1891

BIOLOGY

LIBRARY

G

COPYRIGHT, 1891

BY
H. W. CONN

ttbe ftnfcfeerbocfeer press, t\ew iJorfc

Electrotyped, Printed, and Bound by
G. P. Putnam's Sons

PREFACE.

THE world of to-day is the product of the past
and the foundation of the future. We who live to-
day can only judge of the past by inference from the
present, and our only knowledge of the future is ob-
tained by observing the direction in which the world
seems to be trending as judged from its history.
Since life is the crowning phenomenon of nature its
study leads us closest to the hidden secrets of crea-
tion. The history of man has long been studied ;
the history of nature leading to man has only just
begun to claim attention. It is not long that man
has seen that history could be learned from other
sources than written records, and it is only a few
decades since he has conceived the idea of reading
the history of life from nature itself. For half a
century now have scientists been trying to pierce
the mysteries connected with the life of the past.
The history has not yet by any means been read,
but enough has been revealed to make a sketch of it,
as it appears to the biologist to-day, not a profitless
undertaking.

Of the history which can be written to-day much
is uncertain, much is no more than pure hypothesis

iii

IV PREFACE.

and speculation variously attested by miscellaneous
facts ; but, on the other hand, much is more firmly
established than any recorded history handed down
to us in manuscript. Some parts of the history of
life must rank among our most certain facts of
knowledge. In the following outline of this history
the endeavor will be made to separate carefully those
parts which are speculative from those which are
based on a more firm foundation, and to give to each
part the value it deserves.

It is well to note at the outset that the facts as
collected prove to us that the past history has been
a continuous one, and one in which the facts follow
each other with such logical consecutiveness that it
may be regarded as a logical whole. The history of
life has not been one of isolated facts, but a continu-
ous flow, in which each step is foreshadowed by the
one immediately preceding, and in its turn foretells
the next one. This condition of things has led to a
development in the life of the world and to a series of
facts which science has termed evolution. Whatever
be our philosophical understanding of this term, the
facts, so far as life is concerned, are beyond question.
The history of life has been one of the development
of forms and types from each other, or rather from
common centres. This law we call organic evolu-
tion. The facts upon which organic evolution is
based are beyond controversy, although there may
be still some dispute as to the interpretation of the
facts. No one whose judgment means anything
in the scientific world questions that the history of
life has been one of growth and development of

PREFA CE. V

types from common centres. To this extent the
theory of evolution may be considered as proved be-
yond dispute. In the following pages, therefore, the
development of types from centres will be taken for
granted. This is the most easily described by the
terms development and evolution, and the history of
life will therefore be outlined as one of evolution,
without implying by the use of this word anything
as to the manner of this evolution or as to its philo-
sophical significance.

H. W. CONN.

MlDDLETOWN, July, 189!.

CONTENTS.

CHAPTER PAGE

I. — INTRODUCTION — SOURCES OF BIOLOGICAL

HISTORY ....... i

II. — THE ORIGIN OF LIFE 16

III. — THE ORIGIN OF THE ANIMAL KINGDOM . 59

IV. — THE RECORD FROM FOSSILS ... 87

V. — A VIEW IN PERSPECTIVE . . . .113

VI. — A VIEW IN PERSPECTIVE (CONTINUED) . 141

VII. — HISTORY OF PLANTS 167

VIII. — THE FUTURE OF THE LIVING WORLD . 177

REFERENCES 189

INDEX . . 193

THE LIVING

CHAPTER I.

INTRODUCTION — SOURCES OF BIOLOGICAL HISTORY.

To discover the history of life and to predict its
future is perhaps the chief problem of biological
science. This is the basis of the discussion of or-
ganic evolution ; this gives meaning to the schemes
of classification of animals and plants ; this is the in-
spiration of investigation in embryology and geol-
ogy, of experiments on spontaneous generation, and
is indeed the object of biological discussion gener-
ally. Biologists are in every way trying to discover
how life arose and how it developed into its present
forms. Every source of evidence that can bear on
the question is probed-^the microscope, the chem-
ist's retort, and the geologist's hammer, each lending
its assistance. There is of course no written history
on the subject, no recorded sources from which to
draw anything except a few of the most recent facts.
The evidence from which the history is to be drawn
must be taken wholly from such accidental records
as nature presents charily to our inspection. The

2 THE LIVING WORLD.

indefiniteness and complexity of this sort of evi-
dence make the problem a very difficult one, and it
is not to be wondered at if as yet the complete
history cannot be told. But if the record is dis-
jointed, there is, on the other hand, one advantage
which history drawn from such a source presents.
Recorded history is frequently intentionally falsified,
and more often written so as to give personal im-
pressions of the historian by incorrectly stating the
facts, and thus making it impossible to determine
the truths of which detailed record is written.
There is no such falsifying possible to the record
given of the history of life written in nature. We
must believe nature is true, or give up all hope of
knowledge. While, then, the complexity of the
record makes the interpretation difficult, the impos-
sibility of nature making a false record gives to the
conclusions that are reached a certain security, of
which the biologist is proud and which is the justi-
fication of his claim that he deals only with facts.

There are several distinct sources in the realm of
scientific facts from which the student endeavors to
read the history of life. Recorded history of any
sort, of course, tells us almost nothing. A few facts
concerning the stability of recent species we do
succeed in learning from the monumental records
of Egypt, and a few facts in the history of man are
written, but this is all.

Evidence from Fossils.

The most valuable source of evidence from which
we can trace the history of life of past ages is that

INTRODUCTION. 3

of fossil remains. From the beginning of the deposi-
tion of the stratified rocks it has been constantly
happening that animals and plants which have died
have been preserved in muds and sediments. These
sediments subsequently harden into rocks, and the
buried animals become fossils. From the fossils
thus buried we can get many glimpses of the life of
early times, and we have only to examine the fossils
preserved in the successive layers of rocks to be able
to formulate more or less of a detailed history of
the life of geological ages.

Now this source of history has some decided
advantages. In the first place, when dealing with
fossils we are dealing with actual animals, and not
with a record simply. When we find a fossil we
know something of the size, shape, and appearance
of the actual animal or plant that once lived, and
thus we need not confine ourselves to general ideas
of type. We implicitly trust the evidence given us
by fossils, and do not have to ask if some modifying
circumstances have deceived us. When we find
a fossil oyster it is impossible to question that
oysters were in existence at the time of the deposi-
tion of the rocks in which the fossil was found, so
that the history drawn from this source is positive,
so far as it goes.

But, on the other hand, although fossils give us
abundant details, the history which can be drawn
from them is sometimes nothing more than a history
of details, lacking in the perspective view that makes
up a true history. In the first place, taking all of
the layers of stratified rocks together, we do not by

4 THE LIVING WORLD.

any means have a continuous record of geological
ages. The layers of rocks are the positive points
of history, but they are separated by blank periods
of which no history has been preserved, and of
whose duration even we can get no estimate, ex-
cept that they were extremely long. These periods
must forever be blank, and it was during these
periods unfortunately that the greatest changes in
the history of life occurred. Secondly, fossils can
give us history of those animals alone which have
had a hard skeleton. The hard parts of animals may
readily be preserved as fossils, but not the soft parts.
Consequently, of the orders of animals having no
skeletons, fossils can tell us almost nothing. Un-
fortunately, too, all of the early forms of life agree
in having slight hard parts or none. The Protozoa,
Coelenterata, and Vermes have left fossil records in
only a few cases. We are now learning further that
the animals which formed the beginning of the large
groups were usually without skeletons. The early
coelenterates, mollusks, and even vertebrates proba-
bly had no skeletons hard enough for preservation.
Again, it is evident that the early representatives of
any type were few in number, and their chance of
preservation was, therefore, slight. Hence it fol-
lows that fossils can only in exceptional cases tell
us anything of the early history and development
of groups. Further, since the skeletons alone are
preserved, fossils can tell us little of the develop-
ment of internal structure of animals, and this is,
after all, probably, the principal feature of import-
ance^. for in most cases it seems to have been changes

IN TR OD UC TION. 5

in the anatomy of soft parts which have inaugurated
all of the larger departures in animal history. Again,
the rocks in which fossils have been deposited have
been frequently subjected to the modifying effects
of heat and pressure (metamorphosis). Sometimes
this metamorphosis has completely obliterated what-
ever fossils may have been in them originally, and
very commonly it has so obscured their structure as
to render it impossible to determine very definitely
what they originally were. It is readily seen that,
the older the rocks the more liable they will be to
such metamorphoses, and hence the farther back
into history we go, the less definite becomes the
record. Of the most recent epochs, quite a complete
history is obtainable, but as we go back the record
becomes less and less sure, and finally it stops alto-
gether. Of the earliest history of life fossils give us
absolutely no trace, and it is even true that fossils
give us no satisfactory record of the early history of
any of the groups of animals.

From all this it will appear that although fossil
history is very definite where it is obtainable, it will
at best be a disjointed history abounding in details
at some points and lacking at others. Some small
steps in the history will be given in the most minute
particulars, because of the abundance of animals
with skeletons to represent them, while other im-
mense epochs will be entirely unrecorded from the
lack of proper conditions to produce and preserve
fossils representing the period. Moreover, from the
great metamorphosis of the rocks of the older periods,
the study of fossils will give us absolutely no record

6 THE LIVING WORLD.

of the early history of life and nothing of its origin.
As we shall soon see, the earliest fossil history repre-
sents a period when there was already a well-devel-
oped fauna, and this could of course not have been
the beginning of life.

Evidence from Embryology and Anatomy.

Some other source of evidence, then, is needed to
assist the record of fossils, and especially to aid in
giving the history of the earliest periods. At this
point we find a second important source of his-
torical evidence in the study of embryology. This
we may compare to a written history, since the his-
tory of the race is written in the development of the
individual, and since, like written history, it is open
to certain forms of false statement. Embryology
gives us a record of past animals, and does not, like
paleontology, offer the animals themselves for in-
spection.

The foundation of the value of embryology as a
source of history is based upon the' fact that the
embryology of an animal repeats in outline the his-
tory of its ancestors. This fact is sometimes called
the first biological law. It was announced early in
the century by Von Bear and Agassiz, and has sub-
sequently been investigated and confirmed by scores
of naturalists until .it is safe to say that there is not
a fact in biological science that rests upon surer
basis. It is hardly necessary here to enter into a
discussion of the facts upon which this law is based.
For our purpose, it is sufficient to state that the law

INTRODUCTION. /

is universally accepted in the biological world, and
we may assume with confidence that it will be a faith-
ful guide in the study of the history of life, at least
in so far as animals are concerned.

Embryology, as a source of history, offers certain
advantages over the record of fossiliferous rocks,
and at the same time it is open to many difficulties
of its own. If the embryology of an animal repeats
the past history of its ancestors, and if all animals
have descended from various common points of
origin in the past, it would seem that we should
only have to study the embryology of a few animals
from each great type of the life in order to deter-
mine accurately the history of life in the past ages.
There are two facts, however, which render such a
simple proceeding impossible.

First : The embryological history of an animal
lasts a few days or a few weeks ; the past history of
life has taken thousands of centuries. It is plain
that the embryological history of animals is too short,
and the past history of the life of the world is too
long to make it possible that the former should
retain a record of all the details of the latter. In
the epitome of the history which is presented to
us by the development of an animal, we find only
a few of the salient points in the history preserved,
which we may suppose represent such epochs of
the past as have been of the greatest importance,
and have therefore left the most lasting impres-
sion. In the embryological history we find that
details are left entirely unrecorded. The study of
an embryonic stage can give no idea of the actual

8 THE LIVING WORLD.

appearance of the ancestors which it represents.
Size, habits, shape, are entirely unknown, and fre-
quently we cannot tell whether the early animals
possessed a skeleton or a protective armor, and
numerous other points are left entirely without
even a suggestion. Embryology does give us gen-
eral points of the fundamental structure of the
early types, does give us clearly an outline of
the changes through which animals passed in their
early history, does give us an epitome of the early
development and growth of living things. Such a
history is much like a history of mankind which
should give an outline of the development of social
relations and political institutions, which should tell
that the family relation was superseded by that of
the tribe, this by the absolute monarchy and the
constitutional monarchy, and finally by the govern-
ment by the people. Such a history, dwelling more
or less at length upon these various forms of govern-
ment and showing their relations as well as their
modes of origin from each other, would be much
like the history which the study of embryology gives
us of the early life. A history giving no names, no
ideas of nations, we should regard as a poor history
of nations and people, but it might be a fair history
of mankind in general. So embryology deals in
types and their relations and origins, but tells very
little of actual animals. In dealing with a past
ancestral stage thus represented, we do not have the
same sort of satisfaction as in handling a fossil. We
do not know just what sort of an animal was really
represented by the stage in question, but we do

INTRODUCTION. 9

have the satisfaction of feeling that we are dealing
with a type which was of importance in the history
of animals, and therefore of more far-reaching sig-
nificance than any fossil or score of fossils that we
could pick out of the rocks. The relations expressed
by corresponding stages of different animals are very
suggestive, and a short study of embryology will
tell more of the true history of animals than the
collection of thousands of fossils.

Second : The second fact that renders the study of
history from this source a difficult matter is the very
common falsification of the true record. It is un-
doubtedly the law that animals tend to repeat past
history in their embryology, but it is also true that not
infrequently various modifying circumstances occur
to prevent the history being correctly recorded.
The true record of past history is thus commonly
obscured by numerous departures caused by the
conditions surrounding the embryo, and this of
course renders the reading of the history a difficult
matter. The embryologist has, however, methods
of correcting these errors in large measure, and of
reaching trustworthy conclusions.

In spite of these two objections, embryology is of
the very greatest assistance in enabling us to read
the past history of animals, and to a less extent that
of plants. Especially is this true of the early stages
of the history. As we have above seen, the fossil
records of animals become less and less satisfactory
as we go farther back in history, and they finally
cease altogether, long before we come to anything
like a union of the converging types into a common

10 THE LIVING WORLD.

starting-point. It happens, however, that it is of
just these early periods that embryology gives us the
clearest account. For various practical reasons,
embryologists have largely confined themselves to
the study of the early stages of the development.
These stages teach of the relations of types to each
other, and of the separation of the types from a
common starting-point. In other words, they give
us an idea of just that part of the history of life that
we fail and shall forever fail to get from the study of
fossils.

Closely allied to the study of embryology is that
of anatomy. It has been determined that the em-
bryological history of the higher animals of a class
passes through stages which are represented by the
adults of lower animals of the same class. This of
course renders the study of lower types a valuable
aid in tracing the history of the higher ones. It
is also recognized to-day that relations between
animals represent community of descent. When
we find two animals closely related anatomically we
interpret the fact as indicating a recent common
point of origin. Anatomical relationship thus rep-
resents blood relationship, and if we have the former
we can interpret the latter. It will, of course, follow
that the study of anatomical relations will teach us
history.

The study of anatomy and embryology cannot be
isolated from each other. Together they give us an
idea of the relations of types to each other, and
enable us to draw up a picture of early development
of life.

INTRODUCTION. II

Miscellaneous Evidence.

But even when we carry the history back to the
limits of embryological record, we fail to reach the
beginning. Embryology starts with the living cell,
and traces the growth of the organic world from this
point. Can we learn anything of the origin of this
cell ? In thus going back to the very beginnings of
the history of life, we leave fossils and embryos far
behind, and have to look for our evidence elsewhere.
There is no history yet discovered of the earliest
stages of the living world. There is nothing known
in nature which can tell when life first appeared
in the world, or how. Upon this subject we can
make only vague conjectures, instigated by various
factors of nature. That in some way living things
arose from that which was not living is certain, for at
one time the world was unfitted from its molten con-
dition for the existence of life. But whether life
first came in accordance with the laws of nature
which are still in operation, or in accordance with
supernatural laws, is still a. question in dispute, and
perhaps may always remain so. All the evidence
which can be brought to bear upon the subject is
indirect, and can be comprised under three heads.

i. Experiments upon spontaneous generation, t.e.9
experiments to determine whether life can to-day
arise from the non-living. These experiments have
been performed with a great deal of care and perse-
verance. They are, however, entirely incapable of
touching the real question. A positive or negative
conclusion would have little affected the great ques-
tion under discussion. At the present time they

12 THE LIVING WORLD.

have been decided in the negative, but this only
shows that under certain conditions (which con-
ditions were doubtless not those of the times when
life first appeared) life cannot arise spontaneously.
They tell nothing as to indefinite unknown condi-
tions of the past. If they had been decided in the
affirmative, they could show only that life could
arise under certain conditions, which conditions
again were unquestionably not those of early times.
The experiments have all started with a nutrient
solution, which contained already products which
were the result of life, z.r., meat solutions, etc., arid
such a condition would of course have been impos-
sible at the time when life began. At best, then, the
experiments which have been performed upon spon-
taneous generations could serve only to make us a
little better acquainted with life and the conditions
under which it now acts, without helping us a step
toward answering the question of its primitive origin.
It cannot be denied, however, that if this subject
had been or ever shall be decided in the affirmative,
it would render the origin of life by natural laws
more probable, since it would show that living things
could come from the non-living, and this would be
one step toward the solution.

2. Study of organic and physiological chemistry.
It may seem somewhat strange to count chemistry
as a source of biological history, and of course the
evidence it gives is only indirect. Chemistry has
been for some years now teaching us of the close
connection between chemical and biological laws.
It has shown that many organic bodies can be pro-

IN TR OD UC TION. 1 3

duced by purely chemical processes. It has shown
that many of the vital processes of living things are
simply chemical in their nature, and that some of
them may be imitated by lifeless material in the
laboratory. It has shown that chemical compounds
of great complexity have complex properties, and
that protoplasm is the most complex substance in
existence. In all of these ways has the close union
of chemical and biological laws been made evident ;
and chemistry has thus prepared the way for the
conception of a still closer union, and for the sup-
position that life originally, as now, was a mere
application of natural chemical laws to complex
i conditions, and thus arose by natural and not super-
natural law.

3. The study of the low forms of life. This is of
value in showing the simplest conditions of life, and
therefore bringing us nearer to the condition of the
first life. The simplicity both of structure and
function of some of the lowest forms of life seems
to bring us very close to the inorganic world. The
step from the amoeba to some inert chemical com-
pound is certainly less than from man to the same
compound. From the study of these simple forms,
we can easily conceive of still simpler masses of proto-
plasm with even less organization than the amoeba
and proteomixa. At the same time the complexity of
compounds manufactured by the chemist seems to
be approaching the somewhat greater complexity of
these simplest forms of life. It is plain enough that
the simpler the condition to which life can be

reduced and the smaller the gap between the sim-

?li -^^

OF

'UNIVERSITY))

14 THE LIVING WORLD.

plest living thing and the most complex compound
not alive, the less will be the difficulty in believing
in the natural origin of life. If we could find a
substance that was simply living matter with no
definite characteristics of any specific nature, this
would be the starting-point. From a priori grounds
we might expect that simple living matter would
appear before any definitely formed species. This
fact is the basis of the interest connected with the
study and speculation concerning protoplasm, the
supposed common factor of living things. In the
simplest forms of life we get down almost to pure
and simple protoplasm, and by taking from these
forms all of the common factors, we may suppose
that we obtain the characters of simple protoplasm
itself, and thus presumably the first form of life.

There is another series of facts which can hardly
be called evidence, but which does at the same time
have a great influence in our interpretations of the
past. The long-continued study of nature has led
to the formulation of the law of continuity. Ac-
cording to this law, the processes of nature have
been those of continual slow change, such that the
history of any minute is explained by the conditions
of the preceding minute. The law admits of no
great breaks in the history of the processes in nature,
but assumes that where such breaks seem to exist,
the break is only in our knowledge, and not in the
nature itself. It is impossible that the acceptance
of this law should fail to have great influence in the
interpretation of the life history, for by means of it
a constant development is necessarily substituted

INTRODUCTION. 15

for the series of epochs which the actual facts seem
sometimes to indicate. Especially is this true of
the conclusions as to the origin of life, for here, as
we have seen, direct evidence is wanting, and the
belief in the law of continuity forms the foundation
of all that is to be said on the subject.

Such are the sources of the evidence from which
our history of living nature must be drawn. Of its
beginnings we know nothing beyond such inferences
as may be drawn from experiments on spontaneous
generation, organic chemistry, and the study of the
lowest forms of life, together with the general teach-
ing of the law of continuity. Once established,
however, we can trace in outline at least the early
history of animals and plants through the ages by
means of the record we have of such history in their
embryology and their anatomical relations. Later
the stratified rocks begin to preserve for us here and
there a scattered page of history ; and as we come
to later ages, the leaves thus preserved become more
and more perfect until in the latest times a fairly
complete history may be read from them. Perhaps
by the study of the course of the past we may then
be able to hazard a prophecy as to the course of the
life of the world in the future.

CHAPTER II.*

THE ORIGIN OF LIFE.
What is Life ?

IN taking up the study of the history of life, we
must first ask the question: what is life? This
question is asked not in expectation that any satis-
factory answer is possible, but in order that we may
get as clearly as possible before our minds the chief
facts in modern thought concerning the subject. It
is clearly true that our scientists have by their specu-
lations and experiments so completely changed our
ideas of life that it sometimes seems as if we could
almost grasp the real essence of the matter. The
solution of the life question is said to be close at
hand. Tt is well for us at the outset, then, to review
the question, to see where we stand to-day in our
knowledge of life, and to notice what pointy have
been settled and what points still baffle comprehen-
sion. For, thus far, this life essence has been an
ignis fatuus ; and although many preliminary ques-
tions have been solved in pursuit of it, their solutions

* The substance of this chapter was originally published in the
New Princeton Review.

16

THE ORIGIN OF LIFE. 17

only serve to show us that there is something else
beyond, which is not comprehended. Our first task
is then to find out exactly what is the question now
at issue ; for it is very different from what it used to
be. We shall find that much is now universally con-
ceded which was at one time strenuously disputed.
There are three essential properties possessed by
living things which must be included in any at-
tempted explanation of life : a. Their constant
activity, b. Their power of growth, c. Their power
of reproducing themselves. These being the essen-
tial properties of life, their satisfactory explanation
will bring us far toward the understanding of life
itself.

Relations of Organic Activities to Physical Energy.

First we may say that the activities of organisms
are no longer looked upon as manifestations of a
distinct " vital force " unrelated to other forces. It
will hardly be denied by any one to-day that all of
the energy exhibited by organisms in their various
activities is a part of the store of energy of the uni-
verse, and that all of the forces exhibited by animals
are correlated with physical forces in general. It
has been conclusively proved that every motion
made by animals, every bit of heat a/ising in them,
is simply a portion of the energy which this world
has received from the sun. The process of its trans-
formation is as follows :

Plants, by virtue of the possession of a body called
chlorophyll (i.e., their green coloring matter), have the
power of using the energy of sunlight. By means of

1 8 THE LIVING WORLD.

the energy thus within their reach, they are able to
build complicated chemical substances out of such
simple compounds as water, carbonic acid, and simple

^nitrogenous compounds. It is a well-known principle
of physics that to build a complicated chemical body
out of simple ones requires the exertion of energy,
just as it does to place a number of bricks one on top

^of another. All of the energy thus used is rendered
latent, but it can all be obtained again in active con-
dition by pulling the structure to pieces. Every com-
plex chemical compound may therefore be looked
upon as a store of energy. Plants, then, growing in

/The sunlight are continually making use of the sun's
rays to enable them to build up complex compounds,
and they are therefore storing up the sun's energy
in the form of chemical energy. The energy of their
life therefore, consists in transformed sunlight. Now
animals use as food the chemical compounds thus
built by plants. Animals, unlike plants, are not able
to make use of the sun's rays directly, but they can
make use of the store of energy provided by the
plants. They therefore derive all the energy of their
life by breaking to pieces these products of the
plant's constructive power. Just as the steam-engine,
by breaking to pieces the coal which forms its fuel,
makes use of the energy thus liberated, so the body
by similarly breaking to pieces its food makes use of

ithe energy thus liberated. The steam-engine con-
erts the energy of chemical composition contained
in its coal into motion and heat ; the body also con.
verts the energy of chemical composition contained
in its foods into motion and heat. All of this is

vi1

V

THE ORIGIN OF LIFE. lg

practically granted everywhere, and we need not
attempt to question the conclusion further. All of 1
the energy of the body is a part of the physical
energy of the universe, and its forces are correlated
with other physical forces.

Relation of Vital Activities to Chemical Laws.

Again, it will hardly be questioned to-day that the /
chemical processes going on in the living body are
fundamentally similar to those which may take
place out of the body. The same laws of chemical
affinity govern the changes taking place in the body
and those occurring in experiments in the laboratory.
The chemical processes of the body may be con-
sidered under two classes. The first are the processes
of construction, by which simple bodies are built into
complex ones. This class is chiefly found in plants.
The second are those of destruction, by which the
complex bodies are broken into simpler ones. This
class is chiefly characteristic of animals, though
found also in plants. The destructive changes are
the simpler, and there is no reason to think they are
any different from the destructive processes of the
laboratory. The essential feature is oxidation, ancl^
oxidation may take place anywhere. It is true that
the details of the process of this destruction in
organic beings differs in some respects from that
which chemists have been able to simulate. WherT)
food is thus broken up in organisms, many decom-
position products arise which do not occur when the
process is carried on in the laboratory. These prod-
ucts are thus characteristic of organic beings, and it

2O THE LIVING WORLD.

was for a long time believed that they could never
be obtained except through the influence of vitality.
Modern chemistry has demonstrated the possibility
of making many of them in the laboratory ; many of
the simpler ones have already been manufactured,
and the list is constantly increasing.

Plainly, however, the destructive processes are by
no means so important as the constructive ones. In
plants the simplest compounds, H2O, CO2, and
NH3 are built under the influence of sunlight, into
the most complicated ones. Even in animals this
constructive power is essential, for they do not con-
tent themselves with simply pulling to pieces the
products of the life of plants. They do destroy most
of them, but the energy liberated enables them to do
a certain amount of building for themselves. They
change dead matter into living matter, which must
be looked upon as a constructive process. Now our
chemists tell us that they have reason for believing
that even these constructive processes are purely
chemical, and will one day be simulated in the lab-
oratory. They have, indeed, already shown that
many of these organic bodies can be manufactured
synthetically. Plants manufacture protoplasm, the
most complicated body of which we have any
knowledge. By the decomposition of this body may
be obtained a long series of decomposition products,
which become simpler and simpler until they are
once more resolved into the simple ones with which
the plant started. Now our chemists have begun
with these simple bodies, CO2, H2O, NH3, etc.,
and have begun to climb this ladder of compounds

THE ORIGIN OF LIFE. 21

toward protoplasm, To be sure, they have not yet
climbed very far, but they have made some advance.
Many of the simpler members of the series have
been manufactured synthetically from simple inor-.
ganic compounds. And since they have truly begun*"*
to ascend through this series, it is, of course, an easy
inference to predict that they will some day reach
the top and be able to make the higher members,
even protoplasm itself. In theory they already tell
us what the albumens are chemically, and expect to
be able to make them synthetically before a great
while ; indeed, at the present time chemists are
startled by the recent announcement of the manu-
facture of a proteid in the laboratory of Schutzen-
berger. Scientists are thus looking forward to the
time when they will be able to make protoplasm in
the laboratory, and thus artificially to make living
things. Judging from the general tendency of ad-
vance, it does not perhaps seem improbable that
they may some time make a body which shall
have the chemical composition of protoplasm ; but
whether or not this body would be alive is the very
question at issue.

Of course it is perfectly evident that the methods
used by the chemist in these syntheses are very
different from those employed by living cells. The
chemist uses complicated apparatus and long, round-
about processes to produce the simple organic com-
pounds. A transparent and seemingly structureless
mass of protoplasm builds directly and with ease the
most complicated bodies. No one will, of course,
pretend to compare the organic cell with the chem-

22 THE LIVING WORLD.

ical laboratory in any exact sense. vAll that our
chemists can succeed in showing is that organic
changes are governed by the same chemical laws as
those which regulate inorganic changes. And the
possibility of the manufacture by synthetical pro-
cesses of a number of the simpler organic compounds
gives us undeniable evidence that chemical laws are
the same in the body and in the laboratory.

Properties of Life as Explained by Physical and
Chemical Laws.

Recognizing, then, that all the energy of organ-
isms is derived from solar energy, and that the
chemical processes in the body are essentially sim-
ilar to those outside, the next question to be
answered is how far the vital manifestations of
organisms can be explained according to these laws ;
to see whether or not all of the activities of living
things can be explained by physical laws. And
here too when we reach the real opinion of various
thinkers, we find something like unanimity in many
points at least. Understanding the doctrine of the
conservation of energy, it is at once evident that all
of the energy displayed by organisms must be
transformed solar energy ; and hence all of the mani-
festations of the body which are measurable by
units used in measuring other physical forces must
come under this head. Here will of course be in-
cluded all the forms of motion, both molar and
^molecular. The motions of the body, the heat of
the body, expansion and contraction of protoplasm,
all electrical phenomena, probably also nervous im-

THE ORIGIN OF LIFE. 2$

pulses — all doubtless come in this category. Indeed,
all manifestations of the body which can be matched
by any machine will unhesitatingly be set down as
coming under the head of transformed physical
forces. We can believe that the body will do any-
thing that a machine can do without calling in the
aid of any distinct force. In other words, the
activities of living things, though more complex, are
as truly due to physical and chemical forces as those
of a machine.

It is when we come to the other properties of life
not found in machines that the problem becomes
more difficult. No machine has the power to assimi-
late food and grow. These properties, which really
are one, form the second character which universally
distinguishes living matter from non-living matter.
Now, so far as the mechanical process of growth is
concerned, it is simply chemical change. This is cer-
tainly so in animals. They take into their body certain
complex substances as food. This food undergoes
chemical changes, chiefly those of oxidation. As a re-
sult, decomposition products are obtained, and some
of these products of decomposition are ejected, while
others are retained in the body, and thus the body
grows. But it is not quite so simple as this, for, as
we have seen, a certain part of the changes are con-
structive. Some of the decomposition products of
the food are united together into more complex
compounds, and all of it is more or less altered, so
that none of the food is retained in the body in
exactly the same condition in which it was taken.
In short, the body assimilates its food, converting it

24 THE LIVING WORLD.

from its dead condition into its own substance. Now

•^—x*

it is not possible to imitate many of these changes as
yet in our laboratories, but that they are all chemical
changes can scarcely be questioned. Even the con-
structive changes by which the body raises the
compounds into a plane of greater complexity, must
be regarded simply as chemical processes, the energy
which is required for them being obtained by the
breaking down of other portions of food. And in
plants also growth must be regarded as a chemical
process, for it consists in the combination of simple
organic compounds to form complex ones under the
influence of sunlight.

The third property of living matter — reproduction
— seems at first sight to be a more marvellous power
than that of growth ; but most biologists think that
it is easily derived from the latter. Fundamentally,
reproduction is a direct and necessary result of
growth. In its simplest form, as found in the uni-
cellular animals, it is seen to be nothing more than a
division. The unicellular organism by chemical pro-
cesses continues to assimilate food and thus to grow.
It keeps on increasing in size until it finally becomes
so large that the cohesion between its parts is in-
sufficient to keep the great bulk together, and as a
result it divides into two parts. Each of these parts
is of course like the other, and there are thus two
organisms where there was only one before. This is
the simplest case of reproduction, and heredity in
this case is to be explained as a necessary result of
growth. Now, it is not difficult to see how the more
complex forms of reproduction may have been de-

THE ORIGIN OF LIFE. 2$

rived from this. If instead of a single cell there were
a large number of cells attached together, growth
might lead them all to divide in a similar manner
after the cohesion of parts had ceased to be sufficient
to keep them together. And such a method of re-
production docs occur in large groups of organ-
isms. Or it might be that certain parts, perhaps
a single cell only, would undergo this division, the
parts of this cell becoming free to form new indi-
viduals, and thus spores would arise. Perhaps two
individuals might fuse into one, and thus the vigor
of both would be combined into one. This would
lead to sexual reproduction. And so on. Of course
the details of this process are purely hypothetical,
and it is not our purpose to dwell upon them ; but
it is easy to see that reproduction can probably be
explained upon a mechanical basis as the result of
assimilation and growth.

It is thus seen that the three properties of life can
receive at least a provisional explanation from a me-
chanical standpoint in accordance with the laws of
chemistry and physics, and since we are at present
dealing with life in its simplest form, we need not
here trouble ourselves with its higher properties of
consciousness and intelligence. The explanations
thus offered are accepted with practical universality
by biologists, and we may regard the preliminary
question in the problem of life as definitely settled.

Difference between the Dead and Living Organism.

With all of this explanation and reduction of vital
manifestations to physical laws, no one can fail to

26 THE LIVING WORLD.

realize that something' is lacking ; and that though
scientists have explained much by hypothesis, they
have yet left the real question untouched. It is not
easy to determine definitely this life factor, — a factor
so prominent in the minds of those who disbelieve in
the mechanical theory of life, and so readily ignored
by those who hold this theory. That there is some-
thing more than has been reached by this explana-
tion may perhaps be made evident by a further
consideration of the parallel between an organism
and a machine. The comparison between the dead
body and the machine is exact, for each has the
mechanism which will enable it to transform one
sort of energy into another under the right con-
ditions. But in the body the requisite condition is
the presence of life, whatever that may be, which
guides the chemical changes taking place. In the
machine the necessary condition is the presence of
an engineer, who guides the forces and chemical
changes. The comparison of the living body should
not be simply with the machine in motion, but with
the machine plus the engineer. This difference is
great indeed. A machine may be ever so perfect,
and yet will not perform its work unless its engineer
supply its proper conditions. Food out of the body
will never go through the complicated changes above
mentioned unless subjected to very peculiar con-
ditions by the chemist. Food in the body will not
go through these changes unless subjected to the
action of life. Sunlight may fall upon CO2, H2O,
and NH3 eternally without producing the slightest
tendency toward a synthesis of these elements. But

THE ORIGIN" OF LIFE. 2/

let this occur in any living green plant, and how dif-
ferent the result. In some way living matter causes
a synthesis to take place. The presence of life in an
organism causes certain chemical changes to be set
up in it which result in growth. Remembering that
none of these changes will take place of their own
accord, it is perfectly evident that there is some-
thing in the organism beyond simple chemical
affinity, — some sort of power which directs chemical
changes. Whatever it may be, it is the essence of
life. In almost every sentence used in the com-
parison of animals with machines this factor can be
seen. Even Huxley, the foremost in the mechani-
cal theory, says, " We touch the spring of the word
machine," and the result is speech, and the term
" we " implies something not present in machines.

That there is a difference between organisms and
machines at this point may be made more evident by
consideration of the difference between living and
dead organisms. That the body is a machine, and
that like the machine it converts chemical energy
into mechanical energy will to-day be everywhere
admitted. But a machine cannot die. A machine
may stop its motions, but a machine at rest is not
comparable with the dead body. In both cases, it is
true, there is a cessation of the changes which con-
stitute activity, but in the one case the changes may
be resumed again, in the other this is impossible.
A dead body can never be revivified. It is more
strictly to be compared to a machine which has lost
its engineer, for with this loss disappears all possi-
bility of further action. Its mechanism may be

28 THE LIVING WORLD.

perfect, and it may have all the possibilities of fur-
ther action except a directing power, but without
this it is forever quiet. And so an organism is, so
far as we can see, frequently intact after death, with
all of its mechanism present ; there is just as much
stored energy in a pound of fat in the dead body as
in the living body, L id it is just as capable of being
oxidized. So far as we can see, therefore, every
physical condition may be present in the dead body
which is necessary to produce the process known as
life, if the process could once be started. But with-
out this spark of life to start and direct the chemical
changes no life can show itself. And in like manner
do we find all other comparisons ever made between
organic and inorganic matter failing at this point.
Living things have been compared with crystals, for
both grow, although the process of growth is very
different in the two cases. But a crystal cannot die.
Take it out of the solution upon which its growth
depends and it will cease to grow, forever remaining
stationary. Put it back in the solution, and once
more it will resume its growth. A steel bar may be
magnetized, and under these conditions will exhibit
properties which it did not possess before. But it
may be demagnetized by a blow, and thus lose all of
these properties. This seems indeed to bear much
resemblance to death until we remember that a steel
bar may be magnetized and demagnetized indefinitely,
and never once fail to exhibit its properties. But an
organism, when it has once lost its vitality, can never
be brought to assume a vital condition. It is perfectly
plain that at this point all of the comparisons of

THE ORIGIN OF LIFE. 2g

organic with inorganic processes fail. There are
some conditions supplied by the living organism not
found in the inorganic world, and these conditions,
whatever they may be, direct the play of chemical
forces in the organism.

The Vitalistic Theory.

We have now finally reached the question at issue.
The vitalistic question to-day is not to decide how
many activities of organism can be explained by
chemical and physical laws, but to discover what are
the conditions which regulate these processes; to
decide why it is that a living body can induce
chemical changes which are impossible in the dead
body.

One answer which has long been given to this
question is that the necessary condition is the pres-
ence of a " vital force " ; a force uncorrelated with
other forces, — a distinct entity in itself. This force
is life. It is conceived as having been supplied to
the world at the beginning of life on the globe, and
as having been handed down from one generation
to another, or perhaps created anew at each birth.
Vitality is therefore considered as something apart
from the physical universe, but as capable of exert-
ing an influence upon matter, to direct the changes
taking place in it. According to this view, spon-
taneous generation would be an impossibility, for this
vital force, not being derivable from other forces,
could have its origin only from previously existing
vital force. This theory labors under the disadvan-
tage of being unable to say what is meant by

30 THE LIVING WORLD.

vital force, for of course we can get .no conception
of any force except by its results. But the vitalistic
theory claims that life is an immaterial something
which directs physical processes so as to produce the
activities which distinguish living things. We need
not further consider this view, for it consists chiefly
in recognizing the necessity of something more than
chemical affinity and change, and in acknowledging
our inability to explain it by giving to it the name
vitality.

The Mechanical Theory of Life.

The point of dispute to-day is not whether the
vitalistic theory would explain facts, but whether it
is necessary. A purely mechanical view of life has
slowly arisen from the profuse speculations, which
claims to be able to meet the case without recourse
to any imaginary " vital force." The general ten-
dency of scientific thought gives a certain amount
of a priori bias in favor of such a view. It has
unquestionably been the tendency of science to
explain more and more of the phenomena of the
world in terms of the properties and laws of the
natural universe. The foundation of the law of the
conservation of energy, the conception of forces as
modes of motions, are great steps in this direction.
In the organic world the theory of evolution, the
application of the conservation of energy to the
mechanics of life, the perception that the same
chemical laws govern living things and dead, and
every discovery of likeness between vital processes
and those purely mechanical, are all steps toward

THE ORIGIN OF LIFE. 31

this general unification. It is certainly in a line with
this advance to reach a mechanical view of this life
essence. If, therefore, a mechanical explanation is
possible, there is good reason for believing that it is
in the line of truth.

The mechanical theory is, in brief, that the direc-
tive conditions of which we are in search are simply
those of chemical composition and molecular ar-
rangement. It is pointed out that the properties
of compounds increase in complexity with the in-
crease of the complexity of the compounds, that as
the molecule becomes more complicated its powers
and possibilities become more diversified. The prop-
erties of compounds have, moreover, no traceable
relation to the properties of the elements from which
they are made. Oxygen and hydrogen, when they
unite, form water, a compound with properties not
possessed by either of the elements ; and yet we do
not doubt that they are due to the properties of the
elements. It is therefore easy to make the far-
reaching assumption that,- when the molecule be-
comes as complicated as that of protoplasm, its
properties will be as complicated as those of living
things. One of these properties is to induce chemi-
cal changes in foods. Just as it is the property
of water to dissolve many chemical substances, so
it is the property of the highly complex body pro-
toplasm to cause chemical changes. When it is
possible, we are told, to manufacture the chemical
substance protoplasm, it will of necessity be alive,
for there are no peculiar powers in organisms not
inherent in them as the result of molecular arrange-

32 THE LIVING WORLD.

ment. The directive power which seems to exist is
no directive power at all, but only a property of
protoplasm. Just as it is the property of platinum
sponge to cause, when held in a current of hydro-
gen, the hydrogen to unite with oxygen and burn,
so it is the property of protoplasm to cause more
complicated oxidations to take place, which produce
the fundamental process of growth, and from this,
as we have seen, other vital activities easily follow.
We see, therefore, that the comparison of the body
with the machine plus its engineer is replaced by
a machine that is purely automatic, and finds in its
own complex composition the conditions which
regulate its activities. Death, according to this
idea, is simply the destruction of protoplasm, which
would, of course, destroy its properties. Just as
soon as protoplasm begins to lose its complicated
structure, it loses all of the properties belonging to it
as protoplasm ; and this is death. Demagnetism of
a bar of steel is therefore strictly comparable to
death, the only difference being that it is possible to
cause the steel bar to resume its former molecular
arrangement and once more to possess its magnetic
properties. The possibility, however, does not exist
in living things; the violence of death ruins the
machine. Even a machine cannot be started if its
adjustments are broken, and a living body being
more complex than any machine has its harmonious
action more readily ruined. What is lost in death is,
therefore, not any directing force but chemical or
molecular composition. The dead body is to be
compared not with a machine which has lost its

THE ORIGIN OF LIFE. 33

engineer, but with a broken machine which cannot
be mended. Life is thus only an abstraction from
the properties of living things, just as aquosity would
be an abstraction from the properties of water.

This mechanical theory of life is not at present
open to direct argument. The dynamics of proto-
plasm may be studied carefully ; it may perhaps be
shown that all the activities of protoplasm are easily
explained as the result of chemical and physical
forces. Already scientists are beginning to compre-
hend how the movements of protoplasm, which have
proved so puzzling, are intelligible as the results of
chemical change, whereby the density of the sub-
stance is altered, and consequently its shape. In-
deed appearances seem to indicate that perhaps all
the activities of protoplasm may be explained thus
easily. But all of this fails to reach the real ques-
tion at issue, which asks for the directive cause of
these changes. The only direct argument would
be to manufacture protoplasm and have it begin to
assimilate food, or to show in some other way that
a purely automatic machine is a possibility, which
shall, as organisms do, supply itself with its own con-
ditions of activity. Until this is done the mechani-
cal theory can only be an inference from the general
tendency of scientific advance.

The Origin of Life.

It is very plain that our verdict in regard to the
origin of life in the world will depend largely upon
what position we assume on the question just dis-
cussed. If we assume that life is a distinct force
3

34 THE LIVING WORLD.

unrelated to other forces of nature ; if, in short, we
accept the vitalistic standpoint, the matter becomes
very simple. Such a force could not have come into
existence except by a creative fiat. We should
simply say then that far back in the history of the
world some supernatural power introduced the first
germ of life into the world, and that this first germ
was the simplest form of protoplasm. To one who
holds this view all the attempts to find a natural
explanation of the origin of life are useless.

If, however, we are willing to accept, even pro-
visionally, the view that life is not a distinct essence,
but, as the mechanical theory of life would tell us,
simply an abstraction from the complex properties
of the substance protoplasm, then the question of its
origin assumes a new aspect. It becomes then a
legitimate question to ask how life arose in the
world. Life by its inherent qualities is self-perpetu-
ating, and if once it makes its appearance in the
world its remaining here so long as the conditions
admit is a matter of course. Geology tells us, how-
ever, that at one time the earth was so heated that
no living thing could have existed on its surface. It
follows from .this fact, that life on the globe must
have had a beginning. What then was the nature of
the forces which brought the first living matter into
existence ?

Spontaneous Generation,

First we must ask if experiment or observation
gives us any reason for believing that living matter
can arise from that which is not living. Ever since

THE ORIGIN OF LIFE. 35

living matter has been studied it has been believed
by many that living organisms could arise spon-
taneously, i.e.j without having any direct living
ancestors. Aristotle held this view, and from his
time for centuries no one presumed to doubt that
most of the smaller organisms could, and usually did,
arise spontaneously. It was not until the sixteenth
century that the matter became one of discussion.
At that time Redi discovered that fly-maggots were
not produced spontaneously from decaying flesh as
had hitherto been believed, but came from some-
thing deposited by adult flies. This discovery led
him to further observation, and finally to the con-
clusion that there was no such thing as spontaneous
generation. Since that time this doctrine has been
the ground of many a hard-fought battle. The fol-
lowers of Redi speedily began to show by careful
study of the facts that numerous cases of so-called
spontaneous generation were simply due to careless
observation. It was soon proved that at least all
the higher animals arise by the method of reproduc-
tion only. The adherents of the belief that life can
arise from the non-living were thus driven to base
their claims upon the origin of the smaller organ-
isms, and finally upon microscopic forms, which
can be studied only with extreme difficulty. But
so far from admitting this to be a retreat from their
position, they have shown that it is the most natural
conclusion possible. For a priori grounds should
serve to convince us that if living things can arise
from the non-living, this would be true only of the
very lowest organisms, those which approach the

36 THE LIVING WORLD.

nearest to the condition of simple protoplasm. The
disproof of the claims of the earliest biologists who
believed in the abiogenetic origin of the higher
animals, is therefore no proof or even indication
that this does not occur in the lowest organisms.
The question, therefore, finally settled around the
origin of the lowest and smallest forms of life.
From this point the matter has been chiefly one of
care in experimenting. It was found by some that
low organisms, bacteria, infusoria, etc., would arise
in closed flasks filled with various material for food,
even after all apparent precautions had been taken
to exclude everything alive. But other experiment-
ers employing greater precautions for the exclusion
of living matter obtained opposite results. The ver-
dict vibrated from one side to the other, as different
experiments were made known, until at length Pas-
teur and Tyndall showed that the negative conclu-
sion was the only tenable one. Tyndall, more espe-
cially, by a series of careful experiments conducted
in a manner beyond reach of criticism, so conclu-
sively proved that with the proper precautions no
living organisms could arise in any solution without
the access of previously living organisms, that no
one has seriously questioned the matter since. This
result, indeed, is only a negative one. It simply
shows that no living organisms did arise under the
conditions of the experiment. But it is so conclu-
sive that scientists have, with practical unanimity,
given up all claim that there is the slightest evidence
for the possibility of spontaneous generation. And
this is admitted by the very men who still insist that

THE ORIGIN OF LIFE. 37

spontaneous generation must have occurred at some
time in the history of the globe.

While it is thus true that scientists have somewhat
reluctantly given up this fascinating theory, it by
no means indicates that they have given up the be-
lief in the possibility of life arising from the non-
living under the right conditions. Although no one
has as yet been able to produce conditions under
which life can arise, this by no means proves that
under different conditions a different result might
not be reached. Protoplasm will not arise in closed
flasks, but this does not show that it cannot do so
at the bottom of the sea. If it could be shown that
life arises spontaneously nowhere on the globe at the
present time, this would by no means prove that in
other ages, under different conditions, it may not so
have arisen. And, indeed, now that the possibility
of spontaneous generation to-day is practically de-
cided in the negative, it is beginning to be recognized
that the experiments thus far are utterly futile to
settle the primary question at issue. Even if a
positive result had been obtained, it would have had
scarcely any bearing upon the question of the origi-
nal appearance of life. This will be evident from
the following considerations. The first living things
must have been able to make use of inorganic ma-
terial for food, since there could of course have been
no organic food existing at that time. Our experi-
menters on spontaneous generation have, however,
always used organic solutions in their experiments.
Now, to-day only organisms which are supplied with
chlorophyll are able to raise inorganic matter into an

38 THE LIVING WORLD.

organized condition. At the present time, at least, all
organic life depends upon the action of chlorophyll.
But in the experiments upon spontaneous generation
it has only been claimed that such organisms as bac-
teria and infusoria could arise spontaneously ; and
these organisms containing no chlorophyll have no
power to live upon the inorganic world. Our ex-
perimenters have found it necessary to supply them
with an abundance of organic food. Such organisms
certainly could not have been the first ones to appear
upon the earth, since they would be capable of exist-
ing only so long as organic food was supplied to them.
Indeed, if we could imagine the ocean filled with
albuminous food before any life appeared, and then
assume that these organisms could arise spontane-
ously, we should be no nearer to a permanent origin
of life than we were before. The only result would
be a rapid multiplication of these bacteria until the
ocean was filled with them ; the food would be con-
sumed, and then all would die of starvation, since
they would be unable to make food for themselves
out of the inorganic world as green plants can do.
The first living things must have been able to make
use of the inorganic world, and plainly, so long as
experiments deal only with chlorophylless organisms
arising in organic solutions, they have no direct rela-
tion to the question of the primary origin of life.

Should we then place the origin of life in the same
category of insolvable mysteries as the origin of the
universe in general? Looking at the universe in the
most extreme mechanical manner it is impossible to
think of it without some original creative power.

THE ORIGIN OF LIFE. 39

Behind the whole we must posit something which
no thought can comprehend. If we must find crea-
tive power somewhere, perhaps the beginning of life
may be an instance of its action. It may be well
then, inasmuch as it seems probable that the origin
of life can be nothing but a matter of speculation, to
class it with the origin of matter and force, and thus
to cease to explain it.

The question, then, stands something like this.
There is not the slightest evidence to-day for be-
lieving that life can arise in any other way than
through the influence of other living protoplasm.
From the earliest living thing to the present there
seems to have been a direct continuity of protoplasm,
generation after generation resulting from the normal
processes of reproduction, but in no other way. May
it not be best to abandon the question of the origin
of life and to say that it first appeared as the result
of a creative fiat ?

But this science refuses to do. Science grants
that there are insolvable mysteries, and that the
mechanical conceptions of the universe cannot ex-
plain all things. The origin of matter and force, the
origin of motion and consciousness, are utterly in-
solvable mysteries, and are hence outside the realm
of science. But it is thought that the origin of life
is not one of the transcendent mysteries, but is one
which will in due time be solved. This belief has
been more especially prevalent among scientists
since the precipitate advance of speculation in the
last twenty-five years, due to the growth of the ideas
comprised in the theory of evolution. This theory

4O THE LIVING WORLD.

or group of theories has led to a belief in the general
efficiency of natural law to account for natural phe-
nomena ; and from this conception has arisen the
claim that there must have been a natural origin of
life. While then biologists have somewhat reluc-
tantly given up their beliefs in the present possibility
of spontaneous generation, many of them even the
more strenuously assert that at some time, in some
way, life must have arisen from the non-living.

Speculations as to a Mechanical Origin of Life.

Unable, therefore, to obtain direct evidence either
for or against its proposition of a natural origin of
life, science endeavors to meet the question by
speculation. Having shown that vital processes are
closely related to chemical and physical conditions,
suggestions as to a possible causal connection be-
tween the two are of some significance. Speculations
as to the origin of life can, therefore, hardly be
called absurd, though they are almost unfounded in
fact. Although they cannot be regarded as having
much value, nevertheless modern scientific beliefs
are in a measure founded on them.

We may pass over as irrelevant the suggestion
that life may have been brought into the world by
meteors. This does not of course assist in the slight-
est degree in solving the question of the origin of
life. While different thinkers will hold different
views upon the general question, nearly all will de-
pend upon unknown conditions of the past for aid.
A line of speculation something like the following
would probably not be far from expressing the gen-

THE ORIGIN OF LIFE. 41

eral outline of the thoughts of most biologists to-day
who attempt to formulate any conception of the
primal origin of life in accordance with natural law.

Assuming provisionally that life is simply a prop-
erty of the complex composition of protoplasm, we
can go on to ask ourselves how this complex com-
position could ever have been reached. Now certain
facts of geology assist us much in this matter.
During the early history of the globe the tempera-
ture was so high that few, if any, chemical com-
pounds could exist. As the earth cooled by radiation,
the elements hitherto kept apart began to come to-
gether in chemical union. All during the long
process of cooling conditions existed which have
never been matched since. Even after the tempera-
ture had reached a degree which admitted the
existence of organic compounds, every circumstance
was utterly different from what is found to-day.
Different temperature, different relations of mois-
ture, different electrical conditions, an atmosphere
containing vastly more carbonic acid and oxygen
than ours ; all these factors, and thousands of others
of which it is needless to speculate, combined to
make the conditions of chemical union widely differ-
ent from any that can now occur. Under these
circumstances it is plain that, with the universal
chemical laws, chemical processes would be carried
on of which we can know nothing, but which would
be very different from any taking place in the world
at present, or which can be simulated in the labora-
tory by the chemist or biologist. In these early
times we thus see the possibility of production of

42 THE LIVING WORLD.

an almost infinite variety of compounds, each with
its own peculiar properties. Some of these com-
pounds were so stable as to continue to exist down
to the present day, almost unchanged. Others were
constantly changing. The compounds of carbon
especially were varied and unstable, as we may con-
clude from the compounds of that element known
to-day. Many of these carbon compounds doubtless
would disappear with a change of conditions, break-
ing up, to enter into other combinations and form
other unstable compounds. Now amid this continued
succession of changes, the conditions of heat, elec-
tricity, etc., might at one time have been such as to
cause the elements, carbon, oxygen, hydrogen, and
nitrogen, all of which were present in the atmosphere,
to unite into certain complex bodies approximating
organic compounds. That this is a possibility be-
comes evident when we remember that our chemists
have already begun to make these elements unite
by laboratory methods. Many organic compounds
have been synthetically manufactured from inor-
ganic material. Most of these compounds of early
times probably did not continue to exist very long,
since they were unstable, and had no power of self-
preservation.

' Thus far perhaps no one will hesitate to follow
the scientist, since he is dealing with authentic facts
rather than with speculation. But now he takes a
step into the dark. He supposes that at one time
these elements united into a compound which was,
owing to its peculiar composition, capable of causing
other bodies to change. By virtue of this power

THE ORIGIN OF LIFE. 43

other carbon compounds then existing were caused
to assume the composition of the new one, according
to the laws noticed in a previous section of this
chapter. Once this power is acquired the compound
possessing it would not disappear like the other
unstable compounds, but would remain permanent.
For this substance would assimilate food and grow,
and all the essential features of life can be deduced
from growth. This compound was of course proto-
'plasm in its simplest form. It was only one of a
large number of complex compounds, which made
their appearance under the peculiar chemical condi-
tions of early eras. Numerous others were doubt-
less formed, each possessing its own properties. But
only that compound which was capable of assimila-
tion could continue to exist in an active condition
during the subsequent ages. This substance event-
ually absorbed all other compounds in any way
similar to itself which may have arisen contempo-
raneously with or before it, and it remains, there-
fore, to-day the only living matter, the physical
basis of life.

It will be seen that, according to this speculation,
the first form of living matter was by no means
similar to any organism of to-day. It was rather a
diffused mass of protoplasmic substance, with no
differentiation into cells or parts or individuals. It
will be further seen that it is not necessary to as-
sume that this first protoplasm possessed chlorophyll
as has been claimed, for, according to the hypothesis,
there were many other carbon compounds of high
complexity produced at the same time. These com-

44 THE LIVING WORLD.

pounds, more or less similar to protoplasm, though
not capable of self-perpetuation, would serve the
first protoplasm as food. There would thus be no
lack of organic material for the subsistence of life of
the first protoplasm, even though this first organism
were incapable of feeding upon the inorganic world
directly. Doubtless, too, the conditions which pro-
duced the first living protoplasm existed fcw-srlong
time, and thus living matter would for a long time
be brought into existence by processes other than
those of reproduction. Indeed, there was no defi-
nite beginning of life. Here, as elsewhere, nature
made no jump, but produced life as she produces
everything else, by slow stages. Chemical processes
of early times resulted in the production of many
compounds which, acting upon each other, and acted
upon by the changing conditions, became modified
in an infinite variety of ways. Their complexity
and instability became very great. Finally, some of
the most unstable of all began to effect changes in
others which resulted in assimilation, and thus slowly
the properties became more marked. Simpler and
simpler substances were made use of as food. So
long as the original conditions lasted there would of
course be no need that living matter should possess
the properties of chlorophyll. Nor was this at all
necessary while circumstances were such as to make
possible the natural development of high carbon
compounds. Eventually the power to live upon the
simpler inorganic foods must have been acquired.
But it is only necessary to assume that this power
became fully developed by the time that the con-

THE ORIGIN OF LIFE. 45

ditions had so changed that protoplasm could no
longer be developed by the original spontaneous
method. Perhaps for ages protoplasm existed un-
able to use^organic food, but finding sufficient food
in the surrounding complex carbon compounds.
And when this power did at last become developed,
it was not acquired by all protoplasm. For just at
this point the organic world became divided into two
parts. One part did develop chlorophyll, and has
since been able to live upon inorganic matter, using
the energy of sunlight to build this matter into an
organic compound. Finding its food, carbonic acid,
water and nitrates and sunlight everywhere, this
class of organisms did not acquire the power of
motion. The other half of the living world never
developing chlorophyll became of necessity at last
parasitic upon the plants, and developed an almost
universal power of motion in order to enable it to
seek food. The animal and vegetable kingdoms were
thus finally separated from each other, with the
relations which they hold to-day.

Such, in brief outline, is the substance of some of
the modern speculations concerning the method by
which life arose. It represents one phase of such
speculations, and is subject to great modification in
the minds of different thinkers. It is plainly open
to sufficient criticism, and it is equally clear that it
is not capable of direct proof, at least in the present
state of science. It is as moderate in its terms as
any of the suggestions upon the subject, and makes
as slight claims upon our credulity. It will, at all
events, serve our present purpose of giving an idea

46 THE LIVING WORLD.

of the relation of speculation to the question of the
origin of life.

The Significance of such Speculations.

Now what are these speculations worth? Many
will immediately answer that they are worth noth-
ing. Others may regard them as having a certain
amount of suggestiveness, but no great value. It is
perfectly plain to every one that they are purely
hypothetical. Not only are they unproven hypoth-
eses, but they are further of such a nature that
there can be no evidence either for or against them.
They must unhesitatingly be set down as scarcely
more than bold guesses at a possibility. Even Hux-
ley says : " Of the causes which have led to the
origination of living matter it may be said we know
almost nothing." If, then, science is to confine it-
self to facts, these suggestions may be cast aside as
worthless. Why is it then that we find so many
biologists to-day willing, yes, more than willing,
anxious, to accept them ? Certainly it is not because
they are the simplest explanations, not because a
large number of converging lines of thought point
toward them. Those who seriously discuss these
speculations, or regard them as of any significance,
do so from some cause lying outside of the question
itself.

And this cause is to be found in certain philosophical
conceptions. Science studies the world from one
standpoint only ; a standpoint which its devotees
naturally believe will lead them most surely to the
truth. This study of nature from the exterior has led

THE ORIGIN OF LIFE. 47

to the grand generalization that all nature is gov-
erned by law. The significance of the word law does
not particularly concern science, but is left to other
realms of thought. Science satisfies itself in discov-
ering and applying laws. A thorough study of nature
has made it seem probable that natural law, when
thoroughly comprehended, will explain all natural
phenomena. So many facts formerly relegated to
the realm of the supernatural have been explained by
natural law that science has determined to call in the
supernatural as seldom as possible, and to accept no
breaks in the chain of law unless absolutely forced to
do so. This generalization is at the foundation of the
terms — law of continuity and evolution, as they are
used by science to-day. The significance which this
question of the origin of life has for all evolutionary
theories is at once evident. It is an important link
in the chain of continuity, for unless the spontaneous
generation of life be a fact, the law of continuity is
no law. For even if science does succeed in explain-
ing the development of life from the lowest to the
highest, but does not explain the origin of this first
form, it has only half accomplished that for which it
is striving — viz. : to reduce living phenomena to the
same laws which govern the non-living. It is not
surprising, therefore, that we find biologists observ-
ing, experimenting, and speculating, in order to find
some way to help themselves out of their dilemma.
To any one who is inclined to believe in this law of
continuity, and the efficiency of natural forces, such
speculations as the above, which show a possible
avoidance of a break at the beginning of life, have a

48 THE LIVING WORLD.

certain amount of significance. If we are inclined to
believe that " nature does not make jumps," it follows
that every break which we see in the continuity is not
a break in reality, but simply in our knowledge of
history. Many breaks which formerly existed in our
knowledge have disappeared with advancing dis-
covery. It is natural, then, to believe that the
present chasm between life and non-life was, at the
beginning of the world no chasm, but filled with lost
stages which can never be recovered. Speculations
as to the nature of these lost stages have, therefore,
some meaning in the light of the law of continuity.
Scientists do not look upon any of them as neces-
sarily or even probably true. They do not consider
that we have sufficient knowledge to say anything
definite upon the subject. But science does look upon
these speculations as indicating that the problem of
the origin of life is not an insolvable one. Scientists
take them for what they are — pure speculations, but
think that they tell us that the break at the beginning
of life is one of ignorance and not one of fact.

It is thus only the supposed existence of a philo-
sophical necessity which has created a demand for
some theory of a natural origin of life, and called
into existence the various speculations on the subject.
The conclusion has been reached that the general
advance of thought and investigation has practically
established the truth of the law of continuity. This
law, so thoroughly believed in by modern science,
demands the destruction of the chasm between the
living and the non-living. Science has, therefore, set
to work to destroy it. It has shown that the chasm

THE ORIGIN OF LIFE. 49

is not so great as was once thought ; it has proved
that the animal body, and protoplasm in general, is
a machine making use of the chemical energy of its
foods ; it has shown that growth is little more than
chemical change, and that throughout the organic
world the same physical and chemical forces are at
play as in the inorganic world, only under more com-
plex conditions ; and it has rendered it probable that
most if not all of the vital properties are directly
dependent upon and explained by chemical and
physical forces. Science has, in short, proved that
living processes are a continuous change of chemical
and physical forces, and that what we mean by life is
something to direct this play of force. It then
assumes that this something is to be accounted for as
the property of protoplasm resulting from its com-
plex composition. This assumption is plainly a long
step in the region of hypothesis. But once made, it
becomes easy to posit and to explain by speculation
the spontaneous origin of life. For, indeed, it now
follows as a matter of necessity. The conclusion
which experiment forces upon us, that spontaneous
generation does not occur in nature to-day, is cast
aside as irrelevant to the more fundamental question.
For we ought not to expect, even if life originally
did appear mechanically, that it could do so now,
since the conditions are so different. Concerning the
first origin of life, science, therefore, knows nothing,
and is obliged to rest satisfied with the statement
that its original mechanical origin is an absolute
necessity of thought. " To hold the beginning of
life as an arbitrary creation is to break with the

50 THE LIVING WORLD.

whole theory of cognition," says Zollner. To the
scientist who is convinced of the universal truth of
the law of continuity, therefore, the natural origin of
life, though not possible now, was possible and did
occur in early times under conditions about which
we can only speculate. Carbon in former times
certainly did crystallize in the form of diamond,
because of conditions which then existed, and it does
not do so now because of the absence of those
conditions. So, we are told, the elements carbon,
oxygen, hydrogen, and nitrogen, did in former
times unite together to form protoplasm, under
conditions which then existed, but have long since
passed away.

But this may seem to be attacking the problem
from the wrong end. The law of continuity is the
law to be proved. To any one who is disposed to
question the far-reaching significance of this law the
matter is by no means so evident. If we are willing
to accept the existence of breaks, few or many, in
the history of the universe, we may well place one
at the beginning of life. The break exists to-day, at
least, and no amount of ingenious speculation is suf-
ficient to cover it. Incapable of proof or disproof,
demanded by no bit of evidence, and if we do not
accept the law of continuity, not demanded by any
philosophical necessity, these speculations of science
will be regarded as worthless. They are laborious
searches after something which does not exist. But
on the other hand, when we remember how persist-
ently scientific advance is leading us toward a
mechanical conception of the phenomena of nature,

THE ORIGIN OF LIFE. 51

and how strongly the law of continuity is establish-
ing itself in the light of discovered fact, we must
admit that there is a significance in speculations on
the origin of life which is deeper than at first sight
appears.

Protoplasm Not the Simplest Basis of Life.

In the discussion which has gone before, proto-
plasm has been referred to as if it were a definite
chemical compound, though one of great complex-
ity. It has seemed to many that the first step in
the history of life must have been the production of
a protoplasmic mass like some of our present low
organisms {Myxomycetes}. But nothing is more
certain than that protoplasm is not a definite sub-
stance, and that it is by no means uniform in differ-
ent organisms. It is found to differ widely in its
physical nature, sometimes being almost solid, some-
times very liquid. It is found to differ more or less
in its chemical composition, no two analyses agreeing
exactly, and when we take its functions into consid-
eration, the differences between different specimens
of protoplasms become extreme. Although the
amoeba and the nerve cell of the human brain are
both composed of protoplasm, how wide are the
differences between the powers they possess. Plant
protoplasm is capable of making use of the energy
of sunlight, while animal protoplasm is not. When
we call protoplasm " the physical basis of life," and
try to prove the unity of the organic world by its
universal presence in all living things, we have not
by any means reached the bottom of the matter.

52 THE LIVING WORLD.

We have, it is true, taken one step farther toward
simplifying the life problem when we step from
the study of the cell to the study of protoplasm, but
we have not reduced life to a common factor. There
is, indeed, no such thing in nature as protoplasm in
general, but only particular kinds of protoplasm.
Protoplasm can produce new protoplasm, but not
new protoplasm in general, only new protoplasm just
like itself. The amoeba can make new amoeba pro-
toplasm and no other, while the nerve cell is capable
of giving rise to new nerve protoplasm. The study
of the variations among protoplasm has progressed
until we are slowly ceasing to look upon it as a
definite substance. The microscope investigation of
recent years has shown that not only is it chemically
complex, but it can no longer be regarded as physi-
cally homogeneous. Several distinct chemical com-
pounds have been discovered in it. Within the
simplest protoplasm has been found an extremely
complex structure. There is a dense reticulum
made up perhaps of hollow threads, containing
minute granules and a liquid matter (Fig. i). The
granules are united in various ways to form groups.
To-day we may almost say that the study of the
structure of protoplasm promises to become as com-
plex a subject for the future as the study of the
animal kingdom on a large scale has proved to be in
the past. Instead of looking upon protoplasm as
the primitive life substance, at the present time,
most special students of protoplasm would regard
the granules as more likely the fundamental part and
all else derived. These granules are excessively

THE ORIGIN OF LIFE.

53

minute, requiring the highest powers of the micro-
scope (four thousand diameters) to see them at all,
and yet they are now looked upon as having in
themselves all the remarkable powers of life. We
may perhaps even say that wherever there is a single

FIG. i. Protoplasm, showing its reticulum, very highly magnified.

granule there is life. By the activities of the gran-
ules growth takes place, by their differentiation and
union and by the products of their growth is formed
the compound vhich we have called protoplasm.
By their aggregation into definite groups are formed
the cells. The differences between cells are due to
the differences in their arrangements. Protoplasm

54 THE LIVING WORLD.

of itself is thus ceasing to play so important a part
in biological discussion as it promised to do ; its
place as a primitive life substance is being taken by
the granules with their various products, deuter-
plasm, microsomata, etc., etc., and in scientific litera-
ture to-day we hear less and less of protoplasm.
Protoplasm is no longer the physical basis of life, but
if indeed scientists should to-day recognize anything
junder this term, it would probably be the granules
which are found in such abundance within the proto-
plasm. And yet even these minute bodies, small as
they are, can hardly be said to have reached their
ultimate analysis. They show different functions,
in different cases, and cannot be looked upon as
alike. Whether the analysis can ever be carried
farther, of course we cannot say. But certain it is
that protoplasm cannot be any longer strictly re-
garded as the physical basis of life. To-day the
only universal properties of protoplasm seem to be
that it is reticulated and has certain powers of mo-
tion, growth, and reproduction. It is thus seen to
be always a secondary rather than a primitive sub-
stance, and the term protoplasm seems to be an
abstraction from the idea of many organisms, just
as the term mankind is an abstraction from many
individual men.

There are therefore many steps between the sim-
plest protoplasm yet discovered and the inorganic
world, or even between it and the simplest form of
living matter. Hence it is impossible to look at a uni-
formly diffused mass of protoplasm as the first step in
the life history. But however this may be, it will be
seen that it does not materially alter the significance

THE ORIGIN OF LIFE. 55

of the scientific speculation of the present chapter.
In showing the complex nature of protoplasm, the
microscope has perhaps given a little more founda-
tion to the supposition that its properties are due to
its complex composition, physical as well as molecu-
lar, but has not of course removed the difficulty of a
perpetually automatic machine, acting without any
directing power outside of itself, and has made its
mechanical origin a little more difficult of compre-
hension. The substitution of these granules for pro-
toplasm, moreover, only throws the question of the
origin of life a little farther back into the microscopic
field, but does not alter the problem as to the forces
which brought it first into existence.

Life Appeared in the Ocean.

Although, as seen above, the beginnings of life
are shrouded in darkness, there is one point that can
be stated with certainty. There is no d®ubt that the
first forms of life appeared in the ocean. Protoplasm
itself is largely water, and in the water it must have
appeared. At the early periods, when life appeared,
there was probably little or no dry land, and there
could have been no bodies of fresh water to serve as
the starting-point of life. Add to this the fact that
all of the earliest forms of life found as fossils are
marine, and it becomes certain that the ocean must
have been the place where the first living thing made
its appearance.

Summary.

Life, as we commonly use the term, is certainly
an abstraction from the properties of living things.

56 THE LIVING WORLD.

Whether in its ultimate analysis it is still to be so
regarded is an open question. The marvellous pow-
ers of living things, and the great mystery surround-
ing the processes of growth and reproduction, have
caused thinkers to believe that the property of life
was something more than a simple abstraction, and
to regard it as. a distinct and mysterious force resid-
ing in organisms, capable of indefinite expansion, and
having its origin in some direct creative power.
This conception has been retained while the belief
in the general mechanism of nature has been gaining
universal acceptance.

This conclusion, that of the vitalistic school, has
for some time been subject to criticism. The mys-
te'rious activities and powers of living things have
been in a measure explained as the results of known
forces of nature. The discovery of the law of con-
servation of energy, the discovery of the close rela-
tion of chemistry to vital processes, the manufacture
of organic products in the chemist's laboratory, and
the study of the various properties of the complex
compounds of carbon have all united in proclaiming
a close relation between the laws governing the liv-
ing and the non-living world. It was inevitable that
the conclusion would be reached that what we call
life is nothing more than a manifestation of ordinary
forces of nature under special conditions furnished
by the substance protoplasm. Such a position is
assumed by the mechanical theory of life.

Even when this position is taken, the fact remains
that at the present time protoplasm does not arise
except through the agency of other protoplasm.

THE ORIGIN OF LIFE. 57

We are thus forced to ask what could have been the
origin of the first protoplasm from which the rest
has arisen. After all the discussion, however, we
must finally admit that we do not know when life
first arose or how, nor do we understand the causes
which brought it into existence. Many secondary
problems have been and are being solved, but the
real question remains as yet untouched, except by
hypothesis and speculation. Vital processes may
all be shown to be chemical and physical processes,
but this will never explain why they are carried on
automatically in protoplasm and here alone ; and
granting, if we are inclined to do so, that it is one of
the physical properties of protoplasm to direct this
play of force, there still remains the fact that to-day
protoplasm can only come from other protoplasm.
Whence came the first protoplasm ? To this ques-
tion science offers in answer, first : the law of con-
tinuity, in terms of which the original spontaneous
generation of life from the non-living is a necessity ;
and, second, various speculations, which, though ac-
knowledged to be entirely unproved, are regarded
as showing that in the boundless possibilities of the
past, spontaneous generation might well have taken
place, provided always it be granted that life is
simply the result of complex chemical and molecular
composition.

From all of this discussion and speculation we can
isolate two facts: I. Life arose in the ocean. 2. The
first form of life was the simplest possible condition
of living matter, certainly simpler than any living
organisms with which we are acquainted to-day, and

58 THE LIVING WORLD.

very likely simpler than the simplest mass of diffused
protoplasm. Amid the confusing and unsatisfactory
mass of fact and speculation which centres around
the primal origin of life these two points may stand
prominently before us as certain, while all the rest is
hypothesis, with various degrees of probability.

CHAPTER III.

THE ORIGIN OF THE ANIMAL KINGDOM.

WE have now the history of life fairly started.
Living matter made its appearance in the geological
seas of the far distant ages, and was at first probably
an undifferentiated mass with the simplest powers of
assimilation and growth. Out of this the higher
orders of life were to arise, and we have now to study
the first steps toward the modern world.

Unicellular Life.

The first trace of anything leading toward the
modern world, of which we find evidence, is in the
formation of cells. It has been suggested in the last
chapter that there first existed a diffused mass of
living matter without differentiation into parts.
Such a conclusion has not been proved, and if such
a mass did exist, it probably continued only a short
time. It soon became broken up into small masses
which were more or less independent of each other.
Our reasons for this conclusion are these : The
simplest and lowest organisms which exist to-day
consist of just such little independent bits of proto-
plasm which we call cells (unicellular organisms).

59

60 THE LIVING WORLD.

Secondly, from the study of embryology we find
that all animals and all plants begin with their de-
velopment as single cells, and since embryology re-
peats past history this of course points to a former
unicellular condition of early animals. Here, then,
is our first sure starting-point. For a diffused mass
of protoplasm at the beginning of life we have only
a speculative foundation, but for a unicellular con-
dition of early life we have direct evidence.

Cells.

Before tracing the history farther, it is well there-
fore to say a word in regard to the meaning of the
term cell, as it is understood to-day by naturalists.
As originally used, the word cell was a good de-
scription of the object so named. The microscopic
study of plants showed long ago that they are in all
cases composed of a larger or smaller number of
little separate compartments or little boxes, like a
mass of honeycomb. Each box has a wall and con-
tains a mass of the living substance, the protoplasm.
These were named cells. With such objects as a
starting-point, for which the term cell was a good
description, the word has been extended in its appli-
cation until it has come to be applied to a very
different set of objects. It was soon found that all
plants and all animals were composed of somewhat
similar independent parts and the name cell was
naturally extended to animals as well as plants.
Inside of these cells there was subsequently shown
to be frequently present a bit of dense protoplasm
quite distinct from the rest, which received the name

THE ORIGIN OF THE ANIMAL KINGDOM. 6 1

of nucleus. In many cases, however, especially
among animals, the independent bits of protoplasm
were found to have no wall to surround them, and
many cells were found in which no nucleus could be
demonstrated. Still they were seen plainly to be

FIG. 2. A cell.—/' Protoplasm. N Nucleus. Ne Nucleolus. Civ Cell wall.

independent units and surely to deserve the name of
cells. Thus the term cell came to mean any inde-
pendent mass of protoplasm, with or without a
nucleus, with or without a cell wall. More recently
however, it has been quite definitely shown that,
though the cell wall is not always present, and there-
fore is not necessary to the constitution of a cell, the
nucleus is probably always present in active cells,
either as a distinct body or as a diffused mass. A
cell thus becomes an independent bit of protoplasm
with a nucleus ; or better, perhaps, since recent study
shows the fundamental importance of the nucleus, a
bit of nuclear matter with surrounding protoplasm.

62 THE LIVING WORLD.

Fig. 2 shows a cell in the modern sense. All of the
living parts of animals and plants are composed of a
mass of such cells. The cells however, are far from
being all alike. Indeed, they differ extremely in
shape and size and in many other respects, but in all
cases each consists of a bit of nuclear matter sur-
rounded by protoplasm, and each is thus more or
less independent of the others. The larger and
higher animals are composed of a larger number of
cells, and always of a larger variety in their shape
and function.

The cell has thus been for many years regarded as
the unit of life, and the life of the individual as the
sum of the life of its cells. When we go among the
lowest and simplest of animals, we find one large
group in which the individual consists of only a
single cell. These are the unicellular animals {Pro-
tozoa) and plants (Protophyta). They are all aquatic
organisms, all small, usually microscopic, all of course
extremely simple, but withal extremely abundant :
they are found in the ocean, in rivers, brooks, ponds,
pools, ditches, swamps, gutters, puddles ; in short,
wherever there is any water, there we may be pretty
sure to find some representatives of the unicellular
organisms. Fig. 3 represents a few types of these
animals.

It was a great simplification of our knowledge of
animals and plants when it was discovered that all
were to be regarded as complexes of these simple
cells, for it gave a unit of life. The unicellular animals
and plants were naturally regarded as the simplest
possible organisms, since they consisted of one such

THE ORIGIN OF THE ANIMAL KINGDOM. 63

unit. It was therefore extremely significant to find
that in their embryology all animals and plants alike

FIG. 3. Types of Protozoa.— I Amoeba. 2 Actinospherum. j Paramecium.
4 Vorticella dividing. 5 Euglena. 6 Trichomonas. ' 7 Podophyra.

began life as a single cell, the egg and the germ cell
of the plant being always single cells. To find the
cell as the unit of life, to find a group of unicellular

64 THE LIVING WORLD.

organisms occupying the lowest position in the scale
of nature, and to find all animals and plants begin-
ning their embryology as single cells, were coinci-
dences of remarkable interest. There can be only
one interpretation of this. Since embryology is an
epitomized account of past history, the fact that all
animals and plants begin life as single cells, of course
must mean that, if we could follow back the history
of animals and plants, we should in each case finally
come to some unicellular organism. A unicellular
organism was therefore a common starting-point of
all animals and plants, and the Protozoa and Proto-
phyta of to-day are of interest as being close repre-
sentatives of the earliest organisms of which we have
any suggestion in our recorded history.

This conclusion of a unicellular starting-point of
all animals and plants is a significant result of bio-
logical study. At the same time, as was seen in the
last chapter, more recent and exhaustive study of
cells is beginning to show that the simple cell is not
itself the unit of life, but is a complex body. Pro-
cesses are found to take place inside the cell which
indicate that we are still far from the unit of life. It
is seen that of the whole cell the nucleus is really the
essential part, and that it regulates the activities of
the rest (Fig. 2). The nucleus itself is moreover a
complex. No fewer than five different chemical
compounds have been found to exist in it. In struc-
ture also we find various fibres, liquids, and granules,
all of which seem to undergo definite cycles of change
in the activities of the cell. All of this indicates that
we are still far from the unit of life when we have

THE ORIGIN OF THE ANIMAL KINGDOM. 6$

reached the single cell. There is, perhaps, as broad a
series of phenomena to be studied between the single
cell and the real unit of life, as those which we have
found between the higher animals composed of many
cells and the single cell. Animals have all been
reduced to complexes of cells, but the cell bids fair
to be still farther reduced to a complex of inconceiv-
ably small granules. Be that as it may, it still remains
a fact that in the history of life we have not yet
discovered any direct evidence of an earlier condition
than that of the unicellular organisms. Embryology,
which is our first source of evidence, gives us a
record of a unicellular condition, but of nothing
earlier than this. The first step that we can take in
narrating the history of life is, therefore, to establish
the period when unicellular animals were the highest
condition of organic life in existence. The diffused
protoplasmic mother of life, if ever such existed, had
become broken into independent masses.

Of course, nothing can be determined in regard to
the variety of form of these early organisms, nor can
we say whether they were all alike or whether they
showed variety like that of the unicellular organisms
to-day. Of their habits we know nothing. That
each organism was independent and capable of car-
rying on all the functions of life within itself follows
from every source of belief. That these organisms
multiplied by simple division is certain from the fol-
lowing reasons : Multiplication by division is a uni-
versal phenomenon in all living things to-day. In its
simplest form it is shown by most of the existing
unicellular animals, as is. illustrated in Fig. 4. As
5

66 THE LIVING WORLD.

there shown, multiplication simply consists of the
division of the original cell into parts, each of which
is like the other, and each of which is henceforth
independent of the others. Now such multiplica-
tion is universal in the unicellular organisms ; such
a multiplication is nearly universal among the cells

FIG. 4. Amoeba in the act of dividing. — The nucleus, N, has already separated
in two parts.

of the bodies of higher animals ; such a multiplication
is the universal method by which the single-celled
ovum begins to grow and develop into the many-
celled adult. From all of this there can be no room
for doubt that such a method of multiplication was
possessed by the earliest unicellular animals, which
we must assume lived at the very beginning of the
history of life in early ages.

*

The history of the living world has been like that
of a branching tree, a main trunk dividing into
branches, and these in turn subdividing until they

THE ORIGIN OF THE ANIMAL KINGDOM. 67

become lost in thousands of small twigs. From the
time of branching it is of course impossible to follow
the history as a whole, and only possible to follow
that of the various branches. At the point which
we have now reached we have come to the end of
the trunk, and we find the first division into two large
branches, animals and plants, as mentioned in the
preceding chapter. The separation between these
two kingdoms seems to have occurred even while
the living world consisted of unicellular organisms.
From this point we must follow the animals and
plants separately. We shall first take up the his-
tory of animals, leaving that of plants for later
consideration.

Early History of the Animal Kingdom. — The Origin
of Multicellular Animals.

The Protozoa as they exist to-day may be fairly
supposed to illustrate the early unicellular animals
of pre-historic times. The next stage in the history
of life was the appearance of the multicellular organ-
isms. It is again the study of the unicellular ani-
mals to-day which gives us suggestion as to how
this change arose. Among most of the unicellular
organisms, when a cell divides, the parts completely
separate from each other and live subsequently inde-
pendent lives. Among a few living forms, however,
the parts do not completely separate from each other,
but remain together in some sort of connection and
are more or less dependent upon each other. Fig. 5
shows such a group of cells, the members of which
have arisen from the division of a single one, and which

68

THE LIVING WORLD.

have not completely separated. There is a small
amount of dependence of the cells upon each other ;

FlG. 5. A Colony of Protozoa.— Each cell is independent, a and b show manner
of division to form colonies.

they share each other's food, and all seem to be con-
nected by some physiological bond. Still, each is in
itself complete individual, having a complete system of

THE ORIGIN OF THE ANIMAL KINGDOM. 69

organs, and each can live by itself perfectly well if
separated from the others. Such a group does not
form a multicellular animal, since the cells are each
complete individuals. It is rather a colony of uni-
cellular animals. Nevertheless, such a colony is the
first step toward the production of the multicellular
organisms. From embryological evidence also there
can be little doubt that such was the method by
which the multicellular animals and plants began to
develop in the early history of the world. We have
no means of determining to what extent this forma-
tion of such colonies of independent animals occurred.
It was indeed only a stepping-stone toward the next
stage in the history of living things, a stage of much
more importance, viz., the formation of the first true
multicellular organism.

To form a multicellular organism it is not sufficient
that there should simply be a large number of cells
attached together. This occurs in many of the ani-
mals that are regarded as unicellular. In order that
there should be a true multicellular animal, a meta-
zoan in distinction from the protozoan, it is necessary
that there should be a certain amount of division of
labor among the cells. So long as each cell carries on
all the functions of life in itself the aggregation of
cells forms simply a colony, but when the different
cells in such a group begin to assume different func-
tions— e.g., some of them capturing food and others
digesting it, — then the different cells become strictly
dependent upon each other, and there arises a true
multicellular animal. A multicellular animal is one
in which there is an aggregation of cells, each per-

70 THE LIVING WORLD.

forming only a part of the functions of life, and thus
each dependent upon the others. The whole forms
a unit. In such a community it is no longer possible
for a single cell to be separated from the rest and
still continue its life, for such a cell would be able to
perform only those duties for which it was adapted,
and it would therefore soon die.

The Gastrcza, the Common Trunk of the Animal
Kingdom.

That such a multicellular community arose in the
early history of life from the unicellular forms, is of
course evident, and that it arose by cell division
which did not become complete enough to separate
the individual cells from each other, is also almost
certain. Exactly how a division of labor first arose,
or what that first division may have been, it is perhaps
impossible to say. Indeed, it is not improbable that
in the early history of life there may have arisen
many different types of division of labor giving rise
to different kinds of true multicellular communities.
But whatever may have been the early varieties of
such differentiation, the very important fact is true
that only one of these early types of multicellular
animals perpetuated itself so as to affect the subse-
quent development of animals. The study of embry-
ology shows that all of the subsequent types of the
animal kingdom arose from one definite type of early
differentiation. In other words, of all the types of
cell aggregates or communities which may have been
produced in the early history of life only one of them
proved itself of value enough to take a definite place

THE ORIGIN- OF THE ANIMAL KINGDOM. 7 1

in the history of animals, and from this one all other
types of animals have developed.

The early type of animal thus referred to as of so
much importance in the history of living things is
one of which we have abundant evidence in embry-
ology, though perhaps the type does not exist to-day
as a distinct animal. Since embryology is the only

FIG. 6. A Typical Gastrula.— The shaded cells, en, are those connected with
the digestion of food, en Endoderm. ec Ectoderm.

evidence of this stage in the history of life, our
knowledge of this early form is, for reasons shown,
confined to its fundamental structure with practically
no sure knowledge of its details. The type in ques-
tion has been named the Gastraea by Haeckel, who
first clearly saw that embryology taught that such a
common ancestor formerly existed. The evidence for
this conclusion is as follows : It has been found by a

72 THE LIVING WORLD.

long-continued study of embryology that all multi-
cellular animals pass through a stage which embryol-
ogists have called a gastrula. A typical gastrula is
shown in Fig. 6. It is seen to consist of a cup made
of two layers of cells, one of which forms the outside
of the cup and the other the inside. When the two
layers are thus formed they assume different func-
tions ; the outer layer of cells becomes especially
connected with the powers of motion, sensation, and
the other functions directly connected with the outer
world. The inner cells being removed from direct
contact with the exterior, and being in a measure
protected, take for their function the duty of the
digestion of food which is supplied to them by the
outer layer of cells, the opening serving for both
mouth and anus. Now all animals show evidence of
having passed through some such stage as this gas-
trula, and the interpretation of the fact can mean but
one thing. If embryology repeats past history this
must mean that near the beginning of the develop-
ment of the aru'mal kingdom, there was a common
ancestor which corresponded in its fundamental feat-
ures with this gastrula stage, and further, that all
animals which at the present time show traces of this
stage in their development have descended from that
common ancestor. To the common ancestral stage
thus indicated, Haeckel gave the name Gastraea, a
name which is seen to correspond to the embryologi-
cal stage, the gastrula. This name we will retain in
the following discussion, although it is very probable
that the Gastraea, as Haeckel understood it, never
existed. The idea as Haeckel conceived it has

THE ORIGIN OF THE ANIMAL KINGDOM. 73

undergone some considerable modification within
recent years, but these modifications do not alter the
wide significance of the common appearance of this
stage in the embryology of animals. While then the
Gastraea as originally conceived may be modified in
future years, there is no probability that future study
will make any less significant the early ancestor
whose fundamental structure was essentially that of
the gastrula.

We may start the history of multicellular animals
then by supposing that the unicellular animals first
became aggregates of cells, and then that those cells
which were situated on the outside of the mass
acquired special functions connected with the rela-
tion of the animal to the external world, while
the cells which were in the interior of the mass took
for their duty the digestion of food which was
passed to them from the exterior. In other words,
there was a division of labor between the cells of the
animal, and when this had appeared, there arose the
first true multicellular animal. While we have no
fossil evidence of the form of the first multicellular
animal which appeared in the world, the evidence
derived from embryology tells us pretty surely that
it must have been one which could be described as a
two-layered sac, probably open at one end, for the
injection of food and the ejection of waste, whose
inner lining served for digestion, and whose outer
covering possessed motor and nervous functions.

As a further confirmation of the conclusion thus
reached from embryology, it is highly important to
notice that there is still in existence, among the

74

THE LIVING WORLD.

lower marine and fresh-water animals, a type scarcely
more than such a two-layered sac. The Coelenterata
is perhaps in some respects the lowest group of
multicellular animals, and among this group some
orders have a structure which is almost exactly that

FIG. 7. Diagram of the structure of Hydra. — It differs from the gastrula of
Fig. 6 only in having the body around the mouth expanded into tentacles, t.

above described as characterizing the first multi-
cellular animal. Hydra (Fig. 7) is readily seen to
be simply a two-layered sac, with an opening at one
end. The functions of the two layers of Hydra are

THE ORIGIN OF THE ANIMAL KINGDOM. 75

the same as those supposed to have been in the
early primitive animal. Among other coelenterates
this simple type is more or less modified, but it can
always be seen to have the same fundamental plan.
Since, then, the lowest existing group of multicellu-
lar animals is essentially the same in structure as the
Gastraea, to which embryology teaches us to refer all
animals, we must conclude that the former existence
of the Gastraea, as the starting-point of multicellular
animals, is one of the best attested facts of biological
science. It is a conclusion that can be gainsaid only
by denying completely the value of embryological
and anatomical evidence, and this would of course
be to deny the cogency of biological science com-
pletely.

The first step in the history of life toward the
development of the higher animals may then be
briefly summarized as follows : Some original uni-
cellular animals, in the course of their repeated
divisions, failed to separate completely into single
cells after dividing, but the cells thus produced
remained attached to each other. At first the cells
were all alike, and had similar functions, but after a
time the cells on the outside and those on the inside
of the mass began to perform different duties.
Those on the inside could no longer use any powers
of motion, and the possession of sensitive functions
would be useless, since they had no direct relations
with an external world to excite the sensations. It
is a law of nature that any power which is not used
begins to degenerate, and therefore the internal
cells soon lost their motor and sensory functions.

76 THE LIVING WORLD.

Since they were protected from internal injury, they
could, on the other hand, better perform certain
functions connected with the preparing of the food,
provided that they could succeed in getting hold of
the food itself. The outer layer of cells, coming into
direct contact with the world, retained all of the
powers which enabled it to be stimulated by that
world, and soon learned to pass food to the inner
layer of cells for digestion.

We are still in the dark as to the exact manner in
which the special form known as the Gastraea arose.
According to some embryologists, the cells first
formed a hollow sphere, and then one side of it was
infolded, as one would push in the side of a hollow
rubber ball. According to others, the mass of cells
was solid, the outer ones soon becoming different
from the inner ones, and later a cavity appeared in
the middle, which broke through to the exterior at
one end, and this opening formed the mouth. Ac-
cording to still another view, the cells at first formed
a flat mass of two layers of cells, and this by folding
up into a cup, or going through other modifications,
became the two-layered sac which forms the first
distinct stage in the development of the multicellular
animals. According to others still, and this is the
most recent view, a hollow sphere was formed, and
thus cells from the shell migrated into the interior,
one by one, to form the internal layer. The mouth
opening was developed later. But whatever differ-
ence there may be in our ideas as to the details of
this formation, no biologist questions the fact that
very early in the history of the living v/orld there

THE ORIGIN OF THE ANIMAL KINGDOM. 77

was developed an animal with an outer and an inner
layer of cells, and a mouth opening, and that this
form has been the starting-point of most, if not all,
of the subsequent multicellular animals. We shall
call it the Gastraea, although it may not have been
exactly the same sort of animal as that to which
this name was originally applied.

Divergence of Types.

The next step in our history of animals was one of
great importance. The Gastraea became moulded into
types which foreshadowed the animal world of to-
day. Of the modifications of the Gastraea by which it
became changed into the various types of higher
animals, embryology and anatomy alone give us evi-
dence, and even here the evidence is clear only in a
few cases. There seems to be conclusive proof that
the Gastraea was the last stage which was shared in
common by the different groups of animals, and
possibly some groups branched off even earlier than
this typical Gastraea stage. From this point cer-
tainly a divergence took place which soon resulted
in the formation of a number of animal types which
we now recognize as the sub-kingdoms of animals.
The details of this divergence embryologists have
not yet fully mastered. The simplest case seemed
to be along the one line which gave rise to the cce-
lenterates. In this line of descent the original Gas-
traea attached itself by the end opposite the mouth,
and by then undergoing various small changes in
form, such as production of tentacles and elongation
of the body, produced the group of animals of which

^8 THE LIVING WORLD.

Fig. 8, g, is an example, the group of hydroids
including corals, jelly-fishes and sea anemones. In
nearly all other lines of descent from the Gastraea
there was soon acquired a new fundamental feature.
The mouth, which originally served for the entrance
of food and the excretion of waste matter as well,
was replaced by a pair of openings, one serving for
each function (Fig. 8, b). Just how this took place
is perhaps still a little problematical. The proba-
bility seems to be that the one opening of the Gastraea
elongated into a slit and then closed in its middle.
The two ends of this elongated slit remained open,
however, one for the entrance of food, and the other
for the exit of refuse. With the subsequent elonga-
tion of the body these two openings became widely
separated from each other. After this further
modifications of the body arose. Developing a shell
on its back (Fig. 8, e), it started along a line which
has produced the type which we have called mol-
lusks (snails, oysters). Along one line of descent
this shell assumed a spiral twist, giving rise to the
snails (Fig. 8, /). In another the shell became
divided into two pieces, one on either side, giving
rise to the clams, oysters, etc. ( ' Lamellibranchiata).
In another line it became elongated and divided into
segments, and this gave rise to the large type of
animals known as segmented animals (Fig. 8, d\
(segmented worms, Crustacea, Insectd). An elonga-
tion and segmentation and subsequent modification
in many important particulars, the chief of which
was the development of an internal skeleton, pro-
duced the type of Vertebrata. The details of these

THE ORIGIN OF THE ANIMAL KINGDOM. 7

79

changes, however, need not delay us. They are
technical in nature and could only be understood by
one acquainted with comparative anatomy and em-

FIG. 8. Diagrams illustrating the origin of the Coelentera Mollusca, and
Annelida from the gastrula. — To produce the Ccelentera a becomes directly
modified into^-. To produce the Annelida the line of development is«, £,<:, d.
To produce the Mollusca it is a, b, e^f. In/" the twist that appears in the shell
twists the digestive canal, m, mouth ; «, anus ; j/z, shell ; wy, cells which are
to develop into muscles.

bryology. Nor, indeed, must it be understood that
the embryologist can follow them in all cases even to
his own satisfaction, nor that all embryologists agree.
Various methods of accounting for the origin of the

UJI7H.RSITT;

-«W«

8O THE LIVING WORLD.

vertebrates are advanced. The relations of the star
fishes (Echinoderms)) the group of low worms
(Vcrmes), and some other types still prove a puzzle
to him, and it will doubtless require much investiga-
tion still before the embryological history is fully
elucidated.

The history of the development of the primitive
Gastraea into our modern types is a matter of specu-
lative interest to the specialist, but to the general
reader is too complex to make it worth while to
dwell upon it. There is, however, concealed in this
subject a general principle of development of the
most extreme importance. From the history of this
Gastraea we learn that the divergence of the great
types of animals must have occurred early in the
history of animals, and that no new great types have
appeared except in the early history. The reason for
this is easily seen. The differences which separate
the great types of animals from each other are in
points of fundamental structure and plan. Such dif-
ferences could have appeared only in the descend-
ants of some early form in which no special type had
appeared. After the line of descendants had assumed
a definite type, it is not likely that they would ever
afterwards have changed their type though many
changes in minor details may have occurred. In-
deed, all evidence shows us that the types, after
they have once become fully established, have not
changed their plan but have remained constant. A
tree when it starts from the ground as a seedling
soon gives rise to several branches. Now this early
branching which takes place in a few days after the

THE ORIGIN OF THE ANIMAL KINGDOM. 8 1

seed springs from the ground, really determines the
subsequent shape of the tree. No matter if the tree
lives to be a hundred years old it will always be pos-
sible to see in its giant limbs the early branching of
the seedling. These early branches become larger
ones, and they in turn give rise to smaller branches
and twigs, but after the first few weeks' growth it is
impossible for the plant to produce any more pri-
mary branches. So in a modified way it seems true
in the animal kingdom. Very early in the history of
animals, the Gastraea trunk branched into several
primary divisions, and these have continued to the
present time. They have grown larger, they have
produced many subdivisions, but the animal king-
dom did not, after the early periods of its history,
produce new primary divisions. In other words, no
new sub-kingdoms have arisen since the earliest
periods of the development. (For Vertebrata see
Chapter IV.)

It is possible to reach this same conclusion from
another standpoint. The development of the ani-
mal kingdom has been in all cases from undifferen-
tiated to differentiated. Organs originally all alike
and adapted to simple functions, have become dif-
ferent from each other and adapted to more com-
plex functions. For instance, the mass of muscles
which in the fish's body are adapted only to the
flexion of the body from side to side, and the simple
motions of its fins, become in the more developed
vertebrates, like man, divided into several hundred
separate muscles, each with its own function, all result-
ing in the complicated powers of motion shown by

82 THE LIVING WORLD.

his body. The development of animals has always
been thus a differentiation. But we must bear in
mind that only the undifferentiated can become
differentiated, and it is plain that after an animal
has once become differentiated in any direction the
possibilities for further differentiation become imme-
diately limited. Let us notice a single familiar ex-
ample in illustration. At one time in the history of
the horse family the possession of five toes on each
foot was a common character. From a five-toed
condition a large number of lines of descent were
possible. But the actual animals entered upon a
line of progress which resulted in the successive loss
of four of their toes. Now with each step in this
progression the possibilities for further development
became limited. The animal with four toes \vas
forever debarred from all lines of progression that
required five, and the horse of to-day, having only
one toe, has practically ended the possibilities of de-
velopment in this direction. This one toe he must
of course retain ; and since it is a law of biology that
an organ once lost is never redeveloped, any further
differentiation of the toe of the horse is impossible.
Now the same principle is true everywhere. As
soon as the first step in any line of differentiation is
taken, the possibilities for further development be-
come immediately limited.

The most undifferentiated of all types of multi-
cellular animals is the Gastraea. In this simple
structure there are possibilities of an immense
amount of modification, simply because it has
developed no structure of its own. But just as

THE ORIGIN OF THE ANIMAL KINGDOM. 83

soon as the descendants of this simple type become
modified in any direction, the future development is
immediately limited to the type thus produced.
When some of the early animals modified their
digestive organs so that there were two openings to
the digestive tract instead of one, from that time
their descendants were required to conform to this
type. When some of them developed an elongated
segmented body, it was no longer possible for them
to return to the Gastraea stage, and start in a new
direction. Advance in structure comes from as-
sumption of different functions on the parts of
organs originally alike, and as the parts of the body
acquire a wider and more varied development, the
possibility of further differentiation is more and
more restricted. The further the development of
animals progresses, therefore, the less will be the
modifications of structure that take place, and the
more must development be confined to the elabora-
tion of details. This fact we find constantly illus-
trated throughout the history of animals, and it will
be frequently referred to hereafter.

We can thus understand why the sub-kingdoms
which arose early in the life of animals could con-
tinue to advance and elaborate each its own type by
the production of many minor branches, but why
none of them could give rise to new types which
would present as great differences as the sub-king-
doms first appearing. If the Gastrsea ancestor had
remained in existence to serve in future ages as the
starting-point of new lines of differentiation, then
the production of great divisions would seemingly

84 THE LIVING WORLD.

have been indefinitely continued. But such does
not appear to have been the case. In other words,
the evidence of nature seems to indicate that the
different sub-kingdoms did not develop from each
other, but all (?) diverged early in the history of the
world from a simple undifferentiated animal which
bore some resemblance to the hypothetical Gastraea.
This conclusion is one of no little significance, for it
indicates that the origin of the large types of ani-
mals was a matter much more rapid than their sub-
sequent elaboration. At the origin of the multi-
cellular animals, a comparatively short time probably
produced divergence in the type which gave rise to
the great sub-kingdoms, while the millions of years
that succeeded only sufficed to elaborate and differ-
entiate these types. This understanding gives us an
explanation of the interesting fact that no new
types have appeared within the geological periods
since the Silurian, and it assists very much in under-
standing what seems to be the sudden appearance
of life in the oldest fossiliferous rocks.

Summary.

In the study of the early history of life three
points of importance stand prominently forth.

First : Far back at the beginning of life on the
globe, there was a period during which the unicellu-
lar organisms were the highest that existed. Al-
ready the plant life and the animal life had become
separate. We do not understand, however, that
this unicellular stage was the beginning of the his-
tory of life, for we are learning that there is a vast

THE ORIGIN OF THE ANIMAL KINGDOM. 8$

field lying below the simple cell : but none of our
sources of evidence give us as yet any sure traces
of this earlier history. Of the unicellular stage of the
history we are sure. Without being able to deter-
mine much in regard to them, we may perhaps look
upon the unicellular organisms existing to-day as a
tolerably correct picture of those of early times.

Second : We learn to picture to ourselves these
early unicellular forms as multiplying by division,
and we see among them many instances where the
multiplication did not produce complete separation
of the cells. The cells remained attached to each
other and formed colonies of unicellular animals.
Among these colonies a differentiation of the cells
and of function began to make its appearance. To
what extent different types of this early differentia-
tion appeared, we do not know, but finally there
arose one which has proved permanent. This was
the two-layered cup to which has been given the
name of Gastraea. This simple type with its mouth
and stomach, proved itself strong enough to battle
for life with nature, and it probably became the
starting-point of all subsequent animals. The
kingdom of plants separated itself permanently
from the animals, and can no longer be traced with
them.

Third ; The Gastraea was the last point of com-
mon origin to which the different sub-kingdoms can
be traced. From here there arose a divergence
which soon produced the types of the existing
animal world. Moreover, we- have seen that not
only is it impossible to trace the whole animal world

86 THE LIVING WORLD.

to any later point of origin, but probably most, if
not all, of the sub-kingdoms can be independently
traced to this one point, indicating that this was the
starting-point of all the animal kingdoms. It is
doubtful whether even two of the great types exist-
ing to-day have had a common point of origin later
than the Gastraea. This has been seen to be due to
the fact that the great types are separated from each
other by differences in fundamental plan of structure,
and such fundamental differences could only have
arisen from an undifferentiated form which had as
yet developed no distinct type of its own.

CHAPTER IV.

THE RECORD FROM FOSSILS.

WE have thus far traced the history of animals
through the unicellular stage to the Gastraea type,
and we have seen that this Gastraea probably diverged
rapidly, by various modifications in the shape and
structure of its body, into different sub-kingdoms.
Thus far our evidence has been only sufficient to
indicate such general facts with no details. Embry-
ology, with the assistance of comparative anatomy,
has told us all that we know of this early history of
living things, and these two sources of evidence, as
already pointed out, can give no details. Still it is
plain that the ground on which we are treading is
much more sure than that which served as our foot-
ing, while trying to discover facts in regard to the
primitive origin of life. While our knowledge as
to the origin of life is all hypothesis, and we are
not even sure of general facts, our knowledge of the
divergence of the great types from the Gastraea is
more than hypothesis, and of the general facts out-
lined in the last chapter we are tolerably sure.
Their truth is commensurate with that of the em-
bryological argument in general, and if we accept

87

88 THE LIVING WORLD.

the teachings of embryology as indicating the past
history of animals, we may regard the general facts
of the derivation of the Gastrasa from the unicellu-
lar animals, and the subsequent modification of its
descendants into the different sub-kingdoms which
soon filled the world, as substantially proved.

We come now to the ages represented by fossi-
liferous rocks, and immediately the record is much
more definite. When we strike the fossiliferous
rocks we seem to be dealing with a more tangible
subject. The animals which were buried in the
ancient muds and sediments, and which we are now
unearthing, were actual animals and not hypotheti-
cal ones. With all of the cogency of argument from
the embryological standpoint, it cannot be denied
that the steps in the history of animals which it
points out are more or less hypothetical, and the
stages in the history of life of which it speaks
are stages of type only. But when we take a
fossil in our hands, we cannot question that the
fossil was once an actual animal, and an inhabitant
of the world in the earlier ages. It is now possible,
moreover, to determine many details in regard to
early life, for by means of fossils we deal with actual
animals, and not simply with structural types. For
reasons already pointed out, however, it is still
impossible to obtain connected history.

The Geological Ages.

Before proceeding with the history of animals as
taught us by fossils, it will be necessary to summarize
briefly the geological ages. The accompanying figure

Quarternary do)

Tertiary (9) -

Cretaceous (8) .

Jurassic (7) -

Triassic(6) -j
Permian (5) •{

Carboniferous (4) *

Devonian (3)

Silurian (2) -

Archean (i)

Recent

II

Piocene

Miocene

Eocene Mammals

Birds

Reptiles
/

Reptiles

Amphibia
Coal Measures

Sub Carboniferous

Fishes

Cenozoic

Mesozoic

Upper Silurian

Vertebrates
Lower Silurian

Invertebrates

Cambrian

Paleozoic

No life

FIG. 9. — Diagram illustrating a section of the earth's crust to show the sue-
cession of the geological ages.

go THE LIVING WORLD.

(Fig. 9) represents the ages in the order of their
occurrence. In the subsequent pages the ages will
be referred to by the names given in the left hand
column in the figure, and for convenience of the
reader who may not be familiar with them, a num-
ber will be placed after the name of the age which
will indicate its relative order. Thus Archean (i) in-
dicates that the Archean age was the first of the
geological ages.

The following brief description of the life of the
different ages will serve as an introduction to their
more careful study :

PALEOZOIC.

The Archean (i) is characterized by having no
traces of life. This does not mean that there was
no life on the world ajhthat time, but either that the
conditions were not favorable for the preservation of
fossils, or that the subsequent changes in the rocks
(metamorphosis) have obliterated them. There is
every certainty that life must have been in exist-
ence, but no fossil remains of animals assist us to
this conclusion. All of the history dwelt upon in the
last two chapters must have occurred during the
Archean age, for, with its close, all of the early
history had been past.

The Silurian (2) is characterized by the presence
of animals in great profusion. The animals charac-
teristic of the age were the invertebrates ; for while
it is certain that vertebrates were in existence before
its close, they were but slightly developed in com-
parison with the invertebrates, and did not appear at

THE RECORD FROM FOSSILS. $1

its beginning, at least so far as present evidence
goes.

The Devonian (3) is characterized by the appear-
ance of the vertebrates in large numbers. It is fre-
quently called the age of fishes, from the abundance
of these animals. No higher vertebrates are known
to have been in existence.

The Carboniferous (4) was an age characterized by
its abundant vegetation. It was during this age that
most of the coal beds were deposited. The most im-
portant additions to animal life were the Amphibia,
and probably the Reptilia. The Amphibia were large
animals, and were especially abundant.

MESOZOIC.

The Permian (5) was an age of which only slight
records are left. It was an age in which important
changes in the animal kingdom took place. The true
Reptilia became more abundant, and it is probable
that at this period the Mammalia first separated
from a reptilian stock.

The Triassic (6) was especially characterized by the
development of numerous reptiles, some of great size.

The Jurassic (7) again found the reptiles the
most prominent animals. New reptiles appeared,
many of them being of immense size. At this time
we find the first indication of the birds, suggested
by many bird-like reptiles. The first real feathered
animal also appeared, a bird with remarkable reptile-
like characters. In this age the reptiles reached their
highest development.

The Cretaceous (8) was characterized by a slight

92 THE LIVING WORLD.

diminution in the abundance of reptiles, though
they were still the predominant type. Birds more
closely allied to our modern birds began to appear.

CENOZOIC.

The Tertiary (9) began the modern era. Modern
birds were found in abundance, and the reptiles
lost their prestige. True mammals (in distinction
from the marsupials) made their appearance either
at this time or slightly earlier. During the Tertiary
they rapidly developed into the orders which fill the
world to-day, and almost immediately became the
predominant type.

The Quart ernary (10), the age of man, brings us to
the present time.

These long periods were of immense duration, but
we have no means of determining even approximately
how long they lasted. Nor do they by any means
represent the whole of geological time. The geo-
logical history was doubtless a continuous one, and
if we had in our possession the whole of the history
we should hardly have been able to divide it into
ages. The periods outlined above seem very distinct
from each other because they are separated by lost
periods of which no record remains to us. Between
each of the periods above mentioned there is a break
in our record representing the periods during which
most important changes took place in the history
of life, but of which we can never obtain any knowl-
edge. We do not even know whether the lost
periods were longer or shorter than those of which
we have record, but, judging from the amount of

THE RECORD FROM FOSSILS. 93

change that took* place in animal life, they must
have been at least as long. Owing to these lost
records it frequently happens that the opening of
the different periods shows a surprising acquisition
of the new forms of life. The sudden appearance of
new types of life at the opening of the Tertiary (9),
for example, is doubtless due to the fact that we do
not possess the history of the immediately preceding
period ; and so also the sudden appearance of life
with the Silurian (2) is in the same way due to the
absence of any record of pre-Silurian life.

Life at the Beginning of the Fossiliferous Record.

We will now turn our attention more closely to the
development of life during these ages. In the first
place it must be noticed again that it is no longer
possible to trace the history of life along a single
road. The Gastraea was the last point which the
various types of animal life had in common. From
this point many of the types diverged, and the com-
plete study of the history of life would lead us to
follow each of them. This would, however, involve
a mass of detailed statistics which would be largely
unintelligible to the general reader. Instead of fol-
lowing the history of life through its numerous de-
tails, we shall try therefore to take a perspective
view of it ; noting the general truths and laws illus-
trated by the course of development.

Let us retrace our steps, and study more carefully
the history of life during the geological ages. First
we may ask, What was the condition of the animal
kingdom at the time of our first fossil record of it ?

94 THE LIVING WORLD.

Upon looking at the world during the Silurian (2)
age, we are immediately struck with the surprisingly
great diversity of its animal life. The divergence of
types of animals, for which we have found evidence
in the embryology, had already taken place. The
Gastraea had given rise to a number of lines of
descendants, all of which had more or less com-
pletely lost the original Gastraea characters, and had
taken the characters of the animal types of to-day.
Not only had the large types made their appearance,
but there had already been time enough for most of
them to diverge still further, and produce their char-
acteristic classes.

The diversity of life found in the Silurian (2) age
has greatly surprised geologists. This surprise is,
however, disappearing, as we learn from embryology
of the rapid divergence of types at the beginning of
life, and as we remember the long blank Archean (i)
age, during which life existed without leaving any
distinct traces of itself. It must be remembered
also that the Silurian (2) age itself was of very long
duration, and many of the forms of animal life
attributed to this period were not in existence at the
beginning of the era, but were developed during its
progress. The scantiness of the remains makes it,
however, impossible to determine with any degree
of probability which of the animal forms came in
later. In our study of this early period of life's his-
tory, we will consider the age a unit, always under-
standing that it is a unit of immeasurable duration,
and if we had been able to watch its progress, many
of the forms which appear so suddenly would have

THE RECORD FROM FOSSILS. 95

been found to be slowly developing during its prog-
ress. Before, or during the Silurian age, all of the
sub-kingdoms of animals came into existence.
Ccelentera were present in the shape of sponges,
corals, hydroids. Among Echinodermata were found
crinoids, star-fishes, and some extinct forms of sea
urchins. Various mud tracks tell us that marine
worms lived in these early mud flats. Mollusks
were abundant, some related to clams and oysters
(lamellibranchs), and others related to our snails
and cuttle-fishes. Brachiopoda existed in marvellous
numbers, and the closely allied Bryozoa were plenty.
Crustacea there Avere, much like our shrimps and
lobsters, and a special type (trilobites) was highly
characteristic of the age. Scorpions were present
to represent the air-breathing animals, and the exist-
ence of their sting tells us that other air-breathing
animals (probably insects of some kind) had also
appeared. Of the sub-kingdom Vertebrata the
indications are scanty, though there is no doubt that
they were in existence by the close of the Silurian
age, for in the upper rocks of that period unques-
tionable fossil evidence has been found, and recent
discoveries have shown them in the lower Silurian
rocks, thus carrying them almost to the bottom.
The earliest vertebrates could not have had any
hard parts to be preserved, true bones and even
cartilage being of more recent origin, and this
explains their scanty remains in the lowest rocks,
although there can be no doubt that the vertebrates
must have been more or less abundant even in the
lower Silurian times.

96 THE LIVING WORLD.

If we examine this fauna a little more closely, its
high state of diversity appears to be even more
startling. Not only were all of the sub-kingdoms
represented, but, omitting the vertebrates, all but
two of the classes as well. Two thirds of the
orders of invertebrates, and quite a large number of
the families which are in existence to-day, were then
developed. It may be said that all of the inverte-
brates had already developed their large trunks, and
the subsequent ages have in most cases only sufficed
to elaborate the minor branching of the trunks thus
formed. The Archean (i) age had been sufficient
for the divergence of the types of which embryo-
logical teachings have so plainly given evidence.

A few figures will illustrate the fact just men-
tioned, that the divergence of type before the Silurian
(2) age produced greater modifications than those
that have occurred since. Of the sub-kingdoms of
animals, all were in existence during the Silurian.
Of next smaller groups, the classes, all were in
existence whose hard parts enabled them to be pre-
served, with the exception of one or two small classes,
whose shell is but poorly adapted for preservation.
Of the orders, of animals again, that have left any
fossil records, thirty-four are found in the Silurian
rocks, nineteen being in the Primordial, (i. e., at the
very bottom of the series), while only twenty-five
orders have appeared in the more recent rocks.
Since some of these twenty-five orders have only
slight skeletons, their preservation must be regarded
as accidental, and their absence from the Silurian (2)
rocks does not prove that they did not then exist.

THE RECORD FROM FOSSILS. 97

Nine of the later appearing orders were insects which
fed upon flowers, and could only have appeared
after flowering plants ; and in general we notice that
the later appearing orders were largely land animals.
Besides the orders included in the above, there are
thirty-four which, from the soft structure of their
body, have left no traces in the rocks at any period.
Of these, it is certain that a number at least must
have been in existence in the Silurian (2), since their
close allies are known to exist there. Carrying our
study into smaller groups, and including now sub-
orders in our figures, we find the later ages coming
out in more prominence. Of the one hundred and
five orders and sub-orders that have left any evidence
of themselves in the rocks, forty-four are Silurian
and sixty-one have appeared in the later ages. Here
too we find that of the sixty-one later sub-orders
many are terrestrial, and at least twenty are insects-.
It is further noticed that the larger number of sub-
orders which are of recent origin belong to the
higher rather than the lower orders of animals.
Taking families into consideration, a larger number
were late in appearing, though a number of our
modern families date from the Silurian (2).

Thus we see that the Archean age produced the
sub-kingdoms, the classes, and most of the orders of
animals, while the subsequent ages have only pro-
duced the smaller divisions, giving rise to a far less
divergence in type, but a much larger profusion of
minor branches. Now from all this it follows that
the study of fossils is unable to help us to the knowl-
edge of the early history of any of the sub-kingdoms
7

98 THE LIVING WORLD.

except the vertebrates, since practically all of them
had developed before the fossil record began. It is
true that in the subsequent ages most of the other
sub-kingdoms have very much expanded in variety
of forms, and have in general seized upon larger and
larger fields in nature. But in most of them the
real advance has been slight, and in many there has
been actually no advance, but only production of
new genera and species. In the vertebrates alone
has all of the development taken place during the
period of which we have fossil record, and here alone
can we read a definite history of progression, since
here alone can we trace anything like a continuous
history.

With the opening of the Silurian (2), therefore,
the animal kingdom bursts upon us in a compara-
tively high state of development. From this time
on there is a constant widening of types, a constant
succession and disappearance of old forms and the
appearance of new ones.

As already seen, the Gastraea was the last point in
the history of life shared in common by all multicel-
lular animals. From this point there was a parting
of the ways, and it is therefore no longer possible to
follow the history of the animal kingdom as a whole.
It will be necessary to take up the different branches
and follow them separately. Of course, the further
we trace them the greater and more complex will
become the branching, but it is not our intention to
trace this history in very great detail. It will be neces-
sary, however, here to take into brief consideration
the history of the various sub-kingdoms of animals

THE RECORD FROM FOSSILS. 99

as they are known to-day. Inasmuch as this will
necessarily lead to considerable detail which will be
uninteresting to the general reader, this part of our
subject may be omitted without any break in con-
tinuity.

PROTOZOA.

The unicellular animals are all minute, and most of them are
entirely soft. For these reasons they are not well adapted for
preservation as fossils, and only one of the three classes has been
preserved in the rocks to any 'great extent.

The Foraminifera have usually a shell of lime, and therefore have
left traces of themselves in all ages. Even in the Archean (i) there
is a curious body (Eozoan Canadense) believed by some to be the
remains of Foraminifera, though it is usually regarded to-day as a
mineral deposit. With the Silurian (2), however, true foraminifers
appeared unquestionably in abundance, and every subsequent age
shows traces of their presence. There are two geological periods in
which their deposits are of special importance. The immense chalk
beds of Cretaceous (8) are made very largely of shells of foraminifers.
This chalk is almost exactly the same in its formation as the so-called
Globergerina ooze that is being deposited now at the bottom of the
ocean, so that it seems that chalk is still being formed in our modern
seas. The second large deposit of foraminifers is the Numulitic lime-
stone of the Tertiary, an immense bed of European rocks covering
thousands of miles of territory. We must not conclude that in
these two periods the Foraminifera were any more abundant than in
others, but simply that the conditions were more favorable for their
preservation. It is remarkably interesting that the species existing
in the Tertiary (9) and many of those of the Cretaceous (8) chalk are
identical with species found living to-day, and when we go still
further back in history the amount cf change is very slight. Indeed,
Dr. Carpenter says there is no evidence of any fundamental modifica-
tion or advance of the foraminiferous type from the Paleozoic period
to the present time.

A second order of the Protozoa, the Radiolaria, possess a skeleton
of silica which is tolerably well adapted for preservation. These
have been traced back as far as the Silurian (2), though it is not until

100 THE LIVING WORLD.

we reach the Mesozoic (5, 6, and 7) rocks that their remains become
abundant.

The other Protozoa, having no hard parts, could not, of course,
have been preserved as fossils. There can be no doubt now of their
existence through all of the geological ages, since, as we have seen,
the Foraminifera and Radiolaria, which are the highest of the Protozoa,
have lived during all these periods, and the lower orders of any group
always appear before the higher ones. In general, then, we may
conclude with little chance of error that the Protozoa were in exist-
ence at the beginning of the Silurian (2), in practically the same
condition that they are now, and that the long ages since have seen
very little modification in their structure. They have been practically
stationary, and their development was almost wholly pre-Silurian.

CCELENTERA.

Four classes of animals are to-day included under the head of
Ccelentera.

Port/era (sponges). — These are the lowest existing multicellular
animals. As would be expected, therefore, they are very ancient in
origin. They are found in the lowest Silurian (2) rocks and have
been found in every age since that time. Here also the probability
seems to be that the sponges of early times were much like those of
to-day, so that there has been, very little real advance in structure.
The variety of type was however much smaller in early times than
to-day.

Hydrozoa. — This class includes the hydroids (see Fig. 7) and the
well-known jelly-fishes. The jelly-fishes being so soft, it is, of
course, impossible to say when they really appeared, their absence
as fossils proving nothing. The first traces we have of them are
in the Jurassic (7). Many of the hydroids, however, have a shell,
either of lime or of some horny material, and they have been found
in all ages. The Silurian (2) rocks contain them in abundance. But
the hydroids of this period were quite unlike those found at the
present day. One entire class found in the greatest abundance at
that time has since entirely disappeared (Graptolitoided), the last traces
being found in the Silurian (2). Several smaller groups have also
disappeared. Of our modern forms, only one class (Thecophora) was
in existence, and this was quite different from its representatives
to-day. This class continued with little change during the whole of

THE RECORD FROM FOSSILS. IOI

the Paleozoic (2-4) age, and it was not until the later part or the
Mesozoic (5-8) that the modern forms began to appear, and the class
assumed its present condition. How far this late origin of most of our
existing groups is due to the lack of preservation of animals which
really existed in the early ages, cannot be positively determined. The
great probability is that most of our modern orders are much older
than their fossils would lead us to believe.

Aciinozoa (this includes the corals and the sea anemones). — The
corals have commonly a lime skeleton, and are quite well adapted for
preservation. They have been very abundant in all ages. The
Silurian (2) rocks contain five of our modern orders, though most of
the corals were quite unlike their modern representatives. The orders
most abundantly represented at that time are, however, not the
orders most abundant to-day. During all the Paleozoic (2-4) they
continued to live in abundance, a large variety of forms being in
existence all through that time. With the Mesozoic (5-8) however,
we find the modern forms of corals becoming more abundant, and
from that time the production of the modern coral was a matter
of slow and constant growth.

The Ctenophora are wholly unrepresented as fossils, owing to their
soft jelly-like composition.

In general, then, the Coelentera form an ancient group, appear-
ing in abundance in the earliest rocks, and being numerous in all
ages. During the Paleozoic, however, the types represented were
not those most abundant to-day, and many of them were confined to
the Paleozoic (2-4). The modern Coelentera seemed, so far as our
record goes to-day, to have appeared with the Mesozoic (5-8), some
of the ancient types disappearing entirely at that time and others
becoming of subordinate importance. At this time also there was a
great multiplication of sub-orders and families.

ECHINODERMATA.

This sub-kingdom is divided into five classes : Crinoidea, Aster-
oidea (star-fishes), Ophuroidea (brittle stars), Echinoidea (sea urchins),
and Holothuroidea.

Crinoidea (stalked echinoderms). — This class may be especially
regarded as the fossil class of the group, for although some are still
in existence to-day, they are very few in numbers (only eight genera),
and mostly confined to deep seas. In early geological times they

102

. THE LIVING WORLD.

were, on the other hand, by far the most abundant of the echino-
derms. With the beginning of the Silurian (2) great numbers and
varieties were found in existence. The class was then divided into
three orders (Brachiata, Blastoidea, Cistoidea), ail of which reached a
high degree of expansion in the Paleozoic (2-4). After thus reaching
a culmination they gradually become less numerous and continued
to diminish until to-day, when there exists only the small number

D

FIG. io. Diagram illustrating the history of the Echinodermata. — The letters
on the left indicate the different geological ages. E Echinoidea. O Ophiuroidea,
A Asteroidea. H Holothuroidea. C Crinoidea. B Blastoidea. Cy Cystoidea.

above mentioned. Two of the former orders disappeared com-
pletely, and thus these that are left are simply surviving fragments of
a once predominant type. The modern crinoids, which are in some
respects different from the older ones, made their appearance in the
Triassic (6).

Asteroidea (star-fishes). — This class was also in existence in Si-
lurian (2) times. The type of star-fish found in these rocks is slightly

THE RECORD FROM FOSSILS. 1 03

different from that of the modern animals. The latter appeared,
however, in the Devonian (3), and the ancient type disappeared with
the Paleozoic (2-4). The modern forms of star-fish appearing in the
Devonian (3) continued with little modification until the Jurassic (7),
when the more strictly modern families began to appear.

Orphiuroidea (brittle stars). — The brittle stars were quite abundant
in the Silurian (2), some of the genera being identical with those
living to-day. Their history has been of little special interest. They
have simply continued to expand slowly into the present condition.

The Echinoidca (sea urchins). — The sea urchins were well developed
in the Silurian (2) rocks, but were of a type quite distinct from the
modern forms (having more or less than twenty rows of plates). This
order of Palechinoidea continued to exist during the Paleozoic (2-4),
practically disappearing with its close. With the Mesozoic (5-8) the
modern urchins (with just twenty rows of plates) appeared, quickly
expanding, and reaching a profuse state of development in the
Jurassic (7) and Cretaceous (8). Since then they have been on the
wane.

Holothuroidea (sea cucumbers). — The sea cucumbers have either no
shells, or sometimes a few calcareous plates. As fossils they are
known only by specimens of these plates which are occasionally
found. No traces 4)L them are found earlier than the Carboniferous (4),
though the difficulty of determining their presence makes it not im-
probable that they existed at an earlier period.

In general, then, the echinoderms were very early developed. All
of the classes, with the possible exception of the holothurians, were
found in the Silurian (2), and were as radically distinct from each
other then as they are now. The Paleozoic forms were on the whole,
however, quite distinct from the modern representatives. The
modern types appeared during the Mesozoic, and the group is at
present on the wane.

MOLLUSCOIDEA.

Under this head are included to-day two groups of animals, whose
position in the animal kingdom is unsettled, though they are certainly
related to each other.

Brachiopoda. — These include animals externally resembling bi-
valve mollusks. To-day they are few in numbers and do not form
a very important group, but in earlier times they were very abundant.

104 THE LIVING WORLD.

In the Silurian (2), indeed, the Brachiopoda were more abundant than
any other group of animals, and the age is therefore sometimes called
the age of Brachiopoda. At this time they reached their culmination,
and have been declining ever since, though quite a number of them
are still in existence. It is especially interesting that some of the
genera existing to-day are not to be distinguished from those of the
very earliest rocks. (Lingula, Terebratula.)

Bryozoa or Polyzoa. — These form a group of small animals not
generally familiar except to students of natural history. They are
abundant to-day at the sea shore, and usually are mistaken for plants
by those unacquainted with them. They are small animals, but many
of them have a calcareous shell. They appeared in the Silurian (2),
and existed with no special change, though as the modern era
approached, the animals gradually assumed more the type of the
existing Bryozoa. Like the Brachiopoda, they form a fossil order
whose chief development occurred in the past, although they have
not yet become so widely extinct.

Mollusca.

Of all animals, we have the most complete geological record of the
mollusks. They were well developed as a group at the beginning of
the Silurian (2), and therefore their fossil record cannot tell us any-
thing of their early history, but of their history since the Silurian we
have a very full account. Their hard shells and aquatic habits have
adapted them especially well to preservation. We recognize five
classes of mollusks, whose history has been as follows :

Lamellibranchia (oysters, clams, etc.). — This group is characterized
by having two shells, and is common both in fresh and salt water.
The class was abundantly represented in the earliest Silurian (2),
many of our modern families being already in existence. Indeed,
quite a number of the genera then living still exist. By the close of
the Paleozoic (2-4) are found many others not so well known. With
the Mesozoic (5-8) there was a change. Many of the old forms dis-
appeared and new ones took their places. During the rest of the
geological ages there was an approach to the modern fauna. The old
forms did not entirely disappear, however, and some of the oldest fami-
lies continue to exist to-day. The Mesozoic is marked by the develop-
ment of the clams, quohogs, and other less well-known mollusks. The
modern forms have especially developed what is known as a siphon.

THE RECORD FROM FOSSILS.

105

In general the Paleozoic (2-4) lamellibranchs were commonly without
siphons, although some of the siphonate type even then existed ; the
modern forms, on the other hand, are siphonate as a rule, though
many of the asiphonate forms still exist. The most remarkable
point in the history of this class was the development of a very pecu-
liar order in the Cretaceous (9). This order (Rugosites) is very

FIG. ii. Diagram illustrating the history of the Mollusca. — C Cephalopoda.
P Pteropoda. H Heteropoda. G Gasteropoda. L Lamellibranchiata.

unlike any other mollusks, and it suddenly appeared without warning
in the Cretaceous and disappeared at its close.

Gasteropoda (snails). — The history of this class is quite parallel to
that of the last. Beginning with the Silurian (2), they have been at
all times very abundant. With the Mesozoic (5-8), the older forms

106 THE LIVING WORLD.

began to disappear and the modern type became abundant. The first
water and land snails are found in the Carboniferous (4).

Pteropoda (winged mollusks). — This is a little-known class, found
to-day chiefly in the high seas. Pteropods were abundant in the
Silurian (2) rocks, the genera seeming to be identical with those of
to-day. The group has remained practically unaltered till to-day,
except that some of the early families have entirely disappeared. At
no time has the class been of much importance in the world.

Scaphopoda (tooth shells). — This is also a small and unimportant
class, consisting to-day of only three genera. It is of great antiquity,
however. The first remains are found in the Devonian (3), although
the class probably existed long before that era. It has remained
practically stationary.

Cephalopoda (squids, cuttle fishes, etc.). — This is by far the highest
of the mollusks, and it has at all ages been an important group. The
class has two well marked orders, one having four gills ( Tctra-
branchiata), and the other two gills (Dibranchiata). Beginning
with the early Silurian (2), the Tetrabranchiata were quite abundant.
Fourteen orders were then in existence. They gradually increased
in abundance, size, and complexity, although only one new type was
introduced during the Devonian, and no others until the Tertiary (9).
They reached a very high state of development in the Mesozoic (8).
From that time they rapidly diminished, and to-day only a single
species (Nautilus) is left as a remnant of this once predominant
type. Nautilus is in itself interesting as an example of a long-
persistent species, being abundant as far back as the Silurian (2).
Just before the tetrabranchs reached their culmination the first repre-
sentative of the dibranchs appeared (Triassic). These have con-
tinued to increase until the present time, one large section of them,
however, disappearing with the Cretaceous (Belemitcs). To-day
the class, though existing in great numbers, is impoverished as to
variety, only a few types remaining.

The mollusks in general will thus be seen to be a very old group,
just about as well differentiated at the beginning of the Silurian (2)
as they are to-day. The long ages have seen them always in abun-
dance, and have witnessed slow changes and slight progression.
There has been a constant expansion of the group, but it has consisted
in the multiplication of families and genera. As elsewhere, the be-
ginning of the Mesozoic (5-8) saw the older types giving way before
the modern ones.

THE RECORD FROM FOSSILS.

JO/

Of the history of the unsegmented worms we know nothing through
geology, for their soft bodies have never been preserved. From their
anatomical position we conclude that they are very ancient animals,
and have doubtless lived through all geological ages.

CO M Br A PI

FIG. 12. Diagram illustrating the history of the Crustacea. — C Cirrepedia.
O Ostracoda. M Macroura. Br Bracyura. A Amphipoda. P Phylocarida.
/ Isopoda.

ARTICULATA.

This large division of animals includes two provinces.

Anarthropoda (segmented worms). — The worms have soft bodies, not
adapted for preservation as fossils. Some of them, however, live in
tubes of lime or sand, and these cases are often preserved. Occasionally
we learn of their existence by their tracks on the ancient mud. From
such sources we know that they were in existence, well differentiated,

IO8 THE LIVING WORLD.

during the Silurian(2) age, and have lived continuously until now,
probably with little change.

Arlhropoda, — This province again consists of three classes.

1. Crustacea (crabs, lobsters, etc.). — With the Crustacea we meet
for the first time a class of animals, a considerable portion of whose
development has occurred since the Silurian (2). Still the class is
an old one, and most of the lower orders were distinct at our earliest
record of fossils. The Cirrepedia (barnacles), Ostracoda (water fleas),
Amphipoda (sand fleas), Trilobita, and Euripteridca, as well as one
order (Phyllocarida) which seems to have been the ancestor of the
higher Crustacea, were all in existence during the Silurian (2). But
the trilobites and euripterids attained their culmination at that time,
and soon after disappeared in the Carboniferous (4). The higher
orders certainly appeared later. The Isopoda (sow bugs) and Phyllo-
poda first appeared in the Devonian (3). The shrimps and lobsters are
found in the Devonian (3) and Carboniferous (4), the crabs probably
are in the Carboniferous. Now since the crabs are undoubtedly derived
from the lobster group, and this group from the Phyllocarida above
mentioned, we have in these orders an instance where we can trace
by fossils the origin of the modern orders from the earliest lower types.
The Crustacea have always been abundant, and, with the exception of
a few orders that have disappeared, are as abundant to-day as ever
before. The higher orders indeed were never so diversified as at the
present.

2. A rachnoida (spiders and scorpions). — Under this head are found
the oldest air-breathing animals. The scorpions were in existence in
the later Silurian (2), and true spiders are found in the Carboniferous
(4). That these have been in existence during all the later ages is
therefore certain, though only scanty records have been found.

Myriopoda (thousand legged worms). — Being mostly land animals,
these have left scanty remains. They are found in the Devonian (3),
and doubtless appeared earlier.

Insecta. — Though the insects form about five sixths of the animals*
of the world to-day, their habits prevent their ready preservation as
fossils, and our history of the group is quite meagre. Some of the
lowest orders (cockroaches) were undoubtedly in existence in the Si-
lurian (2). During the whole Paleozoic (2-4), insects were abundant.
All of them had a greater or less resemblance to the cockroach, al-
though in the Devonian (3) and the Carboniferous (4), they took on
features that allied them to the higher orders of insects, the beetles,

THE- RECORD FROM FOSSILS.

true bugs, and dragon-flies being thus foreshadowed. With the
Mesozoic (5-8) the higher orders appeared, and by the end of the
Jurassic (7), the modern orders of insects were all in existence. Of
the various orders the higher ones appeared last. These orders,
since they feed upon flowers, could not be expected until flowers them-
selves appeared, and this was not until the middle of the Mesozoic
(5-8). [See Chap. VI.].

FIG. 13. Diagram of air-breathing Arthropoda. — / Insecta. A Araneina.
Ar Arachnoida. M Myriopoda.

The insects have thus developed within the period of fossil history.
In the Silurian they were represented by the lowest order and this
order, during the rest of the Paleozoic, gradually diverged in the sev-
eral directions which gave rise in the beginning of the Mesozoic (5-8)
to the beetles and bugs and dragon-flies. Later, when the flowering
plants made their appearance, there was a second divergence and a
production of new forms depending upon flowers for their existence.

110 THE LIVING WORLD.

It seems probable that the remarkable social habit of the ants and
bees did not develop until later than the Cretaceous (8).

VERTEBRATA.

/ The earliest traces of the Vertebrata are in the rocks of the lower
Silurian (2). At that time there is no doubt that some forms of fishes
were in existence, though the remains are rather scanty. The verte-
brates then existing were low fishes whose skeleton was poorly adapt-
ed to preservation. Indeed, in the whole Silurian the traces of
vertebrates are rare, although there can be no doubt that they were in
existence. With the Devonian (3), they became very numerous. The
Devonian seas were filled with large numbers of fishes, chiefly
ganoids and elasmobranchs (related to the gar-pikes, and sharks).
During this period these orders of fishes became very abundant and
highly diversified. Late in the Devonian age some of the ganoids
would seem to have acquired the habit of living partly in the air, and
the habit thus acquired gave rise to the amphibians, which appeared
for the first time in the next age, the Carboniferous (4). During this
age the amphibians became abundant, and towards its close they
seem to have become more distinctly aerial and to have ceased to be
able to live in the water. This produced the true land vertebrates.
In the latter part of the Carboniferous (4) or the Permian (5), we find
that true reptiles had come into existence. Having once assumed a
terrestrial habit, the reptiles rapidly expanded to appropriate the large
field open to them, and in the next two ages (Triassic and Jurassic)
they became more and more abundant, and grew to immense size.
While these first land vertebrates were thus expanding, one side
branch of them gradually acquired wings and developed into birds,
which seem to have appeared first in the Jurassic (7) and Creta-
ceous (8).

From the Carboniferous (4) the reptiles had been constantly expand-
ing in diversity, in number, and in size. But with the Cretaceous (S)
they were slowly, yet surely, giving way to another and better adapted
type of land vertebrates. Way back before the Jurassic (7) period, prob-
ably in the Permian (5), one of the early generalized types of reptiles
seem to have sent off a branch of descendants which produced their
young alive instead of laying eggs, and nourished them for a longer
or shorter time by secretions from the dermal glands of the mother.
These animals were at first small and probably weak. But they
continued to exist during the age of reptiles, as a comparatively unim-

THE RECORD FROM FOSSILS.

Ill

portant group. It would seem that somewhere toward the close of

the Cretaceous (8), or perhaps earlier, these animals made a further

improvement upon their method of producing young. They devel-

M BRA F

C ,

FIG. 14. — M Mammals. B Birds. R Reptiles. A Amphibiane. F Fishes.

oped the placenta which enabled them hereafter to carry their young
in the uterus till it was well developed. The placental mammals thus
arising proved to be a much stronger type than that which had existed

112 THE LIVING WORLD.

before. Perhaps, loo, the reptiles had expended the most of their
energy, so that they were ready to retire before any newcomers. At all
events, we find that with the end of the Cretaceous (8) or the begin-
ning of the Tertiary (9) the new group of mammals rapidly took the
place of reptiles. A large share of the reptiles disappeared and those
that remained took a subordinate position compared to that of the
new monarchs of the world. With extreme rapidity now did the
mammals develop. In no other instance in the animal kingdom has
development been so rapid. Expanding into many types, occupying
constantly new fields of nature, they soon began to assume forms
familiar to us, and our modern families began to appear. The
mammal fauna thus became more and more like that existing to-
day, until by the end of the Tertiary (9) the mammals of the present
period may be said to have been in existence. Finally in the Quar-
ternary (io) period there appeared one animal who seized upon a new
and untried field of nature for its own. This new field was that of
mind, and this new animal soon distanced every other competitor and
became immeasurably superior to all other animals. This of course

CHAPTER V.

A VIEW IN PERSPECTIVE.

AFTER this outline sketch of the history of the
animal kingdom, we may now try to take a perspec-
tive view of the whole, in order to get a better
understanding of the true significance of the history.
Details of science are of little interest or of little
significance until they are collected into groups and
formulated into laws ; and a bird's-eye view gives us
a clearer idea of relations than an elaborate study of
details.

Perhaps the most striking point which forces itself
to our attention is one already noticed, viz. : the
great extent to which the development of the ani-
mal kingdom had taken place even prior to the
beginning of the fossil period. In the outline of the
history of animals as sketched in the previous chap-
ter, it has been seen that all the groups of animals
except the insects and vertebrates had already
reached a high state of development before the
Silurian. In many of them there has been almost
no change since that time. In others, the change
has been chiefly in the addition of new families and
genera without any special modification of type.
8 113

114 THE LIVING WORLD.

Not a few of them reached their culmination early in
the Silurian (2), and soon after became extinct. In
other words, the sub-kingdoms and classes of ani-
mals, and, in many cases, the orders and families,
were as distinct from each other in the beginning of
the Silurian (2) as they are to-day. The divergence
of types had fully taken place prior to the beginning
of our fossil record. Except in the one division of
Vertebrata, the development of classes and orders
must be learned from embryology and anatomy ; the
study of fossils is entirely inadequate to help us to
any connected history.

Paleozoic, Mesozoic, and Cenozoic Epochs.

A second equally striking fact is that in the long
history that has intervened between the Silurian (2)
/ and the present time there have been two specially
prominent dates. The first was at the beginning of
the Mesozoic (5-8). During the Paleozoic, as we
have seen, various types of animals had existed in
abundance ; but, though the three Paleozoic ages
were of an immensely long duration, the modifica-
tions of type and the production of new families
during its progress were comparatively slight. With
the beginning of the Mesozoic, however, there was a
remarkable expansion and development of almost all
classes of animals. Within a comparatively short
time a greater expansion into new forms took place
than had occurred during all of the Silurian, Devon-
ian, and Carboniferous taken together. A glance at
the diagrams (Figs. 10-14) will show this expansion
in a striking manner. The fact that there was such

A VIEW IN PERSPECTIVE. 115

a general development of new forms at this period
would indicate that some highly important change
in the condition of the surrounding living world
occurred to act as a stimulus. Although it is im-
possible to say positively what these new conditions
were, we may perhaps find a partial explanation in
the reduction of the amount of carbonic acid in the at-
mosphere. During the Carboniferous (4) era, vegeta-
tion had been rapidly at work drawing the CO2 from
the air and storing it up in the form of coal. Roughly
speaking, we may say that all of the carbon now
stored in the coal beds was in the atmosphere in the
form of CO 2, previous to the Carboniferous age.
Plainly, the withdrawal of this CO3 by the Carbon-
iferous vegetation rendered the air much better
fitted for the terrestrial life of animals, and it is
interesting to find that during this period or just
after it the first air-breathing vertebrates came into
existence. Though this may not be a sufficient
explanation for the whole of the marvellous ex-
pansion of type in the early Mesozoic, and especially
of marine types, we are nevertheless probably correct
in regarding it as one of the most important factors
in this expansion. At all events, it is true that some
impulse acted upon the animal kingdom at about
the beginning of the Mesozoic which produced an
exceptionally rapid advance and a divergence of
form.

It is especially noticeable that this era, which
was so marked in the life history of animals, was
not an era of especial note in the vegetable king-
dom. It was not until toward the Cretaceous (8)

Il6 THE LIVING WORLD.

age that the plant world received a similar impulse,
causing it to expand into modern forms.

The second date of importance in the history of
animals was at the beginning of the Tertiary (9).
With this age we may say that the strictly modern
order of nature began. From this time we begin
to find in abundance the families and genera of ani-
mals which characterize the world to-day. Of our
existing species of animals, the lower ones were the
first to appear. Or stated in other terms, the lower
species of animals are of longer duration than the
higher ones. In the lowest rocks of the Tertiary
(Eocene), we find representatives of existing reptiles
living contemporaneously with a world of extinct
mammals. None of our living mammals had then
appeared, though modern reptiles were abundant.
Of the mammals, too, the lower orders approached
the modern condition first, the Insectivora and the
Edentata considerably antedating the Primates.
During the Tertiary, the approximation toward
modern fauna was very rapid. Old species disap-
peared, and new ones appeared leading directly up
to the present time. The new era inaugurated by
the Tertiary was not, however, so marked as that
coming in with the Mesozoic, for except in the class
of mammals there was far less expansion of type
than occurred in the early Mesozoic. Still the new
era is sufficiently well marked. Of the causes which
produced it we have no satisfactory knowledge.

I

A Constant Change of Species.

Looking over the whole history as seen by our
collections of fossils to-day, we find that through all

A VIEW IN PERSPECTIVE. 117

this long series of ages there has been a constant
change of species. Few species of animals have
succeeded in remaining in existence very long ; or, it
is probably better to say, few species of animals
have succeeded in breeding true for a very long
time. We are, of course, speaking geologically, and
thus are reviewing time by ages, rather than by
years. Sooner or later the descendants of any one
species died or became so changed from their earlier
form as to constitute a new species. Thus through-
out the past new species have been continually
appearing. It is indeed very seldom that any spe-
cies continues to exist from one age to another, and
thus, as a rule, the species of each age were distinct.
As already noticed, there were long periods between
the geological ages, during which no record of life
has been preserved. These blank periods were un-
doubtedly periods of considerable disturbance in
nature, and they lasted many thousands of years.
The only record we have of the events that occurred
during these times is in the modification which they
produced upon the living world. The disturbances
were usually sufficient to change animal forms so
much as to produce an entirely new set of species.
It is thus usually possible to determine closely the
age of any fossil by its specific characters.

(Nevertheless, we find that no universal rule can
be given ; for while most species change with the
advent of new ages, some of the species of animal,
were much more persistent. Some forms of life
remained with little change through all the geologi-
cal periods. This is true of Lingula, some foramini-
fers, and some mollusks. \ The species that are found

Il8 THE LIVING WORLD.

now are, to be sure, different from those found in
the Silurian (2), but the genera are identical, and the
differences between the species are not great. In
such cases the same species may exist from one age
to another. These are. however, exceptional in-
stances. Generally the particular species character-
istic of each age were replaced in the subsequent
age by a new set, and this same fact is also true of
the genera. Families were longer lived, and many
families living in the Silurian (2) age inhabit our
seas to-day.

It is not common, then, for any species to succeed
in outliving the long periods between the geological
ages. Nevertheless, the different species of animals
have had quite different lengths of life. Some are
confined to a single stratum of a single age, while
others extend through two or three geological sys-
tems (some Brachiopoda). It is found also to be
a general rule that the lowest species are of the
longest duration. For instance, certain species of
Foraminifera (Saccaminina Carteri) appeared in the
Silurian, and have continued to exist until to-day
.with little or no modification. So, too, with the
Lamellibranchiata and Crustacea, the lowest forms
appeared early and continued to live a long time,
while the higher ones were more rapidly modified.
Again, as a somewhat better illustration, we find that
in the Tertiary (9), the reptiles had already assumed
the forms which have continued to exist up to the
present time. Not so, however, with the higher
vertebrates, which have undergone great changes
since that time. The same is true everywhere. Low

A VIEW IN PERSPECTIVE. IIQ

animals with simple structure are not so delicately
adapted to their environment as to require a change
for every little change in their surroundings. The
higher animals, however, are so complex and withal
so delicately adjusted to their environment that any
little change of conditions throws them out of har-
mony, and requires change in structure to meet the
new conditions. A steam engine requires much
more care and gets out of order more readily
than a water-wheel. So the types of low animals
have continued to exist, while the higher ones have
been rapidly modified.

(7 It is to be noticed further that not only has this
Change of species been a constant change, but that
itrias also been a gradual one. There have proba-
bly been no abrupt breaks in the history. We find
that when one species disappears, it is either re-
placed by another^closely related form, or the whole
line becomes extinct. In all cases where the rocks
have preserveo! for us anything like a continuous
history, it is seen that there are no abrupt transi-
tions from one type to a radically different one. In
the few cases where the record is exceptionally com-
plete, it is possible to trace one species into another
by innumerable connecting links. Such evidence
is, however, rarely forthcoming, and new species
commonly seem to appear abruptly. /In many in-
stances, indeed, quite new types of life^eem to come
suddenly into existence, but wherever this is the
case we find that there is usually a break in our
record of history*. Now if we take into account the
long periods of "'unrecorded history that intervened

120 THE LIVING WORLD.

between the ages, we are of course prepared to find
that each age is characterized at its outset by a new
set of species, and in some cases by a distinctly
new fauna. Remembering, then, that all breaks in
the history occur where there are breaks in the
record, and that there are no breaks in the history
where the record is complete, we may unhesitatingly
conclude that the real history of life has been one
of continual slow change, without breaks, with each
age passing imperceptibly into the next.

Animal Types the Same To-Day as in Earlier Times.

With all of this extinction of old forms, it is
remarkable that since the Silurian (2) no great
type of animals has disappeared. All of the sub-
kingdoms in existence in the Silurian are still in
existence to-day, and the same is equally true of the
classes and nearly so of the orders. It is marvel-
lously interesting and surprising to find that all of
the fossils found in the rocks deposited so many
millions of years ago can be readily placed with one
or another of the divisions of animals that are in
existence to-day. The fact gives us a forcible lesson
that the animal kingdom is one, and that during all
the history of the world it has been a unit. It tells
us that the same laws in force to-day were in force
millions of years ago in the world. It indicates that
some bond has united the animals of the rocks with
those of our seas and lands ; and this unity gives us
one of the strongest arguments for the belief that
heredity and evolution have been the laws presiding
over the development of the animal kingdom.

A VIE W IN PERSPECTIVE. 121

A second fact of equal significance is that not only
has no large type disappeared, but no new great type
of animals has appeared since the Silurian (2) age.
This has been sufficiently discussed, and only need
to be mentioned here as correlated with the subject
in the last paragraph.

During the geological ages species differed in
their range as they do to-day. Some were confined
to small localities (certain species of trilobites), while
others had an almost world-wide range (Foraminifera).
As would be expected, the species with a long range
in time were usually the ones that had also a large
geographical range (Saccuminina Carteri). To this,
however, there are exceptions, for some species con-
fined to a single system of rocks have an enormous
range in geographical area (certain ammonites).

In looking over the whole geological period we find
that after a species of animals has once disappeared
it has never again reappeared, no single instance
being known which would serve as an exception to
this rule. The principle may be carried out still
further. No type of animals is ever a second time
developed. It is probably even correct to say that
throughout the wide animal kingdom no organ that
has fully disappeared is ever again redeveloped.
Animals cannot develop a second time that which
they have once completely lost ; when a change of
circumstances requires an animal to perform again a
function formerly performed by a lost member, the
lost member is not redeveloped, but some other
organ assumes a new function. For instance, the
insects once possessed many legs but they have uni-

122 THE LIVING WORLD.

versally lost all except three pairs. Now the larvae
of butterflies (caterpillars) need a larger number of
legs than this, but instead of redeveloping the lost
legs they have simply had a fold of skin modified
to serve the function of legs.

Early Forms Intermediate Between Existing Types.

The next point of significance which we notice in
the examination of the history of animals is that,
although the earlier animals all conform to types still
in existence, it is frequently impossible to classify
them satisfactorily. Among the invertebrates there
is not very much difficulty in this respect, probably
because of the fact that even our earliest rocks
contained the types well differentiated from each
other. When we study the early representatives of
the higher types whose history is more or less com-
pletely represented in the early stages, it becomes
difficult and indeed impossible to determine where
to place them. The early insects of the Devonian
(3) and Carboniferous (4) seem to be related to sev-
eral of the present orders of insects, and it is impos-
sible to determine whether they are to be called
beetles, bugs, and dragon-flies, or are to be regarded
as all cockroaches showing an approach to these
other types of insects. Especially is this difficulty
of classification true in the case of the mammals.
Among the early representatives of this class of
animals in the Tertiary (9) some are so truly inter-
mediate in type that it is impossible to determine
whether they were insectivorans or marsupials, while
others -stand midway between the carnivores and

A VIEW IN PERSPECTIVE. 123

ungulates. In short, many of the forms found in
these rocks are so intermediate in type between the
orders which we find in the world to-day that they
cannot be regarded as belonging to any of them, and
at the same time" they show so many characters in
common with the mammals now existing that it
is equally impossible to regard them as forming dis-
tinct orders. They were, in fact, intermediate types.
It is commonly true that while the fossils require
no new. types created for their classification, the
fossils representing the introduction of any group
show such a complication and combination of rela-
tions that it becomes impossible to classify them
satisfactorily into any of the modern orders. As
we study fossils of the succeeding rocks, however,
we find the similarity of the structure to the modern
orders more and more close, so that the nearer we
approach the present > time the easier becomes the
task of fossil classification. The interpretation of
this is of course found in the fact that the various
orders of animals have separated from common
centres in accordance with the principle of descent,
or otherwise. The nearer we come to those centres
the greater is the similarity of the animals ; it was
only in later times that the different lines of descent
became sufficiently separated from each other to be
recognized as distinct orders.

Introduction^ Development, and Decline of Types.

We must now notice more particularly the history
of some representative groups, studying their method
of introduction, expansion, and extinction. In gen-

124 THE LIVING WORLD.

eral we may say that, at the beginning of their history,
all groups of animals are few in numbers and of
slight variety ; then they expand until a culmination
is reached, when they begin to diminish in numbers
until they reach extinction, and are replaced by
other groups. Such seems to be the general history
of all groups. Taken separately, however, the groups

Q

Te

Cr

Jr

Tr
P
C

D

FIG. 15.

differ widely from each other, some of them having
become extinct long ago, and others not yet seeming
to have reached their culmination. Although the
history of the different orders and classes is almost
as varied as the groups themselves, still we may
recognize several distinct types of development.

Fig. 1 5. — Some groups of animals were in existence
in abundance at the beginning of our record ; they

A VIEW IN PERSPECTIVE. 12$

increased rapidly in numbers and diversity, soon
reached a culmination, and then disappeared with
an almost equal rapidity (see Fig. 15). The tri-
lobites, for instance, were numerous in the Silurian
(2). During this age they became highly diversified,
reached their culmination in the last of the same
age, and then rapidly diminished in numbers and
disappeared. The same is true of euryptids. The
graptolites were abundant in the Lower Silurian (2),
reached their culmination during this age, and dis-
appeared completely with it, no traces of them being
found in any subsequent period. Other examples
of the same kind are the Cystiphilloidea, disappear-
ing in the Devonian (3), the Blastoidea and Cistoidea
in the Carboniferous (4), and also the Palaeechinoidea
ending their history in the Carboniferous (4).

Fig. 1 6. — Some groups not in existence at the be-
ginning of the history seem to have suddenly appeared
and then as suddenly disappeared again (see Fig. 16).
The rugosites of the Cretaceous (8) are the best ex-
amples of this. These remarkable mollusks, so unlike
any other representatives of the group, seem to have
become quite abundant in that age. They completed
their history, however, with the Cretaceous, and no
traces of them subsequently have been found. None
other of the invertebrate orders had a similarly short
life. One order of the dipnoids (Acanthodini), one
of Amphibia (Stegecephala), six orders of reptiles
(Ichthosauria, Plesiosauria, Pythonomorpha, Thero-
morpha, Dinosauria, and Pterosauria) have been
confined to comparatively short periods in the past.
Three orders of birds (Saururae, Odontholcae, Odon-
totormae) and at least five orders of mammals

126 THE LIVING WORLD.

(Toxodontia, Condylarthra, Amblypoda, Mesotheria,
Tillodontia) are confined to the Tertiary (9). The
duration of the existence of the above-mentioned
orders was short. In many cases the orders were
confined to one system of rocks. Of course the
sudden appearance of these various orders is, in a
measure, misleading. They always appear after a

Q

7>
Cr

Tr

P

C

D

FIG. 1 6.

long period of unrecorded history, commonly at the
beginning of a new system of rocks, and this fact
must be interpreted as meaning that their seemingly
sudden appearance is due to their earlier history
having been lost. In some of the orders mentioned,
indeed, the appearance was not very abrupt. But,
nevertheless, all of these groups of animals are
marked from others by their rapid development and
sudden extinction.

A VIEW IN PERSPECTIVE.

127

Fig. 17. — Some groups have appeared in greater or
less abundance in the Silurian (2), have rapidly cul-
minated, and then slowly dwindled away, but have
never become entirely extinct, even up to the present
day (see Fig. 17). The crinoids form our best illus-
tration. They were present in abundance early in
the Silurian (2) rocks, they rapidly expanded, reached

FIG. 17.

their culmination in the Carboniferous (4), and have
since that time been constantly diminishing in num-
bers. They are not yet extinct, but only eight genera
of this once predominant type are known to exist,
and most of these are confined to the depths of the
sea. The Brachiopoda also were so abundant in the
Silurian (2) age that they may be called the charac-
teristic animals of that era. Immediately after the

128

THE LIVING WORLD.

Silurian (2), however, they began to diminish in
number, and have been growing of less importance
ever since. To-day, though not so much diminished
in numbers as the crinoids, they form a comparatively
unimportant group with few genera. The orders
of Phyllocarida form the only other important illus-
tration. This is a small group of Crustacea, quite
abundant in the Silurian, which, with the exception
of a single genus (Nebalia), is to-day extinct.

Q

Te
Cr

FIG. 18.

Fig. 1 8. — Some groups of animals appeared well de-
veloped with the Silurian (2) age, and have continued
to exist in undiminished numbers ever since, or
have even increased in number and diversity (see
Fig. 1 8). This is true of a majority of the orders
found in the Silurian. Many of them seem to have

A VIEW IN PERSPECTIVE.

I29

been constantly increasing in range with greater
or less rapidity. This is true of the hydroids,
corals, star-fishes, mollusks, and especially of the
air-breathing insects.

Fig. 19. — Some groups not present at the beginning
of the Silurian appeared subsequently, perhaps in
small numbers, and from the time of their appearance

FIG. 19.

have continued to expand (see Fig. 19). Their num-
bers have grown larger and their diversity of structure
has constantly increased. This has continued until
to-day, and now they exist in greater abundance
than ever before. Under this head are included all
of the vertebrate orders living to-day, and nearly all
of the insects, at least of the higher orders. Among
the lower animals we find, it true of the dibranchiate
9

130 THE LIVING WORLD.

Cephalopoda (squids), of an occasional order among
the rest of the mollusks, of our common lobsters,
shrimps, and crabs, of the order of modern sea-
urchins, and seemingly so of some of the orders of
Coelentera (Alcyonaria), etc., though of this we can-
not be sure, since the animals appear to be so poorly
adapted to preservation that their early history is
uncertain.

It is plain that all of the orders of animals existing
in great abundance to-day must come under one of
the two classes last mentioned, and it will thus follow
that nearly all of the animals well known to the gen-
eral reader have had the history of constant expan-
sion from the time of their appearance until to-day.

Fig. 20. — Not infrequently in the history of some
groups there has occurred a long period of stationary
condition, followed by a rapid development (see Fig.
20). A very striking illustration of this is shown in
Fig. 10, which represents the history of the echino-
derms. From the figure it will be seen that the sea-
urchins appeared in the early Silurian (2), but during
this and the long Paleozoic ages remained practically
stationary. With the beginning of the Triassic (6),
however, some influence caused the urchins suddenly
to begin to develop into new forms. In a very short
time, comparatively, there were then produced a
large number of sub-orders and families, and the sea-
urchins were brought into existence practically as
we have them in the world to-day. By the time of
the Cretaceous (8) this development had ceased, and
from that time the echinoid group, as well as the rest
of the echinoderrns, has remained stationary, or per-

A VIEW IN PERSPECTIVE.

haps has even been declining. The mammals again
give a very pretty illustration of the same principle.
This, the highest class of vertebrates, appeared first
in the Triassic (6). These early mammals were all
small animals and were of the lowest type, the mar-
supials. During this and the two subsequent ages,

FIG. 20.

Jurassic (7,) and Cretaceous (8), the mammals prob-
ably continued to exist with very little change, the
few remains found during these ages not certainly
indicating any special advance over those of the
Triassic (6). At the close of the Cretaceous (8),
there seems to have been some influence acting
upon the class which started them into a remarkably
active development. Just when this impulse oc-
curred, we do not know, but with the beginning of

132 THE LIVING WORLD.

the Tertiary (9) we find abundant fauna of .the
higher true mammals, indicating that by this time
the mammals had been for some time under the
influence of an expanding force. From this early
Tertiary (9), the development and expansion of
the mammals occurred with great rapidity and in
a very short time, short at least compared with the
long period in which the class seems to have re-
mained dormant, the higher mammals had diverged
into the modern ones.

Causes of the Sudden Expansions of Type.

The question, of course, arises why any group of
animals should have remained so long in a compara-
tively stationary condition and then have suddenly
expanded with such rapidity into a widely diversified
fauna. It is hardly ever possible to give a definite
answer to this question. A somewhat general answer
can, however, be given, which is very suggestive
as indicating one of the important laws of animal
life. It would seem that the comparatively sud-
den expansion of type has occurred when a group
of animals in some way begins to occupy a new
field of nature. Where conditions remain constant,
animal life is held in comparative equilibrium, and
therefore is more or less stationary. But any change
in conditions will disturb the equilibrium, and the
result is always a change in the structure of the
animals themselves. Now any change in climate,
in temperature, in atmospheric constitution, in con-
figuration of the land and sea, etc., will be sure to
affect the equilibrium of life. That such changes

A VIEW IN PERSPECTIVE. 133

have been constantly occurring in the past we have
abundant proof, and it is certain they always induce
changes in living nature. Indeed, the periods of
greatest geological disturbances have always been
those of most rapid evolution of animals (e. g., the
end of the Carboniferous (4) and Cretaceous (8). It
would seem, however, that more restricted causes
may frequently have been the origin of organic
change. A group of animals may migrate into a
new territory, and finding there a new field with a
less powerful set of competitors, it will be able to
expand itself to a much greater extent than in the
old home. That this is a result of such migration is
well known from the study of animals to-day. Un-
doubtedly such local changes have occurred in the
past and have constituted one of the factors which
have caused the periodic expansion of animals.

There is, however, another factor probably of even
greater importance, but one which cannot be so
easily understood or explained. Internal changes in
the organism itself will produce new fields of expan-
sion. It is well known that animals are constantly
varying, now in one direction and now in another.
These variations are continuous, and seem to have
no direct relation to any change in environment.
Of the many variations that appear, some are use-
less, and soon disappear. Others are of some value,
and will be preserved by natural selection. Now it
will occasionally happen that the variations may be
of such a character as to make a radical change in
the organism and fit it for entirely new conditions.
For instance, we may suppose that all during the

134 THE LIVING WORLD.

Triassic (6), Jurassic (7), and Cretaceous (8) ages the
mammals were undergoing constant changes, and
were subject to numerous vicissitudes. Nothing
occurred, however, to give them any special ad-
vantage over their conditions or their enemies, and
they therefore continued for this long period with
little modification. But toward the end of the Cre-
taceous age it chanced that among the multitude of
variations some of the individuals acquired a new
character in regard to the habit of reproduction.
This new character gave them the power of retaining
their young in the uterus for a longer time than before,
carrying them indeed until they were well developed.
Now we know from the study of animals to-day that
this character, together with others correlated with it,
make the animal thus favored very much more pow-
erful and far better adapted to contend in the strug-
gle for life. We may not be able to say why it is,
but it is certainly true that the mammals with this
new habit in reproduction always triumph over the
marsupials. It is easy to suppose, therefore, that
as soon as this variation occurred in the mammalian
stock there was an immediate impulse given to the
development of the group. The mammals became
more active, and soon proved themselves more than
a match for the large reptiles. In the struggle for
food they soon triumphed over their marsupial pre-
decessors, and finding thus the whole world open to
their conquests, they multiplied rapidly and gave
rise at once to the multiplicity of types found in the
early Tertiary (9). A new field had thus been offered
by a chance (?) anatomical variation.

A VIEW IN PERSPECTIVE. 135

Now in the explanation thus given we must of
course deal somewhat with hypothesis. We do not
know positively that it was the occurrence of the
new variation in the reproductive habits that gave the
stimulus to the mammals, and thus caused the rapid
divergence in the Tertiary period. We know, how-
ever, that the mammals did at that time receive a
new impulse from some source, and did immediately
expand to fill the new field of nature open to them,
and we have pretty good evidence, moreover, that at
the same time the above-mentioned change in the
reproductive organs occurred. From all of this we
learn to picture to ourselves the animal kingdom as
constantly searching after new fields for expansion.
The different types of animals are constantly migra-
ting here and there, constantly subjected to new
conditions, and therefore constantly undergoing
change. Generally the severe competition with
nature and with enemies keeps them in restraint
by cutting off all sportive branches, and allowing
only the central strongest forms to continue to exist.
But occasionally a change in the conditions produced
by geological forces gives certain classes the advan-
tage over others, or changes in the configuration of
the land produce new fields for expansion. Or,
again, some one of the sportive branches proves
to have inherent qualities of great vigor, and in this
way offers a new field of organic type. In any of
these cases the opportunity is seized
ately, and shows itself in rapid expj
tainly a fact that the multipli<
greatest in the early life of a gei

136 THE LIVING WORLD.

As already stated, it is very seldom that we can
determine what were the circumstances which have
produced the rapid development of certain types.
We may suppose that the marvellous expansion of
the reptiles in the Mesozoic was due to the fact that
these were the first true air-breathing vertebrates,
and had therefore the whole land to themselves.
Such an unoccupied field would undoubtedly have
had a tendency to stimulate them into rapid expan-
sion. We may suppose also that the rapid appear-
ance of the numerous order of insects in the Jurassic
(7) and Cretaceous (8) was correlated with, if not
caused by, the appearance of flowers and the acquir-
ing of the habit, on the part of some insects, of
feeding upon them. So too the rapid expansion of
birds in the Cretaceous and subsequent age was due
to the acquiring of aerial powers, which powers were
of course due to internal variations in the direction
of the production of wingsl Perhaps the expansion
of mammals was due to the change in reproductive
habits as above mentioned. In a few other cases it
is possible to make a guess as to the causes which
produced expansion of certain groups of animals,
but in general we must rest satisfied with ignorance
of the details, and with only the general explanation
that expansion is due to the occupancy of a new
field in nature.

A General Advance from the Earliest Ages.

The next point for us to notice is that there has
been a general advance in the animal kingdom from
the earliest ages until now. Taken as a whole, there

A VIEW IN PERSPECTIVE. 137

is no doubt that there has been an increase in com-
plexity and diversity, and this is what is meant by
advance. At the same time, when we attempt to
follow this law into particulars it is by no means
always possible to do so. In the sub-kingdom Verte-
brata, there is no doubt of the facts. Beginning in
the Silurian (2) with the lowest form, there followed
in the Devonian (3) multitudes of fishes ; in the Car-
boniferous (4) came the next higher class, the Am-
phibia, to be followed in the next era by the
reptiles ; these by the birds in the Cretaceous and
the mammals in the Tertiary (9), and lastly by the
appearance of man in the Quaternary (10). In all
this we see a continuous advance. In the Articulata,
too, it is possible to trace the same advance. Ap-
pearing at first in the form of low generalized
trilobites and phyllocarides, the Crustacea developed
the higher orders of lobsters, shrimps, and crabs con-
siderably later. The insects seemed to appear in the
Silurian (2) as cockroaches, which belong to the
lowest order. In the other Paleozoic ages there
appeared indications of the Neuroptera, and Coleop-
tera, and Hemiptera, which are always recognized as
the lowest orders. It was only in the later periods
that the higher orders made their appearance.

In the other groups of the animal kingdom, how-
ever, it is difficult to recognize any striking advance
in structure. It is questionable whether we can say
that the echinoderms of to-day are as a whole of a
higher type than those of the Silurian, and a like
question arises with respect to the Ccelentera,
Mollusca, and Brachiopoda. At the same time, the

138 THE LIVING WORLD.

general increase in complexity even in the case of
these types is indicated by two facts at least. We
see that in early periods it was the lower orders that
were most abundant and widely diversified, while
to-day it is the higher forms that are predominant.
For instance, among the Articulata it was the low
trilobites and cockroaches that existed in abundance
and diversified profusion in early times, while to-day
it is the higher orders that are the most abundant,
even though in the insects the lower orders are still
in existence. So in the case of the echinoderms, the
most abundant order of early ages was the Crinoidea
although the other classes of echinoderms were all in
existence. To-day though the crinoids are still in
existence, it is the higher orders that are the most
abundant.

The second fact indicating the general advance is
the greater diversity of life to-day than in the earliest
times. Some groups have indeed passed into de-
cline or disappeared, but most of those that do exist
are to-day in greater profusion than in any of the
past ages. Taken as a whole, it is one of the most
evident teachings of the history of life that there
has been through all the ages a constant increase
in the profusion of living things, and a continually
growing diversity of form. Even though it is possi-
ible to say that many of the families are really no
more highly developed than were their represen-
tatives in the past, still the fact of the increase of
diversity of type is a plain indication of a general
advance. Thus the fact of the evident advance in the
structure of the representatives of some types, the

A VIEW IN PERSPECTIVE. 139

increase in the profusion of the higher orders of all
classes, and the general increase in the abundance
and diversity of the families of animals which we can
trace through all ages, shows conclusively that there
has been a distinct advance in complexity ef type
from the earliest times up to to-day.

Although there has been thus a general advance,
it is certainly true that it has not affected all orders
of animals alike. We have already noticed that some
have remained practically stationary in type. Some
have actually gone down hill instead of up. Many
groups of animals have advanced to a culmination
and then begun to disappear ; and in the disappear-
ing of animals we can frequently discern evidence of
a reversal of the process of development, the later
appearing animals becoming of a distinctly lower
type than their relatives at the period of culmination.
This has been specially well shown in the ammon-
ites which passed out of existence in the Cretaceous
(8). In many orders of animals the degeneration has
been produced from other causes. Degeneration is
always certain in the animal kingdom when any
organ ceases to be used. Many orders of animals by
becoming parasitic upon others cease to have any
use for some of their organs. The result is a very
general degeneration until the structure of the ani-
mal as a whole has become markedly degraded.
Any of the orders of parasitic animals will serve as
illustrations of this fact, and all of the sub-kingdoms
show examples of such degradation. Among fami-
liar examples may be mentioned fleas, which have
lost their wings ; tape worms, which have lost all of

140

THE LIVING WORLD.

their digestive organs, etc. Even the barnacles may
be included under this head, for although not para-
sitic, they have acquired stationary habits, and have
lost the locomotive organs they once possessed.
While, then, the general history of the animal king-
dom has been an advance, this advance is compatible
with many cases of retrogression. All groups ad-
vance to a culmination, and then decline through
decaying forms, and innumerable instances of para-
sitism have produced thus their inevitable degrading
effects.

CHAPTER VI.

A VIEW IN PERSPECTIVE. — (CONTINUED.)

Soft-Bodied A nimals.

LOOKING at the animal kingdom as a whole, we
are now in position to trace something like a general
outline of its history. Life appeared, very probably,
before the land was fit for habitation, and conse-
quently all animals were marine ; at all events, it is
certain that the earliest animals lived in the ocean.
Now, drawing our conclusions from the study of
embryology and comparative anatomy, we learn that
the earliest animals were soft-bodied, and were pro-
vided with no hard parts for defence or protection.
Their method of defence was probably the same as
that of the low soft-bodied animals of to-day. The
smaller forms were endowed with marvellous insensi-
bility to injury, and with great powers of multiplica-
tion. Cutting them to pieces simply caused their
multiplication, and this power of resisting injury
served in the place of protective organs. The larger
forms, like Ccelentera, were provided with stinging
hairs for defence, and they also possessed remarkable
powers of recovery from injury. Having no sup-
porting organs, they were never large animals, but
they doubtless filled the early seas in abundance.

141

142 THE LIVING WORLD.

Development of Hard Parts.

The next step in the history was the development
of hard parts for support and protection. This for
the first time made the existence of large animals a
possibility. Almost immediately after the diver-
gence of types of animals, the development of some
form of skeleton began. The echinoderms developed
plates of lime in their skin ; the mollusks secreted
upon their bodies a thick shell of the same material ;
some of the hydroids developed an external shell,
and the corals a calcareous framework ; the Articu-
lata (except the so-called worms) developed upon the
outside of their body a light but tough shell of
chitin. Such skeletons as these, while they were
certainly of great value, were in one respect a disad-
vantage. In the mollusks, for instance, they were
large and clumsy compared with the size of the
animal ; they prevented rapid motion and effectually
checked any great increase in size. It was only in
later times, when the mollusks got rid of their clumsy
shell, that any very great size could be attained, as
in the case of the great giant squids. In the Articu-
lata the shell, made of chitin instead of lime, was
lighter and tougher, and therefore better adapted to
activity. As a result, the Articulata succeeded in
producing such active and well-protected groups as
the insects. But the external shell of insects and
crustaceans is not fitted for any large animal, the old
trilobites (some of which were over two feet in
length) reaching the extreme in this direction.

A little later another division of animals hit upon
a new device for the support and protection of its

A VIEW IN PERSPECTIVE. 143

soft parts. An internal skeleton was developed.
This first appeared as a soft but resisting rod (noto-
chord), inside of the back, and reaching from head to
tail. This rod gave some strength, but could not
become resisting without destroying the flexibility
of the body. Therefore it soon became broken into
segments to give the body flexibility, and then it
hardened into a series of bones and formed the spinal
column and the basis of an internal skeleton. Of
this early history we have no trace from fossils, for
it was not until this rod had hardened into cartilage
that it was possible for vertebrates to be preserved
in the rocks. The early history of the vertebrates,
therefore, must be read from embryological evidence.
This type of an internal skeleton immediately proved
to be a success. This means of support gave strength
and rigidity to the soft parts, and at the same time
did not burden them so much as to render them
unwieldy, for it was adapted on mathematical prin-
ciples for furnishing the greatest strength with the
least bulk.

The value of this skeleton immediately showed
itself by the increase in size of the animals possess-
ing it. We find that there followed now a new
phase of animal history. For a long series of succeed-
ing ages we see an increase in size among the higher
members of the animal kingdom. Doubtless these
early vertebrates had fierce contests with other ani-
mals, but the large and active vertebrates soon
proved themselves superior to the clumsy mollusks
and the sluggish trilobites. The struggle of verte-
brates with each other was severe. Size was a factor

144 THE LIVING WORLD.

of the utmost importance in these contests, and
through all the history of the vertebrates we find
that the development of each class was marked
by an increase in size. This tendency toward in-
crease in size continued until the Jurassic (7), when
it reached its culmination in the huge reptiles of
this period, some of which were the largest animals
that ever lived. Of course, we do not mean that
every order of vertebrates became of large size, for
many of the smaller ones did establish their right to
live. Many small fishes and small reptiles did suc-
ceed in perpetuating themselves. Nevertheless we
can recognize the truth of the general fact that from
the Devonian (3) to the Cretaceous (8) or Tertiary
(9) a tendency towards increase in size marked the
history of the animal kingdom.

This increase in size, however, was sure to reach
a limit. Great size is only to be possessed at the
expense of activity and agility, and the great reptiles
of the Jurassic probably became so large that it was
a matter of greater and greater difficulty for them to
procure sufficient food. As we have already seen,
from the Tertiary period the mammals began to
supersede the reptiles as the monarchs of creation.
Even among the h'ighest class of animals we find for
a long time a tendency to produce animals of great
size. Some of the mammal orders of the Tertiary
(9) attained a size which almost made them rivals in
bulk with the reptiles. But we soon notice in the
history of this class a tendency to develop small
size and agility instead of huge. bulk. Of the eden-
tates, the huge Megatherium disappeared, and only

A VIEW IN PERSPECTIVE. 145

the small animals, such as armadillos and sloths
succeeded in perpetuating themselves. The gigantic
Dinotherium and the Mastodon were exterminated,
and only two species of elephants remain to repre-
sent this once abundant type. In general, among
mammals, it was the smaller animals, those which
developed speed, as in the ungulates, or immense
activity and strength in capturing prey, as in the
Carnivora, that gradually became the most abundant,
and thus eventually the predominant types. The
development of bulk had been superseded by a new
phase of progression. Increased activity, as shown
by the power of flight, and the development of claws
and teeth for defence, took the place of size as the
most potent factor in the struggle for existence.

The Mental Factor.

But this phase of nature was soon to yield to
another higher force. From the earliest record we
have of animals we can see an indication of the final
era in the history, i. e., the era of mental activity.
The brain of the lower vertebrates was small and
indicated the possession of little intelligence. The
same was true of the brain of early mammals. In
some of them the brain was so small that it could
be passed through the neural canal of the lumbar
vertebrae, and was thus only a little larger than the
spinal cord. From the early Tertiary (9) period,
however, the size of the brain has been increasing.
At the same time with its increase in size, the brain
has been increasing also in complexity, the cerebral
lobes becoming larger and showing more of a ten-

10

146 THE LIVING WORLD.

dency toward convolution. There is hardly a more
significant fact in the history of animals, when we
remember the approaching advent of man, than this
increase in the size of the brain of mammals from
the early Tertiary (9). Undoubtedly this increase
in the size of the brain was connected with the in-
creased activity of the mammals already mentioned,
for activity and agility imply delicate control of the
lower motor centres by the higher centres of the
brain, and this requires more brain power. Active
animals always have relatively large brains. But
undoubtedly also the increase in the size of the
brain was accompanied by an increasing amount of
intelligence. In short, with the development of the
mammals there had appeared a time when bulk had
given place to muscular activity and agility, but at
the same time the growth of the brain was gradually
preparing the way for the time when intelligence
should take the place of brute force.

«
Instinct.

Already, in an entirely different line of descent, do
we find the mental nature becoming predominant.
The higher order of insects has as its chief
character the habit of living in colonies, and the
consequent development of remarkable instincts.
As a rule, the insects seem to have depended upon
their powers of rapid multiplication as their chief
means of defence. Most insects are weak animals,
and all are small, but their immense powers of re-
production safely defend them from extermination.
Some of the higher orders, however, learned to band

A VIEW IN PERSPECTIVE. 147

themselves together into colonies, and then devel-
oped a most complicated set of instincts, mostly
adapted for the preservation and integrity of the
colony. Now while instinct is indeed a mental
factor quite distinct from intelligence, it is, like in-
telligence, a function of the nervous system. The
insects thus first inaugurated the development of
the nervous system to an extent which made its
powers prominent factors in the preservation of the
race. At what geological date instinct became such
a prominent factor in the life of insects we cannot
say. The individuals found in the Cretaceous (8)
were all sexual individuals. Now the formation
of colonies is usually accompanied by the produc-
tion of sexless individuals (neuters, workers). It
would therefore seem that the insects of the Creta-
ceous had not yet acquired their social habits, and
that the high development of instinct was subsequent
to this period. At all events it was a late event in
the history of insects, just as the development of
intelligence was a late event in the life of verte-
brates, and it seems probable that instinct reached a
high development in the Tertiary (9) before the
special development of intelligence began.

Intelligence.

We are now prepared to recognize a period in
the history of animals when intelligence had be-
come the predominant feature of nature. This of
course brings us to man, and we are now in posi-
tion to consider the important question of the
relation of man to this history of life. For our pur-

148 THE LIVING WORLD.

pose there is no need of deciding whether man is
a final step in the line of evolution or is to be
regarded in some measure as a special creation.
Man, by virtue of the powers he possesses, holds an
important position in this life's history, and his posi-
tion is the same whether his unique powers be
regarded as evolved from those of animals or as
newly created. The hint has already been dropped
that the essential feature of man is the importance
of his mental nature, and if we add to this his ethi-
cal nature, we have included all features of distinct
importance that belong to him. Man may, indeed,
be not incorrectly defined as the animal in which
everything has become subordinate to the nervous
system. For his existence in the world he depends
not upon his physical force nor anatomical structure.
He proves his right to live neither by defensive nor
offensive armor, neither by superior strength nor
superior powers of multiplication. The possession
of mind alone is his pride. No sooner did his men-
tal nature become a predominant feature than his
physical nature ceased to undergo any considerable
development. It is one of the curious facts of na-
ture that man, who possesses powers of mind so
infinitely superior to those of the highest animal
that there is really no comparison between them,
should at the same time be so much like the
apes in his anatomical structure that naturalists
think he ought to be classed in the same genus with
them. Anatomically, then, man is an ape, while, so
far as mental powers are concerned, he deserves a
new kingdom to himself.

A VIEW IN PERSPECTIVE. 149

It is not difficult to find an explanation for this
surprising fact. Among animals physical power is
usually the only feature by which one animal con-
quers another. With them, therefore, when a new
species is established it must be for some superiority
in physical ability. As a result the anatomical struc-
ture of animals changes with every advance, for
each new species must be in some respects better
adapted to its conditions than the older one from
which it came. With man, however, the physical
side of nature is comparatively of no importance.
From the moment that he proved himself master of
animals by his intelligence, the development of his
physical nature became a very subordinate matter.
He made artificial weapons for defence and offence
far superior to any that could be supplied him by
nature, and by intelligent use of them he became
far more efficient than any amount of muscular
energy could have made him. Now nature never
supplies animals with organs for which they have no
use, and therefore the development of man's body
practically ceased with the beginning of the develop-
ment of his mind. His hand, it is true, has become
more delicate, since that is an organ of great use to
his intelligence. Some other changes, perhaps, also
appeared, but except in a few superficial features
man remained in anatomy essentially as he was at
the outset, while the growth of his mental powers
soon produced between him and the animal king-
dom a wide chasm which we cannot bridge even in
imagination.

Here, then, we readily see why it is that there has

ISO THE LIVING WORLD.

been so much confusion and disagreement in the
attempts to place man in the scheme of classifica-
tion. Relying upon his bodily structure, he is un-
doubtedly to be placed with the higher primates,
and he is therefore ranked with the apes in all
schemes of natural classifications. But he differs
from all other animals in having as his essential
character the development of a new side of his
nature which is not primarily anatomical. When in
the pre-Tertiary times certain of the vertebrates
acquired a new character connected with the repro-
ductive system, there soon arose a type of animals
which we call mammals. Now classification is al-
ways based on structure, and we therefore call this
new group of animals a new class, because of their
new anatomical character and from the fact that
connected with it were other changes in anatomy
which radically modified the type of animal. When,
however, the final race acquired his new character of
mental development, we do not regard the resulting
animal as a new class, for in this case the new de-
parture did not involve any great changes in
anatomical type. We can therefore readily sympa-
thize with both classes of naturalists, one of which
regards man as a species of the ape family, while
the other recognizes three kingdoms — animals,
plant, and man. Either extreme of classification is
perhaps more justified than an intermediate position.

Divergence of Character Marks the Development of
Animals.

With the appearance of man there is the begin-
ning of a new law in nature, and one of great sig-

A VIEW IN PERSPECTIVE. 15 1

nificance. The development of mind, and especially
of the ethical * nature of man, is producing a result
in the human race which in a measure reverses the
results of the law of natural selection, and the other
laws governing animals.

As we have seen, the history of the animal king-
dom is such as can be best explained in the form of
a branching tree. In accordance with the laws of
nature, the most important of which is the law of
natural selection, the descendants of any line of
animals gradually diverge from each other like the
branches of a tree. For example the descendants of
the early type of mammals gradually assumed dif-
ferent characters along different lines until there
was produced the abundance of mammal orders
found in the early Tertiary (9). The exact way in
which the laws of nature work to produce such a
divergence is a matter under discussion to-day by
naturalists. Darwin tried to show that his law of
natural selection was in itself sufficient to explain
this divergence. Further study makes this more
doubtful, or at all events requires the addition of
certain other factors favoring the isolation of indi-
viduals. But whatever be the difference in our ideas
of the details, there is no question that the laws of
nature under which animals live and multiply, result
in the production of what is known as divergence of
character.

Now the essential feature of a divergence of char-
acter is isolation and separation. In some way the

* By the development of the ethical nature we would not mean to
imply that conscience has been evolved from the other attributes of
man, but simply that since the appearance of man the ethical element
has developed to a higher grade.

152 THE LIVING WORLD.

descendants of one pair of animals become asso-
ciated into groups. Each group has characters of
its own, and since it remains isolated from the
others, at least so far as interbreeding is concerned,
it soon establishes for itself a distinct line of descent,
a new race or a new species. Later in the history
perhaps its own descendants in like manner become
separated into still other groups, and so on, the
divergence becoming wider and wider as the cen-
turies roll by. Without some sort of isolation and
separation into groups divergence of character is
impossible.

Convergence of Character the Result of Human
Development.

The mind and ethical nature of man, and especially
the law of Christ, under which man is slowly learn-
ing to live, is producing a slow but sure modification
of the history of development. Instead of producing
isolation aud divergence, they are producing union and
convergence.

Man is distinctly a social animal. Among the
lower animals there are some that have what are
known as social instincts. Instead of living as
isolated individuals they associate in groups for
mutual protection. Among animals of low intel-
ligence, like fishes, this has little significance in the
development. With higher animals, however, social
habits produce more effect. With the higher insects,
these social habits have been developed to their
highest point. The marvellous series of instincts so
well known among ants and bees are unquestionably

A VIE W IN PERSPECTIVE. 153

the results of these social habits. But even in insects
the social instincts are very narrow in their applica-
tion, for although they produce association of indi-
viduals into colonies, although they are frequently
so far developed that the individual sacrifices his life
for the good of the colony, they do not give rise to
anything like an association of the colonies with each
other. Each colony of insects still keeps up the
natural warfare with other colonies.*

Man is also universally a social animal, and during
all his history has lived in communities. In early
history, as among savages to-day, men united for
mutual protection into communities or tribes. But
while the members of each tribe show friendship for
each other, the separate tribes have retained the
same sort of mutual enmity that is possessed by the
colonies of social animals. Hostility is the constant
relation of the tribes to each other, a hostility per-
haps even more constant than among animals. This
constant hostility produced a more or less complete
separation of the tribes from each other, and the re-
sult, as in the rest of the animal world, was a gradual
separation of the tribes from each other. These are
exactly the conditions necessary for the production
of divergence in character and the formation of new
races. To this extent then the development of the
races of men was similar to the development of races
of animals.

But almost from the beginning of the development
of man we notice a new law dependent on the pos-
session of conscience and the feeling of love. The

* To this statement there are a few exceptions.

154 THE LIVING WORLD.

ethical element of man's nature, though very rudi-
mentary in early history, has always belonged to
man. Its basis is love. Now love is really a new
feeling in nature, at least man is the first animal in
which it becomes of importance enough to influence
his development. Among the higher animals, such
as birds and mammals, we do find a maternal love of
the mother for her offspring, but the love ceases with
the maturity of the young. Even among social
animals there is found nothing that corresponds to
the love of one individual for another in its broader
sense. Animals combine for mutual protection- but
seem perfectly indifferent to each other except in so
far as the advantage of the whole community is con-
cerned. A soldier ant may sacrifice his life for his
colony, but he shows no feeling for another indi-
vidual in distress. We would not deny that occa-
sionally there are, even among animals, slight traces
of what must be regarded as the feeling of love in
a higher sense. It is certain, however, that such in-
stances are few, and that the feeling of love is not
one which can be regarded as having any considera-
ble influence upon the development of the animals.
With man, however, intelligence and conscience
have produced different conditions. Even in the
savage tribe a feeling of love for one's relatives or
friends, patriotism for one's tribe, self-sacrifice for
the good of the community, are characters which
receive the highest honor. While then with man, as
with animals, the tribal relations may be primarily
for mutual protection, it is certain that the feeling
of love for one another, the ethical element of human

A VIEW IN PERSPECTIVE. 155

nature, has had at all times the important effect of
cementing the tribes into a rigid unity. Union for
mutual protection is impossible without it. Among
the Jews we early find the command to love one's
neighbor as one's self. Their interpretation of the
word neighbor was to be sure very narrow, but the
presence of such a law shows that love was recog-
nized as one of the noblest attributes of man.

The tribal relation was thus originally assumed for
protection, but a mutual love cemented the tribes
into units. By further application of the same feel-
ings, the size of the tribes increased, and they were
finally submerged into nations. Now it is the feel-
ing of love which alone makes great nations possible.
The increase in the size of tribes and nations has
undoubtedly been brought about by conquest rather
than by love. But it is the feeling of love alone
which enables a large tribe to hold together. The
tribe whose sympathies were confined within narrow
limits would always disappear before the tribe whose
broader love made possible the union of larger
bodies. Those tribes in which this feeling of mutual
love and sympathy was the broadest obtained the
mastery over the others and increased at the ex-
pense of the others. Now since all tribes and
nations have recognized a demand on man to love
other members of his own nation, the scope of man's
obligation to love his fellow men has constantly ex-
panded with the increase in the size of nations.
Finally, with Christ there was announced the com-
plete law for man, the law of universal love. By
Christ was man's obligation to love extended to his

156 THE LIVING WORLD.

enemies, by him was the word neighbor defined so
as to include every one who needed help, whether of
the same tribe or nation or another, whether friend
or foe. Thus it was that love, the special attribute
of man, was so broadened as to include all mankind
and to bring about a universal brotherhood. This
law of Christ looks towards the destruction of the
tribal relation, and the national relation as well, and
when it is fully established as the law of man, it will
produce one nation, one association, which shall
combine all of mankind into one union of mutual
assistance and love. We are far enough from such a
condition at present. It is the millenium of which
we sometimes dream, and toward which our progress
seems slow enough. But the development of man
is tending in this direction, now that he has once
recognized that universal love is the law of his life.

Looking forward then into the coming centuries
we see the vegetable world remaining practically as
it is, except as it is modified by the interference of
man in exterminating plants not of value to him,
and improving those which he uses. We see the
animal kingdom on the land largely exterminated by
the power of man, though life in the ocean may for
a long time remain unchanged. But the marine
types offer no chance for the future, for they are all
low ones which reached their culmination in the past
ages. But we see mankind left, the creature of God,
advancing in intelligence, knowledge, and morality,
to the end which we do not see and cannot imagine.
Whether there be a phase in our nature superior to
mind, which shall in the future ages be brought into

A VIEW IN PERSPECTIVE. I 57

expansion and produce a new race of beings we can-
not tell, but the era of animal life ended when that
of man began.

It was not to be expected, of course, that this law
as announced by Christ would be at first understood.
The human race in his day had hardly entered into
the conception of the beauty of love in its narrower
sense of love to one's neighbor, and it was certainly
not ready to accept the definition of neighbor as in-
cluding one's enemies. It is not our purpose here to
enter into a consideration of the history of the slow
growth of this new law. Even yet we fail to accept
it, for the successful general receives our highest
honors, and the soldier is our greatest hero. We even
fail to understand the law. The larger our nations
grow, the more comprehensive become our obliga-
tions, but we do not yet believe that the time will
come when the Chinaman, the African, and the Cauca-
sian will actually unite into a common brotherhood.

All this lies beyond our present purpose. The
relation of the new law to the history of life, how-
ever, is very important for us to understand. This
law, as soon as it is applied to human life, produces
no longer divergence of character but convergence.
We have seen that the essence of divergence is sep-
aration and isolation of groups of individuals. The
races of mankind which we find to-day have been
produced by such a lack of intercourse among the
tribes of men and by the constant enmity and war-
fare of early nations which have tended to keep up a
constant isolation. But with the broadening of
man's application of his obligation to love his neigh-

158 THE LIVING WORLD.

bor, the nations become larger, and the increase in
the size of the nations acts against the increase in
their diversity. As fast as the members of a nation
become cemented together, so fast do they begin to
assume common characters. The American nation
is absorbing into itself a great variety of people,
perhaps faster than it can assimilate them. But as
fast as they are absorbed, just so fast does the char-
acter of the nation change. The American nation
no longer possesses the character of the men who
struggled for independence a century ago. The
history of civilization has been always marked by
the absorption of the small races into the larger
ones. It is only enmity and a narrow patriotic love
that prevents all of the smaller nations of Europe
from becoming parts of the larger ones. Thus it is
that to-day the formation of new races has been
checked, at least among the higher classes of man-
kind. The nations are growing larger, their hos-
tilities are becoming lessened, intercourse of com-
merce and friendship between them is increasing,
and with all this a tendency toward unification and
concentration is plainly seen. Not divergence but
convergence of type is the history of to-day.

It is therefore plain that the ethical nature of man
is producing a new phase in the development of the
world. It is checking the tendency to formation of
new types, and is tending to unite into one the
members of the race. To-day intelligence is uniting
all men into closer and closer relations of commerce
and education. In the future we can see it destroy-
ing all desire of conflict and victory ; we can see it

A VIEW IN PERSPECTIVE. 159

doing .away with race prejudice, and we can see
a united mankind advancing to higher and higher
planes. Thus it is that the intelligence, conscience,
and social habits of man, and above all his approxi-
mation toward the law of Christ, have produced a
new era in the history of life, and, as a result, the
development of mankind in the future is not to run
parallel to the development of animals in the past.
No longer are we to find a divergence and produc-
tion of numerous species of animals or even numer-
ous species of intelligence. There is to be one
human race, a race of marvellous complexity, ad-
vancing to higher and higher planes. Mankind is
to remain a unit, and so long as his chief character
is the development of his intellect, he will still con-
tinue to be man, whatever be the changes that the
future may see either in his physical or mental
attributes.

Each Type a Master of the Preceding.

Before leaving this sketch on the outline of history,
there are several other points of general interest to
be mentioned. First we must notice that in all of
this history the predominant animal type of any age
is, as a rule, more than master for all of the animals
that have preceded it. With their hard shells the
mollusks have no fear of the lower animals. The
activity of the articulates makes them more than a
match for the mollusks or other lower animals. The
vertebrates are always superior to the invertebrates,
and so, as a rule, the amphibians, reptiles, birds, and
mammals are seen in turn possessed of powers that

l6o THE LIVING WORLD.

make them masters of all the lower forms. Finally,
man supersedes them all.

The Rarity of Terrestrial Life.

Again we must notice how few are the types
of animals that have ever succeeded in adapting
themselves to a life on the land. Beyond the
vertebrates and the air-breathing articulates (insects,
spiders, etc.), there are almost no land animals.
Among the mollusks there are a few snails which
have become adapted to a thoroughly terrestrial
life. There are indeed quite a number of species of
snails thus breathing air, but they are all closely
allied to each other, and commonly have a much
restricted habitat. Evidently, then, only a few
mollusks have really been able to acquire the power
of breathing air. None of the other invertebrates
have been able to acquire the power of living in the
air, and we may say, therefore, that there are only
two types of animal structure which are adapted to
life out of the water. The Protozoa all require water
as a medium through which they can feed and carry
on respiration. Ccelentera have a body too soft to
resist gravity, unless assisted by the buoyancy of the
water. The same may be said of the Echinodermata,
together with the fact that their characteristic sys-
tem of organs, the water system, requires the presence
of water to make it of any value. The mollusks are
too clumsy, as a rule, to be adapted to terrestrial
life. The Vermes mostly respire through the surface
of the body or by gills, and either method makes it
necessary for them to have the exterior of their body

A VIE IV IN PERSPECTIVE. l6l

constantly moist. They are therefore aquatic, or
occasionally live in moist earth. The Articulataand
Vertebrata alone are well adapted to a terrestrial
life, and of these immense groups only five classes
have really become terrestrial animals (Insecta,
Arachnida, Reptila, Aves, Mammalia). Five classes,
then, out of the whole animal kingdom are all
that have acquired the power of living in the air.
These classes have more to contend with than the
aquatic animals, since they have gravity to resist,
and must accommodate themselves to the climate ;
they must adapt themselves to varying temperature,
and to many other conditions to which aquatic ani-
mals are not subjected. Now it is a law of nature
that the being which surmounts the greatest obsta-
cles is the one to rise to the highest plane. It is no
wonder, then, that these five classes of animals have
become predominant types, and surpass the other
animals in variety and numbers. They have had
the whole land in their possession.

We cannot definitely say when the terrestrial fauna
appeared. In the Silurian (2) there were certainly
in existence some scorpions, and probably, therefore,
'other land animals. Indeed, traces of insects have
been found in these rocks. Terrestrial vertebrates
did not appear until the end of the Carboniferous (4),
and not in any very great abundance until the Meso-
zoic (5-8). From the beginning of the Mesozoic,
however, terrestrial fauna has played the most impor-
tant part in the history of the world, and in the more
recent times it has been the terrestrial forms of life to
which evolution has been chiefly confined.

1 62 THE LIVING WORLD.

It is also a fact of great interest that both of these
types of animals which acquired terrestrial habits, the
vertebrates and articulate types, have ended their
history with the development of mind. The devel-
opment of insects has ended in the production of the
complicated insects familiar to every one acquainted
with the habits of the bees and ants. The develop-
ment of the vertebrates produces the intelligence
which has reached its culmination in man. Both
types of terrestrial animals have developed the ner-
vous system, and in each type the mental nature is
the special character of the highest orders. It will
also be noticed that the development of instincts
among insects has been entirely independent of the
development of the intelligence of the vertebrates.
The two have not even progressed in parallel lines.
Each group has developed the functions of the ner-
vous system, but one has developed the reflex func-
tions to an extreme (instinct), and the other the
reasoning powers. The intelligence of the verte-
brates cannot then be regarded as a more highly
developed condition of the instincts of insects,
although both intelligence and instinct are func-
tions of the mental nature. That terrestrial life has
had the effect of stimulating both types of land
animals into the development of two distinct types
of mind, becomes therefore a matter of even greater
interest.

The Modern Fauna an Impoverished One.

Finally, we must notice that the present age is one
of short duration and comparatively impoverished

A VIEW IN PERSPECTIVE. 163

fauna. The Quaternary (10) has certainly lasted a
great many thousands of years, but in comparison
with the immense periods of the earlier ages, it is
very short. It was with the Quaternary, indeed with
the later part of the Quaternary, that the strictly
modern fauna (i. e. the modern species) appeared,
and we may say, therefore, that the duration of the
present geological age is very short compared with
the immense periods of time that have preceded it.

The modern fauna is regarded as an impoverished
one. Of course there seem to be more animals and
more species to-day than ever before, but this is due
largely to the fact that we have the animals them-
selves to study to-day in profusion, while of the past
we have only here and there a specimen. In variety of
form the geological ages certainly surpassed the pres-
ent. There are but few classes in which there has not
been such a great extinction of orders in the past
that the representatives remaining are to be regarded
as fragments. Most of the invertebrates reached
their culmination in the geological ages, and many
of them have been for a long time on the decline.
Even of the vertebrates, every class has been in much
higher state of development in the past than at
present. The fishes belonged to the Devonian (3),
though one order has subsequently greatly expanded
since then (bony fishes, in the Cretaceous (8). The
amphibians belonged to the Triassic (6), the reptiles
to the Jurassic (7), while the mammals existed in
greater profusion in the Tertiary than they do to-
day. More than one third of the orders of the ver-
tebrates have become extinct, and of those that

164 THE LIVING WORLD.

remain a larger number have become reduced to a
few unimportant representatives of orders which
in former times were abundant in number and diver-
sity of form. Some of the orders that are still left, it
is true, are perhaps more abundant, so far as number
of species is concerned, than any orders of the past,
but as a whole the fauna of to-day consists of rem-
nants of the past.

Summary of the Last Two Chapters.

We may compare the history of the whole animal
world to the growth of a giant tree. As members of
the human race our position is among the topmost
branches, and it is only by peeping down through the
foliage that we dimly get an idea of the rest of the
tree. The foliage is dense and our vision is obscured.
Absorbed in that which immediately surrounds us we
fail to see the size and magnificence of this tree of
life, and, indeed, not infrequently we are inclined to
believe that man stands alone, having nothing to do
with the rest of the tree. Only by shutting our eyes
to the dense foliage that surrounds us, and by study-
ing the past do we learn that mankind too is a mem-
ber of the same tree of life that has lived through the
ages and has suffered so much from the storms of
the centuries to make room for his final appearance.

If we can imagine ourselves as removed from our
natural position among the branches and viewing
this tree of life in perspective from the distance, it
will appear something as follows. Its trunk is hid-
den from our view by the primeval mists of the early
ages, though its existence in the dim past can be in-

A VIEW IN PERSPECTIVE. 165

ferred from the convergence of the great branches as
they approach this region of obscurity. All above
the top of the trunk, however, is in sight, and we
can see the tree growing with an ever widening ex-
panse of its branches. But the storms of the ages
have played great havoc. Many limbs have been
torn off completely, many more have been so shat-
tered that only a remnant is left to mark the position
of a once mighty member. The prunings that have
thus occurred have frequently served to give more
room to the branches that are left, and these have
taken advantage of the opportunity to develop large
numbers of small twigs and fill the space formerly
occupied by a fallen member. Occasionally we see
a branch that has grown with marvellous rapidity for
a short time and then suddenly died ; or another
that grew for a long period as a single trunk and
then suddenly expanded into minor branches and
twigs. This tree of life is old and most of it has
long since spent its energy. Many of the branches
are dead, while others continue to live only in the
shape of scraggy twigs. Death and destruction have
played such havoc that the tree has become very un-
symmetrical and seems from our distant view almost
a wreck. But still some of the branches that are left
alive are very vigorous, and when we look simply at
the top of the tree the vigor displayed there almost
conceals from us the wreck of the past. There at
the top we see a single branch of this venerable tree
that has in recent times begun to grow to an enor-
mous size. A new law regulating its growth has
stimulated it in a new direction and caused it to de-

1 66 THE LIVING WORLD,

velop foliage at the expense of branches. So rapidly
is this branch growing to-day that it bids fair to ab-
sorb into itself all of the vitality of the whole trunk,
crowding out of existence its neighbors and allowing
only such of the lower branches to exist as do not
come in direct conflict with it. This vigorous branch
is man, and although it is far above all the others,
although it has expanded far more than the rest and
grown so far away from the trunk that its connection
with the great tree of life is sometimes obscured,
still our study by the light of history shows us that
this branch, too, is part of the same tree to which
other animals belong, and is simply the crowning
top of this tree of the ages.

CHAPTER VII.

HISTORY OF PLANTS.

THE history of the plant kingdom from early ages
to the present time requires but brief notice. The
problems connected with this kingdom are much
simpler than those relating to animals. While plants
certainly constitute important factors in the develop-
ment of life, their relation to the history of mankind,
which is after all the primal object of study, is very
distant. Like animals they have had a history, and
it is one of even more constant progression. Very
early in the history of life plants became separated
from animals by acquiring the power of utilizing
sunlight as a source of energy, and though some of
them have subsequently lost this power, it is never-
theless probable that this was the real point of
separation of the two kingdoms. Whether plants
or animals were the first to appear cannot be de-
termined, though it is certain that animals, as they
exist to-day, could not have preceded plants. We
have seen, however, that probably neither of them
preceded the other, and that the first organic life
was neither animal nor plant.

Of the early history of plants we know little or
nothing. The general simplicity of their structure

167

1 68 THE LIVING WORLD.

and their lack of any complicated system of organs
makes their embryological history less significant
than that of animals. We can, it is true, determine
that all plants start their history as single cells, and
this would seem to indicate that, like animals, they
have been originally derived from unicellular ances-
tors. But it is impossible to find traces of anything
in the history of plants that correspond to the
Gastraea of the animal world, nothing that can be
regarded as the central starting-point from which the
various groups diverged. Indeed, the history of
plants seems more like the history of a single de-
veloping line of life than of a series of diverging
lines.

Although embryology gives us little help in study-
ing the history of plants, still by combining its
teachings with the facts derived from comparative
anatomy, some few points can be determined in re-
gard to the line of descent through which plants
have come. That the unicellular plants were fol-
lowed by the low algae cannot be questioned. That
the higher plants were derived from these seems
also sure. The mosses, ferns, club mosses, cycads,
cone bearers (pines), and angiosperms (common
flowering plants) follow each other in something like
the order given, with increasing grades of structure.
Now in the embryology of the cone bearers (gym-
nosperms) we can still find traces of an earlier stage
in the history of plants corresponding to the club
mosses, and even in the highest plants of all, the
angiosperms, there are indications of a like history.
It is significant to find that the fossil history of

En

Cr

FIG. 21. Diagram illustrating the history of plants—/5 Pteridophyta. Ly Ly-
copoda. G Gymnosperma. Ex Exognens. En Endogens. A Algsc.

I69

I/O THE LIVING WORLD.

plants tells somewhat of a similar story, and we may
thus conclude that even in plants, embryology re-
peats past history, at least in a measure. Of the
early history of plants, however, we know practically
nothing, and of the history of later ages it is neces-
sary to combine all sources of evidence in order to
get anything like a connected account.

Plants have not left so complete a fossil record as
have animals. Particularly is this true of the lower
forms which almost universally agree in having no
hard parts adapted for preservation. So imperfect
is their preservation that it is impossible in many
cases to determine whether a given specimen in
the earliest rocks is a plant or simply a crystalli-
zation, a worm track, or mud crack. There is no
doubt that plants were in existence during the long
Archean (i) age. The immense beds of graphite
belonging to these times give us almost certain
proof of the fact. But no traces of them except
the graphite beds have survived the metamorphosis
of the rocks.

In the Silurian (2) age, however, plants were un-
doubtedly abundant, and our first record of them is
thus nearly contemporaneous with that of animals.
But the plants were all of the lower types.

Marine algae were doubtless in great numbers in the seas of this
period, and everything seems to indicate that they were not unlike
their descendants of to-day, which form the slimes of fresh water and
some of the sea mosses. Their poor preservation makes it impossible
to say much about them. During the Silurian (2) there were also in
existence at least two types of land plants, living perhaps in swamps
or shallow water. There was a delicate little plant named Psilophy-
ton, thought to be a link between two later groups (rhizocarps and

HISTORY OF PLANTS. I/I

lycopods), and also a plant of a size so large as to resemble a tree,
though in structure of so low a type as to belong to the algae (Nema-
tophyton). Other land plants were living also, but they were all of
a very low type. The land vegetation was marvellously different
from that of to-day, and we may almost look upon it as a flora of
marine algae which had been transferred to the land and then en-
larged. Still there was, even at this time, a differentiation into stem
and leaf, and this does not occur in ordinary algae. The Silurian
flora was in one marked respect very different from the Silurian
fauna. The latter has surprised us with its diversity and high
grade of development, while the former may equally surprise us with
its scarcity and its low grade. The development of plants had not
reached such a high state as the development of animals at this time.

In the next age (Devonian) there was an undoubted advance.
Algae were still in abundance, as indeed they have been in all ages
up to the present time. But with the Devonian, undoubtedly, higher
plants appeared. True rhizocarps and lycopods (horse tails) were
abundant at this time, though possibly they may have begun in the
previous age. In the Devonian also we find true ferns, some of them
small and others of large size like our tree ferns. In this age, too,
appeared the first indication of the flowering plants in the form of
gymnosperms or conifers (yews and cordiates, an extinct family).
The flora of the Devonian was thus on the whole composed of the
highest of the cryptogams (lycopods) and the lowest of the phanero-
gams (evergreens and conifers).

In the interval between the Devonian (3) and the Carboniferous (4)
ages there were great changes in the level of the land. After various
submergences it finally arose clothed with a flora of precisely the
same general character as that of the Devonian, but with new species
and genera. The changes had been sufficient to obliterate old and
produce new ones. The flora still consisted of the same groups of
plants with no advance in structure. The ferns became very diversi-
fied, and the horse tails and lycopods reached their greatest size, but
there was no advance in structure. The age was characterized, how-
ever, by the great development of the cone bearers (Gymnosperma),
ferns (Pteridophyta), and the horse-tail group (Lycopoda). The Car-
boniferous was an age of especially abundant vegetation, and to the
plant growth of that time we owe our beds of coal.

Coming now into later periods, we find with the beginning of the
Mesozoic again a new flora, but again no advance in structure of much

172 THE LIVING WORLD.

importance. The typical vegetation of the Carboniferous became less
prominent and the cycads appeared. The cycads are the prominent
type of the Mesozoic. During the passage of the Mesozoic, however,
we see a change in the flora of a radical sort. Even with the Trias-
sic (6) the giant horse tails began to wane, and the more modern forms
of moderate size appeared. In the Jurassic (7) the cone-bearing plants
reached their highest development, from which culminating point
they have steadily declined. Some of our modern forms arose at this
time, the genus Pinus being in considerable abundance. During the
Jurassic also the first of the endogens made their appearance in the
form of screw pines and grasses, and a little later the palms appeared.

The Cretaceous (8) period is, however, the line that marks the
boundary between the older vegetable world and the modern, just as
the beginning of the Mesozoic separated the older animals from the
modern forms. Occasional traces of the higher flowering plants are
found in the lower rocks of the Cretaceous, but it is in those of the
upper Cretaceous that they appear in abundance. Here there sud-
denly bursts upon our view an abundant flora of modern forms.
Most of the plants of that time belonged to the same genera as those
living to-day, and many of them were, so far as we can tell, of the
same species. Our knowledge of them is chiefly confined to the
preserved leaves, and these are a rather unsatisfactory basis for deter-
mination of species, but there seems to be no doubt that the poplars,
willows, beeches, oaks, birches, alders, laurels, sassafras, magnolias,
butternuts, hickorys, and many others were represented in profusion.
There is also pretty good evidence of the existence of the higher
flowering plants, like the Composite, at this time, though doubtless
the plants with inconspicuous flowers predominated. In short, at
this time the modern botanical world was nearly complete, so far as
type was concerned. Its highest forms had been developed even
then, and from that time to the present the growth has been simply
in the expansion of typesUhen existing, and in the relative increase
of the numbers of the highest flowering plants. The gymnosperms
certainly have become less abundant, and the plants with large con-
spicuous flowers have greatly increased in preponderance.

•The endogens have remained nearly constant since then, so that
we may say that in the later Cretaceous the vegetable world reached
nearly as high a position as it has to-day.

We may now ask whether in this history of plants we can trace
the origin of the different groups of plants from each other. As

HISTORY OF PLANTS. 1/3

already pointed out, the similarity in structure would lead us to be-
lieve that the Silurian land plants were simply terrestrial forms of
the marine algae. The same conclusion would follow from the study
of their anatomy and development. The rhizocarps, to which some
of the early land plants seem to be related, are themselves closely
related to various algae. So, too, may the ferns and horse tails
or the Carboniferous be regarded as derived from lower marine
plants (algce), with various modifications, probably through some
lost intermediate steps. The cone bearers, however, give evidence
of having descended not from the algse, but from some high form
of plant belonging to the Lycopoda. Their structure, and espe-
cially their method of reproduction and development, shows them
closely related to certain plants classed with the Lycopoda (Isoetes),
but differing from the ordinary Lycopoda in having two kinds of
spores, a large one and a small one. These micro- and macrospores
correspond to the pollen and embryo sac of the flowering plants, and
from some such source doubtless have the latter been produced.
The endogens again were unquestionably not products of the cone
bearers, but they had an independent origin down the main line, not
unlikely an origin close to that of the gymnosperms. The higher
flowering plants, the exogens, though closely related to the gym-
nosperms, were probably not derived from them directly, and were
certainly not derived from the endogens. Probably they had an
independent origin from some lower group close to the gymnosperms.
It would seem then that some low algae-like type of plant at one time
became terrestrial, and then there occurred a divergence from it
which resulted in the various forms of cryptogams, the mosses, ferns,
and lycopods. These groups soon expanded and reached their cul-
mination in the Carboniferous. But along one line of descent
(Isoetes, etc.,) two kinds of spores were produced, and this line now
expanded and produced the flowering plants, the gymnosperms,
endogens, exogens, all of which plants retain the t\vo kinds of
spores (pollen and embryo sac), but seem to give indication of having
had independent origins from the cryptogams.

It is a fact of no little interest that the flora of the world probably
did not, as would be expected, originate in the tropical regions, but
on the contrary in the Arctic zone. Abundant evidence shows that
in the northern regions the various groups of plants first appeared
and culminated. This must, of course, indicate that the climate of
the Arctic zone was not the frigid one that it is to-day.

THE LIVING WORLD.

The first point that strikes out attention in this
history, is that the life of plants has been more evi-
dently one of progression than that of animals. We
do not find that the Silurian (2) age opened with an
abundant and highly specialized flora to correspond
to the highly developed fauna of the times. The
plants were few and all of the lower orders. Com-
paratively speaking, then, we may say that the ani-
mal kingdom had reached a much higher state of
development by the beginning of the Silurian than
the vegetable kingdom. While all of the classes of
animals (except two) had appeared at this time or
during the Silurian age, there were no plants higher
than rhizocarps, and nearly all of the plants in exis-
tence were algae or still lower types. The whole
vegetable world which is familiar to us to-day was
still to be developed.

We notice next that although in the early periods
the animal world developed faster than the vegetable
world, still we find that before the close of the geo-
logical ages the relation was reversed. The vegeta-
ble kingdom reached its culmination long before the
animal kingdom. With the Cretaceous (8), the vege-
table world had developed its highest types, the sub-
sequent history being only an elaboration of them.
But at that time the highest class of animals had not
appeared at all, for the true mammals came into
existence only in the next age (Tertiary), and very
great advance took place in them even later.

In general it remains to be noticed that with the
opening of the Silurian, the vegetable kingdom had
reached a condition where cellular differentiation was

HISTORY OF PLANTS. 1/5

found. All the powers pertaining to the cell seemed
to be fixed, for there is no reason for thinking that
the cells of early plants were not about like those of
to-day. During the Silurian also the stem and leaf
were differentiated. Sexual reproduction probably
had begun at this time, but it was in a low form, and
the formation of fruit did not appear until later.
Thus the advance has been almost wholly in refine-
ments of parts, and not in the productions of new
features.

Finally, we notice that it is impossible at present
to trace the various types in the vegetable kingdom
to a common centre. We do not find any evidence
of a rapid divergence from one central point of origin.
In the animals we have reason for thinking that all
of the great types arose early as branchings from
one simple type, the Gastraea, and that the different
sub-kingdoms did not arise from each other to any
noticeable extent. In the vegetable kingdom the
reverse seems to be nearer the truth. New types of
plants were constantly arising till the Cretaceous,
and there is every reason for thinking that they
arose from the earliest existing plants in all cases.
In other words, the history of the vegetable world
is rather to be compared to the history of a single
sub-kingdom of animals than to the history of the
whole animal kingdom. Like the vertebrates they
have had most of their development since the Silu-
rian age, although it is true that their history really
began earlier. In the fossil history of plants, there-
fore, we find just such an advance as we have seen
in the vertebrates, and not such a mixture of ad-

THE LIVING WORLD.

vance and stationary condition as we have seen in
the animal world taken as a whole. This difference
is certainly a striking one, as we take a cursory
glance at the fossil study of plants and animals as
they are known to-day. The fact may, however, be
partly due to our incomplete knowledge of the lower
orders of plants. The lowest classes of plants are
almost wholly unrepresented by fossils, and through
all the geological ages it is the higher classes which
have been most preserved and most studied. Per-
haps if our fossil record of lower plants were as com-
plete as that of the lower animals, we should find
that here too there has been divergence from com-
mon centres to a much greater extent than now
appears. But however that may be, the facts as
collected at the present time point to a history of
much more continuous progression among plants
than animals. See Fig. 21.

CHAPTER VIII.

THE FUTURE OF THE LIVING WORLD.*

SCIENCE is at all times trying to read the future
by means of the past. No one questions the right
of astronomy to make predictions, and her success
in this direction is everywhere recognized. It is,
then, certainly a legitimate question for us to ask
here whether the past history of life cannot give us
indications of the direction of the drift of the living
world, and thus enable us with something like prob-
ability to look into its future. Such predictions
cannot of course claim anything like the certainty
of the predictions of astronomy, for the complexity
of the problem is too great. At the same time, a
little study will show that there are some definite
results plainly indicated by the drift of the past, and
a clearer idea of the meaning of past history can be
obtained by trying to see in what direction it turns
our thoughts for the future.

We have just seen that a fair idea of the life of the
world can be obtained by comparing it to a giant

* The ideas advanced in this chapter were first published in the
American Naturalist, 1886, from which part of the following pages
are quoted.

ii , 177

1/8 THE LIVING WORLD.

tree. In this comparison we have seen that the tree
is to be regarded as an old one, all of whose branches
show by their shattered condition, the effects of the
storms of the ages. Comparatively few branches
remain alive, while a larger number have either dis-
appeared or become reduced to a few still vigorous
shoots. The highest branch alone appears to be in
its primal vigor, still rapidly growing and expanding,
and this because of the influence of a new life prin-
ciple, perhaps engrafted into the old tree.

Now if such a comparison is a correct one, it
is evident that the tree must be looked upon as
being near its death. Of course, however, it is pos-
sible to question the correctness of this comparison,
and we must therefore ask whether there are really
any grounds for believing that the life of the world
has passed its prime, that while man is the crowding
creation, his appearance indicates the decline of the
living world as a whole.

Development More Rapid in Early Ages.

In order to answer this question in the affirmative,
it will only be necessary to refer to some of the facts
already noticed regarding the previous ages. First
we may notice again the significant fact that the
development of the animal kingdom seems to have
been more rapid in the earliest times than it has
been in subsequent ages. The diversity of the Silu-
rian (2) fauna has already been noticed, and this, of
course, means that a large part of the evolution of
type had occurred previous to the Silurian age. All

THE FUTURE OF THE LIVING WORLD. Ijg

subsequent development has been only elaboration
and not the production of new types.

Now we are not at liberty to assume an indefinite
amount of time prior to the Silurian. Of course it is
impossible to say just how long a time elapsed
between the origin of life and the beginning of the
Silurian, but it seems hardly possible that it could
have equalled the time that has elapsed since then.
But, upon evolutionary theories, the animal kingdom
must have developed during that period from the
lowest unicellular condition to the complex and
diversified fauna of the Silurian. When we consider,
therefore, that during this time all of the important
groups of the animal kingdom arose (with perhaps
the exception of the vertebrates) and that none have
arisen since that time, it becomes quite evident that
evolution must have progressed with greater rapidity
at that time than it has since. This conclusion is
no new one, for many naturalists have seen the
necessity of making some such assumption. It will,
indeed, be generally acknowledged that evolution in
the earliest ages was more rapid than at present.

Here, then, we see another point of likeness in the
comparison of the living world with the life of the
individual. An individual when it begins life grows
most rapidly, but from the very moment of its birth
the rapidity of growth lessens until a stationary
condition is reached at maturity. So it seems in
the longer history of world life. Its growth was
most rapid at the birth of life and has been decreas-
ing since that time, not with regularity, perhaps,
being frequently interrupted by periods of more

180 THE LIVING WORLD.

rapid expansion, yet nevertheless on the whole
decreasing. If this is so, it plainly implies an end
to the process.

The Organic World Approaching a Limit.

That the organic world is approaching a limit in
its development is a conclusion which does not, how-
ever, depend upon any vague idea of growth for its
support ; for a little thought upon discovered laws
will clearly show us that there must be a limit to
advance. The best definition which has ever been
given to the grade of structure of animals and plants
is the degree to which differentiation of organs is
carried. Evolution as it tends to raise the grade of
animals is constantly increasing the amount of differ-
entiation and specialization. We have already seen
that such differentiation and specialization becomes
self-limited. A low undifferentiated and unspecial-
ized organism has an indefinite possibility in its
lines of specialization. A simple spherical cup of
cells, the supposed common ancestor of the animal
kingdom, may be modified in a very great variety of
directions, each of which may give rise to a different
type of animal. This possibility lies in the fact that
it is as yet undifferentiated and unspecialized. But
just as soon as it does become modified in any one
direction the possibilities decrease. Some of the
descendants of this ancestor becoming vertebrates
are forever precluded from becoming anything else ;
others becoming mollusks must remain mollusks
forever, with all of their descendants. And as later
descendants become further modified in any direc-

THE FUTURE OF THE LIVING WORLD. l8l

tion into definite types the chance for future modifi-
cation becomes rapidly less. It is only the abso-
lutely undifferentiated which has infinite possibilities,
for as soon as a single step is taken in any direction,
the possibilities become finite. Now it is plain that
this continued specialization cannot go on forever.
Since evolution does not retrace its steps, every step
in advance limits the possible lines of development.
All the descendants of the vertebrate line must
conform jto the vertebrate type. The vertebrates
become separated into fish, reptile, and mammal, and
each group is still further fettered in its develop-
ment by the special line which its ancestors have
taken. The descendants of the animals which have
started the order of birds cannot take any new line.
They can develop this type to perfection, or they
may lose their special characters, but there they must
stop. And thus, with every step in advance, the
possibilities of expansion are constantly decreasing.

Now a continued specialization of this sort is sure
to reach a limit ; it must run to extremes and event-
ually stop. Physical laws will of themselves set
limits to every line of advance, even if there be no
such limits determined by the organism itself. It is
easy to find examples which will show that such has
been the general history of groups in the past. Some
have reached the extreme of their development in
the distant past, and have ceased to advance, or,
perhaps, have disappeared. Others seem even now
to be at the summit of their advance, and others
still are yet advancing. The line of development
represented by the trilobites has completely ex-

1 82 THE LIVING WORLD.

hausted itself. It rapidly approached its limits even
in the Silurian (2), and then began to dwindle away,
and has disappeared entirely. The brachiopods had
also at this time reached their point of highest
specialization, and had become a highly developed
group even at this early age. Since then they have
remained stationary as to their organization, having
steadily decreased in numbers, and the few that are
left show no advance over the Silurian forms. The
cephalopod mollusks gradually increased in com-
plexity during the Paleozoic (2-4) ; and finally a
limit of the shelled forms was reached in the ammon-
ites of the Jurassic (7) and Cretaceous (8). The
culmination was followed by extinction. Meantime
a second line of development began, that of the
naked cephalopods (squids, cuttle fishes), and this
has gone on advancing until the present time. The
decapod Crustacea represent a group which is even
now near its culmination. From their first appear-
ance in the Carboniferous (4) there has been a tend-
ency to a concentration of organs toward the head.
As this specialization advanced, the abdomen be-
came smaller, while the head region became larger.
Finally, in the crabs, everything was concentrated
in the head region. The abdomen remained as little
more than rudiment. Evidently we are here near a
limit, and we may look upon the crabs of to-day as
the culmination of the special line of development
which has characterized this line of animals. The
vertebrates in general have been continually advanc-
ing during geological times, with a continued increase
in specialization and in multitude of types. But even

THE FUTURE OF THE LIVING WORLD. 183

here there has been the same story of limitation.
The ganoids culminated in the Devonian (3), and
have advanced no farther. One great line of reptiles
reached its limit in the Jurassic (7). And so every-
where. The study of every group teaches that the
past history has been a gradual specialization which
approaches a limit. In many cases in the past this
limit has been reached, and advance has ceased ; in
others, the animals are still on the road toward it.

It is plain, then, that unless there is some way for
new lines of specialization to arise, the evolution of
the animal world is inevitably approaching its end.
With every advance in differentiation, the possible
lines of development decrease, and since the actual
lines followed are tending to run themselves out, the
whole must eventually stop. Is it, however, possible
that new lines of differentiation may arise, and thus
the development of the living world go on in-
definitely?

New Lines of Specialization Not Now Appearing.

First, we must notice that the development of the
vegetable kingdom practically ceased long ago. As
already noticed, the Cretaceous (8) rocks show us
representatives of the highest orders of plants. At
that time the plants in existence do not seem to have
differed materially from those of to-day, since many
species are identical so far as can be determined.
The vegetable kingdom thus practically reached its
culmination at this time ; for although many new
species have appeared since then, there has been no
advance in type. The time since then has been long

1 84 THE LIVING WORLD.

enough certainly for a great amount of change, if
the limit had not been practically reached. It has
been sufficient for the entire development of the
class of mammals, with its great profusion ; but the
plant world has remained nearly stationary, and this
suggests to us that we may look for no farther de-
velopment along this line.

With the animal kingdom, however, it is different.
Even until the present era we can see that develop-
ment of new and higher forms has continued. Is
there any indication that it has reached its end ?

That there is a theoretical possibility of the origin
of new types cannot be denied. New types, i. e., new
lines for specialization, can arise only from undiffer-
entiated forms. But such undifferentiated forms
still exist in great numbers. Even the most un-
specialized forms of all, the unicellular animals, are
abundant enough, and in all groups we are acquainted
with more or less generalized types. Theoretically,
then, there is no reason wrhy any one of these forms
should not expand itself, and thus form an eternal
source of new world forms. So long as the un-
specialized forms do not become extinct, we cannot
deny the possibility of an infinite number of future
sub-kingdoms, which would of course make the
animal kingdom an example of never-ending evolu-
tion. But all of our evidence indicates that such
a future is probably not a practical possibility,
even though, so far as we can see, it may be
a theoretical one. All biological studies point
strongly to the conclusion that, instead of several
points of origin the animal kingdom has had only

THE FUTURE OF THE LIVING WORLD. 185

one. The sub-kingdoms have not arisen independ-
ently from the Protozoa, but have all had a common
ancestor, the Gastnea, and this means that only once
has the unicellular form given rise to an important
line of multicellular descendants which perpetuated
itself. Though the Coelentera stand very near this
primitive Gastraea, there is no evidence that the
sub-kingdom has the power of further production of
new types ; but, on the contrary, everything tends
to show that, whatsoever differentiation of this
simple type ever did take place to give rise to the
sub-kingdoms, occurred before the Silurian. Since
paleontology shows that no great types have arisen
since the Silurian, it is plain that all of the expansion
of the simple unicellular form must have taken place
before the Silurian. And coming through the later
ages, we find that the evidence is the same in its
tenor. The conclusion everywhere seems to be that
when a generalized form has given rise to one or two
lines of development, it either disappears or loses its
power to originate new forms. Every bit of evi-
dence which indicates a fundamental unity of the
animal kingdom testifies to the same. Without
questioning the theoretical possibility that any or
all of the existing unspecialized forms may in the
future develop, we must acknowledge that the
probability is against it. Nothing in history in-
dicates that these groups retain the power to ex-
pand, and there is, therefore, no reason for thinking
it a possibility in the future. Remembering what a
large number of groups we are learning to trace
back to the Silurian, remembering that development

1 86 THE LIVING WORLD.

in the later geological ages has consisted simply
in the expansion of groups appearing long before,
we must conclude that the power of the undifferen-
tiated forms to expand into different lines of devel-
opment disappears very early in their history.
While then we cannot deny the possibility of an
indefinite future development from the existing
generalized types, it is certainly improbable that
any new great groups will arise. Man, seizing upon
the last undifferentiated faculty, the intellect, is
developing this to the extreme. With him, the
animal world proper has ended and the intellectual
world begun.

If there is anything further needed to convince us
that the evolution of animals ceases with man, we
have only to notice his influence upon the rest
of the animal kingdom. We cannot yet compute
that influence, but it will doubtless be the death-
blow to the evolution of animals. Man is rapidly
causing the extinction of almost all land animals, at
least the larger ones. As the frontiers of civilization
are being extended farther and farther into the
uninhabited regions, he is driving out of existence
all of the large animals and many of the smaller
ones. We have only to look ahead a comparatively
short time to see the extinction of nearly all land
animals, except such as may strike man's fancy to
use or preserve. To what extent this may apply to
other animals — to insects, marine animals, etc. — is
not clear. But in the highest group of animals, the
vertebrates, it is pretty clear that man is eventually
to bring about not only the end of advance, but also

THE FUTURE OF THE LIVING WORLD. l8/

the practical extermination of all animals except such
as he especially preserves.

His mastery over the higher vertebrates is so un-
bounded that he is the only one who has any possi-
bility of farther advance. With the lower animals,
his competition is not so severe, but, as we have
seen, these lower types have practically ended their
evolution in the distant past.

To man alone, then, are open further possibilities
of higher development, and his development must
be wholly mental. His unique position in nature
comes upon us now with double significance. Even
more forcibly than ever do we see the significance of
the new law of love to which he is trying to adapt
his life. Remembering how lines of specialization
exhaust themselves, and how divergence of character
ends in extremes, we begin to get a grand conception
of the law of love which binds mankind into unity,
thus checking the development of castes and forcing
him to advance as a whole. The law of love is the
only law which could produce the highest mental
and moral development.

REFERENCES.

A long list of references would be inappropriate in this work.
The following short list may be of value to those wishing further
reading upon the topics treated in the foregoing pages :

PROTOPLASM AND CELLS.

BERTHOLD. " Studien ttber Protoplasma mechanik," Leipzig, 1887.

NELSON, JULIUS. "Heredity and Sex," American Naturalist, 1887.

PASTEUR, L. " Studies on Fermentation," Macmillan & Co., 1879.

SCHWARZ. " Morphological and Chemical Composition of Proto-
plasm," Zeitr. z. BioL d. Pflanzen, v. 1887.

WEISSMAN. " Essays upon Heredity and Kindred Biological
Problems," Clarendon Press, 1889.

WHITMAN, C. O. " Seat of Formative and Regenerative Energy,"
American Jour, of Morphology, 1 888.

LIFE.

BASTIAN, CH. " The Beginnings of Life," D. Appleton & Co., 1872.
" The Modes of Origin of Lowest Organisms," Macmillan
& Co., 1871.

BEALE, SIR LIONEL. "The Mystery of Life," J. & A. Churchill,
1871. "Protoplasm; or, Matter and Life," Lindsay & Blakis-
ton, 1874.

DRYSDALE. " Protoplasmic Theory of Life," Balliere, Tyndall & Co.

HUXLEY, TH. " The Physical Basis of Life," Humboldt Library;
originally delivered as a lecture in 1868. " Anatomy of Inver-
tebrates," D. Appleton & Co., 1878. Article on Biology
in Encyclo. Britannica. "Animal Automatism," Fortnightly
Review, 1874.

189

1 90 REFERENCE S.

MIXOT, C. S. "On the Conditions to be Filled by a Theory of

Life," Proc. of Am. Association, 1879.

MORRIS, C. H. "Organic Physics," American Naturalist, 1882.
DuBois REYMOND. " Seven World Problems," Pop. Set. Monthly,

1882.
TYNDALL, J. " Floating Matter in the Air," D. Appleton & Co.,

1882.
WARD, LESTER. " Organic Compounds in their Relations to Life,"

American Naturalist, 1882.

EARLY HISTORY OF ANIMALS.

BALFOUR, F. M. "A Treatise on Comparative Embryology,"
Macmillan & Co., 1880.

BUTSCHLI. " Bemerkungen zur Gastraea Theorie," Morph. Jahr.,
1884.

CONN, H. W. " Marine Larvae and their Relation to Adults,"
Biol. Studies of J. H. U. iii., 1883. " A Suggestion from
Modern Embryology," Science, 1885.

HAECKEL, E. " Die Gastraea Theorie," Jena Zeitschrift, 1874-5.

HYATT, A. "Genesis of the Arietidae," Smithsonian Contributions
to Knowledge, 1889. " Larval Theory of the Origin of Cellular
Tissue," Proc. of Bost. Soc. Nat. Hist., 1884.

LANKESTER, E. R. " Notes on Embryology and Classification of
the Animal Kingdom," Quar. Jour. Mic. Set., 1877.

McMuRRiCH. " The Gastnea Theory and its Successors," Biologi-
cal Lectures, Ginn & Co., 1890.

MITSCHNIKOF. " Embryologische Studien an Medusen," Wien,
1886.

SEDGWICK, A. "Origin of Metameric Segmentation," Quar. Jour.
Mic. Soc., 1884.

WILSON, E. B. "Some Problems of Annelid Morphology," Bio-
logical Lectures, 1890, Ginn & Co.

GEOLOGICAL HISTORY.

COPE, E. D. " Origin of the Fittest," D. Appleton & Co., 1887.
DANA, R. " New Text-book of Geology," Ivison, Blakeman, Tay-
lor, & Co., 1883.

REFERENCES. 191

GEIKIE, A. " Text-Book of Geology," Macmillan & Co., 1885.
LECONTE. " Elements of Geology," D. Appleton & Co., 1882.
LYELL. " Principles of Geology," D. Appleton & Co., 1872.
NICHOLSON and LYDECKER. "A Manual of Paleontology," Wm.
Blackwood & Sons, 1889.

PLANTS.

DAWSON. " The Geological History of Plants," D. Appleton & Co.
NICHOLSON and LYDECKER. " Manual of Paleontology," vol. ii.
SACHS. " Text-book of Botany," Clarendon Press, 1882.

MISCELLANEOUS.

CLAUS and SEDGWICK. " Elementary Text-book of Zoology," Mac-
millan & Co., 1884.

CONN, H. W. "Natural Selection and Christianity," Methodist
Review, 1891.

DARWIN, C. " Origin of Species," "Descent of Man," D. Apple-
ton & Co.

FISKE, J. " The Destiny of Man," Houghton, Mifflin, & Co., 1885.
" Outlines of Cosmic Philosophy," J. R. Osgood & Co., 1875.

HUXLEY, TH. " Evidence as to Man's Place in Nature," New York,
1863.

LUBBOCK, J. " Pre-Historic Times," London, 1869.

MIVART, ST. GEO. " Genesis of Species," Macmillan & Co., 1871.
" Lessons from Nature," D. Appleton & Co., 1876.

ROMANES, G. "Mental Evolution in Animals," 1884 ; "Mental
Evolution in Man," 1889, D. Appleton & Co.

REED. " Evolution versus Involution," J. Pott & Co., 1885.

SPENCER, H. " Principles of Biology," D. Appleton £ Co.

TYLOR. " Primitive Culture," John Murray, London, 1871.

WALLACE, A. "On Natural Selection," Macmillan & Co., 1870.
" Darwinism," Macmillan & Co., 1889.

INDEX.

Advance of type, 136

Algae, 170

Amphibia, no

Anatomy, 10

Animals and plants, divergence of,

67, 45

Archean age, 90
Articulata, 107
Assimilation, 23

Beginnings of fossiliferous history,

93

Birds, history of, 1 12

Branching arrangement of organ-
isms, 66, 98

Breaks in continuity, 48, 50

Carbon compounds, 42
Carboniferous age, 91, 115
Cells, 59, 60
Cenozoic, 92, 114
Change in species, 116
Chemistry of organic compounds,

19

Chlorophyll, 17, 37
Christianity, law of, 155
Ccelentera, history of, 100

Convergence of character, 152
Cretaceous age, 91
Cycads, 172

Death, 32

Degeneration, 139

Devonian age, 91

Difference between dead and living

organism, 25
Differentiation, 81
Disappearance of types, 121
Divergence, of character, 150 ; of

types, 77, 80 ; prior to Silurian,

95-97

Diversity of modern fauna, 138
Division of cells, 65
Dominant types, 159

Early divergence of types, 80, 83
Echinodermata, history of, 101
Embryology, 6 ; falsification of,

9 ; imperfection of, 7
Endogens, 172
Ethical nature, 151
Evolution, 47 ; limits of, 180
Exogens, 172
Expansion of types, 132

193

194

Ferns, 171

Fishes, history of, no

Fossils, value of, 3

Gastraea, 70 ; formation of, 76 ;

modification of, 78 ; significance

of, 72

Gastrula, typical, 72
Geological ages, 88
C Granules, 54
Growth, 23
Gymnosperms, 171

Haeckel, 72

Hard parts, development of, 142

Hydra, figure of, 74

Influence of man on animals, 186
Insects, 147 ; history of, 108
Instinct, 146
Intelligence, 147
Intermediate types, 122

Jurassic age, 91

Law of continuity, 14, 47
Life, essential characters of, 17
Limitation of differentiation, 82
Limit of organic development, 180
Love, among animals, 152 ; among

men, 153

Low forms of life, 13
Lycopods, 171

Mammals, expension of, 134 ; his-
tory of, 112

Man, 147 ; capability of develop-
ment, 187 ; classification of, 150

Mechanical, origin of life, 40 ;
theory of life, 30

Mental factor, 145

Mesozoic, 91, 114
Metamorphosis, 5
Modern fauna, paucity of, 162 ;

flora, 172

Mollusca, history of, 104
Molluscoidea, history of, 103
Multicellular animals, origin of,

67

New fields for expansion, 132 ;

lines of specialization, 183
Nucleus, 61, 64

Ocean as starting-point of life, 55
i Organism as a machine, 26
\ Origin of life, 33

I Paleozoic, 90, 114
Permian age, 91
Persistent species, 118
Physical basis of life, 51
Physiological chemistry, 12
Plants and animals, 45 ; history

compared, 184
Plants, as fossils, 170 ; history of,

ID?, 173

Properties of compounds, 31
Protophyta, 62
Protoplasm, 43, 51 ; chemistry of,

52, 64 ; structure of, 52
Protozoa, 62 ; history of, 99

Quaternary age, 92

Rapidity of early development, 81,

178

Redi, 35

Reproduction, 24
Reptiles, history of, no
Rhizocarps, 171

INDEX.

195

Silurian, age, 90 ; life of, 94
Size, increase in, 144
Skeleton, development of, 142
Soft-bodied animals, 141
Specialization, new lines of, 183
Speculations on life, value of, 46
Spontaneous generation, n, 35

Terrestrial life, rarity of, 160
Tertiary, 92, 116

Tree-like arrangement of organ-
isms, 164

Triassic age, 91
Tyndall, 36

j Types, of fossil and living animals,
1 20 ; history of, 123

Unicellular life, 59, 64

! Vermes, history of, 107
Vertebrata, history of, no
Vital force, 29 ; and physical en-
ergy, 17

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