just made. It is this: we do not find _peculiar_ kinds of atoms which
occur exclusively in living matter; the materials are exactly the same as
those of the outer world. In short, the elements of both the organic and
inorganic divisions of the universe prove to be the same. Carbon is
carbon, whether it is part of the substance of a living brain cell, or
black inert coal, or the glistening diamond, or an incandescent part of
the fiery sun. Hydrogen is the same, whether it be a constituent of the
ocean, of the air, or of the living muscle fiber. And so it is with all of
the other elements of the living mechanism. This starts us upon a line of
thought which leads to a significant conclusion, namely, that a living
thing which seems so distinct and permanent is after all only a temporary
aggregate of elements which come to it from the not-living world; existing
for a time in peculiar combinations which render life possible, they pass
incessantly away from the living thing and return to the inorganic world.
Every breath we draw sends out particles which were at one time living
portions of ourselves; every movement we make involves the destruction of
living muscle cells, whose protoplasm breaks down into the ash and gas and
fluid wastes which eventually return to the world of dead things. A tree
loses its living leaves with each recurring season, and the antlers of the
stag are lost annually, to be replaced anew. Indeed the major part of some
organisms is itself actually dead. The bones and hair and nails of such an
animal as a cat are almost entirely lifeless, even though they are
integral and necessary portions of the organism as a whole. They are
constructed by living protoplasm which has died in their making. Thus
without going beyond the boundaries of the individual body, these
substances have passed from the sphere of life, and are dead. The apparent
gap on the other side between the lifeless and living world is equally
imaginary, for our living substance is continually replenished and rebuilt
from the elements of our dead foods. So, as Huxley says, a living organism
is like a flame or a whirlpool, which is an ever changing though seemingly
constant individuality. We look at a gas flame, and we see in the flame
itself those particles of gas which have come through the pipe to be
agitated violently in the higher temperature of the flame as they are
oxidized or burnt. These particles immediately pass off as carbonic acid
gas and water vapor which are no longer parts of the flame. A fountain is
continually replenished by the water which is not-fountain, but which
becomes for the time a part of the graceful jet, falling out and away as
it leaves the fountain itself. Just so a living organism is an ever
changing, ever renewed, and ever destroyed mass of little particles--the
atoms of the inorganic world which combine and come to life for a time,
but which return inevitably to the world of lifeless things. This is one
of the most fundamental facts of biology. The independence of a living
thing like a human being or a crustacean is a product of the imagination.
How can we be independent of the environment when we are interlocked in so
many ways with inorganic nature? Our very substance with its energies has
been wrested from the environment; and as we, like all other living
things, must replenish our tissues as we wear out in the very act of
living, we cannot cease to maintain the closest possible relations with
the environment without surrendering our existence in the battle of life.
From the foregoing discussion, it will be evident, I am sure, that there
is ample justification for the biological dictum that a living individual
is a mechanism. Not only is the organism composed always of cell units
grouped mechanically in tissues and organs and organic systems; not only
are the operations which make up its life constant and regular under
similar conditions; not only is the whole creature mechanically connected
with the inorganic world; but above all the whole activity of a biological
individual is concerned necessarily and again mechanically with the
acquisition of materials endowed with energy, which materials and energy
are mechanically transformed into living matter and its life. Even though
an organism is so much more complex than a locomotive, and so plastic,
nevertheless, in so far as both are mechanisms, the conception of the
evolution of the former may be much more readily understood through a
knowledge of the historical transformation of the latter.
* * * * *
What, now, is life? To most people "life seems to be something which
enters into a combination of carbon and hydrogen and the other elements,
and makes this complex substance, the protoplasm, perform its various
activities." Nearly every one finds it difficult to regard life and
vitality as anything but actuating principles that exist apart from the
materials into which they enter, and which they seem to make alive.
According to this general conception, "life is something like an engineer
who climbs into the cab of the locomotive and pulls the levers which make
it go," as health might supposedly be regarded as something that does not
inhere in well-being, but gets into the body to alter it. But is this
conception really justified by the facts of animal structure and
physiology? Let us recall the steps of our analysis. The living organism
is a collection of differentiated parts, the organs; the life of an
organism is a series of activities of the several organic systems and
organs. If we could take away one organ after another, there would be
nothing left after the last part had been subtracted. In a similar manner,
the activities of organs prove to be the combined activities of the
tissue-cells, and again the truth of this statement will be clear when we
imagine the result of taking away one cell after another from organisms
like the frog or tree. When the last cell had been withdrawn, there would
be nothing left of the frog's structure, and there would be no element of
the frog's life. It is true that the particular way the tissue-cells are
combined is of primary importance, but it is none the less true that the
life of a cell is the kind of element out of which the life of even the
most complex organism is built. And we have seen that the essential
substance of a cell is a complex chemical compound we call protoplasm,
whose elements are identical with chemical substances outside the living
world. Is there any ground for supposing that the properties of protoplasm
are due to any other causes than those which may be found in the chemical
and physical constitution of protoplasm? In brief, is life physics and
chemistry? Nowadays the majority of biologists believe that it is. Just as
the properties of water are contributed by the elements hydrogen and
oxygen which unite to form it, just so the marvelous properties of
protoplasm are regarded as the inevitable derivatives of the combined
properties of the various chemical elements which constitute protoplasm.
Biologists have known for more than a century, since the work of Lavoisier
and Laplace in 1780, that the fundamental process of the living mechanism
is oxidation, and that this process is the same, as they said, for the
burning candle and the guinea pig. Beginning with Woehler, in 1828, scores
of students of physiological chemistry have duplicated the chemical
processes of living matter, which were regarded as so peculiar to the
living organism that they seemed to be due to the operation of a
non-mechanical and vital cause. The investigator mentioned was the first to
construct artificially from inorganic substances the nitrogen-containing
ash product of the living organism called urea. Now hundreds of so-called
organic compounds have been made synthetically and their number is added
to week after week. Therefore, the biologist who finds that a physical and
chemical analysis of some vital processes is possible, and that the
analysis is being extended with astonishing rapidity, finds himself unable
to regard protoplasmic activity as anything different in kind or category
from the processes of physics and chemistry which go on in the world of
dead things.
It is true that even at the present time some biologists are reluctant to
accept the thoroughgoing mechanical interpretation of organic phenomena,
partly because these are so complex that their ultimate constituents
cannot be discerned, but more often on account of the apparently
purposeful nature of biological processes. Some, indeed, have gone so far
as to postulate something like consciousness which controls and directs
the formation of protoplasm, and the exercise of its distinctive
properties in the way of growth, reproduction, and embryonic development
into the adapted adult. But the fact remains that wherever analysis has
been possible the constituent elements of an organic process prove to be
physical and chemical. Protoplasm differs from inorganic materials only in
its complexity and in the properties which seem to owe their existence to
this complexity. As Huxley points out, it is no more justifiable to
postulate the existence of a vitalistic principle in protoplasm than it
would be to set up an "aquosity" to account for the properties of water,
or a "saltness" for the qualities of a certain combination of sodium and
chlorine. We may not know how the elements produce the properties of the
compound, but we do know that such properties are the invariable products
of their respective constituents in combination. As far as the evidence
goes, it tells strongly and invariably in favor of the mechanistic
interpretation.
Under the present limitations, it is impossible to give this subject the
further discussion it deserves. It is not our purpose to review the origin
of life in times past, and the origin of living matter from inorganic
constituents, though the subject is one of the most important in the field
of cosmic evolution. We must begin with the living organism; and how the
first one arose must be of less importance to us than the knowledge of its
mechanical constitution and of its mechanical operation. Of far greater
value is the realization that a living creature is not an independent
thing, but that, on the contrary, it must hold the closest possible
relations with the world of materials and energies constituting its
environment. We must again insist upon the importance of that mechanical
adjustment to the conditions of life which is the universal characteristic
of plants and animals. It is the history of these creatures and the origin
of their adapted conditions that we are called upon to study. We must
scrutinize the nature of to-day to see if we can find evidence that
evolution is true, and if we can discern the forces which, acting upon the
living mechanism as man has dealt with machines, might bring the various
species of the present day to their modern forms.
* * * * *
We have now learned that evolution means a common ancestry of living forms
that have come to differ in the course of time; our common reason has
shown us also that organisms are in a true sense complicated chemical
mechanisms adapted to meet the conditions under which they must operate.
We come now to the evidences offered by the organic world that evolution
is true and that natural forces control its workings. Clearly the
examination of the matter of _fact_ is independent of the question of
_method_. For just as the chemist may experiment with various substances
to see if they will dissolve in water and not in alcohol before it is
necessary or desirable for him to take up the further studies of the laws
of solution, so reasonable grounds must be found for regarding evolution
as true before passing to its method of accomplishment. And in the
following discussions, the animals will be used almost exclusively, not
because the study of plants fails to discover the same relations and
principles, but because the better known animal series is more varied and
extensive, and above all for the reason that the human organism arrays
itself as the highest term of the animal series.
In the complete scheme adopted by most naturalists, five categories
include the evidences bearing upon the fact of evolution. These are
_Classification_; Comparative Anatomy, or _Morphology_; Comparative
Development, or _Embryology; _Palæontology_, which comprises the facts
provided by fossil relics of animals and plants of earlier geological
ages; and _Geographical Distribution_. Each of these divisions includes a
descriptive and analytical series of facts, whose characteristics are
"explained" or summarized in the form of the general principles of the
respective divisions. Such principles, taken singly and collectively,
constitute the evidences of evolution.
The particular nature of any one of these categories, evolved in the
development of science practically in the order stated, depends upon the
special quality of an animal which it selects for comparison and
organization in connection with other similar facts, and also in its own
mode of viewing its facts. One and the same organism may present materials
for two, three, or even all five of these divisions, for they are by no
means mutually exclusive. For example, a common cat possesses certain
definite characteristics which give it a particular place when animals
more or less like it are grouped or classified according to their degrees
of resemblance and difference, in small _genera_ of very similar forms, in
larger _tribes_ or _orders_ of similar genera, and in more and more
inclusive groups of these lesser divisions, such as the _classes_ and
_phyla_, or main branches of the animal tree. The common cat and its
relatives are even earlier to be regarded as anatomical subjects, and
their thorough analysis belongs to comparative anatomy,--a name which
explains itself. The purpose of this department of natural history is to
explore the entire range of animal forms and animal structures, and to
determine the degree of resemblance and difference exhibited by the
general characters of entire organisms and by the special qualities of
their several systems of organs. It provides the data from which
classification selects those which indicate mutual affinities with
greatest precision and surety. But its materials are _all_ the facts of
animal structure, and because each and every known organism can be and
must be studied, the investigator engaged in formulating the evidence of
evolution has at his disposal all the data referring to the entire realm
of animals. The data of embryology are likewise coextensive with the
territory of the animal world, for we do not know of any form which does
not change in the course of its life history. An adult cat is the product
of a kitten which is itself the result of a long series of changes from
earlier and simpler conditions. In so far as it deals with structures in
the making, embryology is a study of anatomy, but as it is concerned
primarily with all of the plastic remodeling which animals undergo during
the production of their final forms, it is an independent study.
Nevertheless we shall learn how intimate are the relations of these two
divisions of zoölogy and how the evolutionary teachings of each body of
fact support and supplement those of the other.
Palæontology searches everywhere among the deposits of earlier ages for
links to be fitted into their proper sequence of time, from which it
constructs the chain of diverse types leading down to the species of the
present. A cat of to-day is therefore viewed in an entirely different
connection, as the last term in a consecutive series of species. Forming
alliances with geology, and even with physics and chemistry, this
department of zoölogy endeavors to reconstruct the past from what it
learns to-day about organisms and the conditions under which they live.
Finally the observations that cats of various kinds do not occur
everywhere in the world, but only in certain more or less restricted
localities, belong to the subject of geographical distribution, and
illustrate its nature.
Our task is to learn the teachings of these several divisions by recalling
and putting together what we know already about the commonest animals, or
noting what can be observed in a visit to a zoölogical garden and
aquarium. On account of the present limitations of time, the subject of
classification will be combined with comparative anatomy; embryology will
be taken up together with these subjects; palæontology will be the main
subject of the next discussion, which will include also a brief statement
of the meaning of distribution. Then we will be prepared to study nature
to see how evolution works.
II
THE STRUCTURE AND DEVELOPMENT OF ANIMALS AS EVIDENCE OF EVOLUTION
In order to become acquainted with the way the structures of animals
provide evidences of evolution, it is by no means necessary to review the
entire range of their forms, because research has discovered that the
principles of relationship are universal among animals, and that any group
of examples will demonstrate what is taught by comparative anatomy as a
whole. The commonest creatures may serve us best in order that we may come
to view evolution as a process that involves each and every living thing
that we know, and not as something which belongs only to the remote and
unknown past.
Let us begin with the common cat and the group of carnivora or
flesh-eating animals to which it belongs. As we pass along the streets of
the city, we will see many cats which differ in some details, though they
resemble one another closely. While they vary somewhat in form, the range
in this quality is not so noticeable as in the matter of color; some of
them will be gray, some maltese, while others will be yellowish or black,
and they will differ in the striped or spotted character of their
coloration. We readily classify them all as "cats" in spite of their
differences, because they are alike in so many ways that we have learned
to associate as the distinguishing characteristics of these animals, and
to label--"cat." The animals which we might see in a walk of several
blocks may reasonably be regarded as offspring of the same pair of
ancestors of a few years back, even though they are dissimilar. We all
know that the kittens of one and the same litter vary: no two of them are
ever exactly alike in color or disposition or voice or size, nor is any
one identical with either of its parents, although it may be necessary to
employ exact means of measuring them in order to demonstrate their
variation. The fact of difference, then, is surely not inconsistent with
even the closest ties of blood, and we do not need to go beyond the scope
of daily observation to find that this is true in nature wherever we look.
Should we extend our observations so as to include the cats of Boston and
Philadelphia and San Francisco, the animals would probably vary over a
wider range, but they would be so similar to New York cats in their
make-up that we would have no difficulty in regarding them and all the
others of the United States as the descendants of a single pairs of
ancestors, perhaps brought over in the "Mayflower." But why does this view
seem justified? Because experience has taught us that the living things
which resemble each other most closely are those which are most intimately
bound by ties of blood and common heritage. It is "natural" for relatives
to resemble one another more than persons not related, and for brothers and
sisters to be more alike than cousins. Science does not refer to something
outside everyday observation when it states that _the possession by two
animals of a great body of similar characters beneath their minor
differences is an indication of their common ancestry_.
Thus at the very outset our simple illustration establishes the most
fundamental principle of comparative anatomy. Let us see how it works
further. The Manx cat possesses an abbreviated tail, although in other
respects it is practically the same as the familiar long-tailed form; the
Angora and the Persian differ in having long hair. All of these animals
are so much alike in so many respects, and so closely resemble the wild
cats, that it is not unreasonable to regard them all as the descendants of
the same original wild ancestors, and as the varying products of lines
which branched out from the same stock in different directions and at
different times. It is, in a word, their "cat-_ness_" which demonstrates
their relationships. But common sense need not stop here. Guided by the
facts of anatomical similarity, it convinces us that the dun-colored lion
and puma, the striped tiger and the spotted leopard are simply cats of a
larger growth whose remoter ancestry is one with that of the previously
cited forms. Not until we explore and compare their several systems do we
see how thoroughgoing is their uniformity in structural plan. And because
reason justifies the view regarding the origin of domestic cats from wild
ancestors, the evolution of all the various members of the cat tribe must
be acknowledged. These animals exhibit a fundamental likeness, which, to
employ a musical analogy, is the "theme" of "cat-_ness_," and they are so
many variations of this theme.
The members of another tribe of the familiar carnivora display in their
own way the same kind of evidences of relationship. The varieties of
domesticated dogs differ far more widely among themselves than do common
cats, yet their community of ancestry is demonstrated not only by
structural resemblances, but also by the striking fact that forms as
diverse as the greyhound and the fox terrier can be crossed. Here again
there are wild forms, like the wolf and fox and jackal, so like the
domesticated members of the dog tribe that we cannot fail to recognize a
common "dog-_ness_" and its significance as evidence of the relationship
in ancestry of all these animals.
Extending our survey so as to include the other tribes of flesh-eaters,
identical principles come to light. One is compelled to regard the polar
and grizzly bears as obvious blood relatives of the brown bear, and even
of the raccoon of our own territory. Instead of walking upon their toes
like cats and dogs, these animals plant their feet flat upon the ground;
and they agree in many other details of structure that place them
together, but somewhat apart from the other tribes. The many kinds of
seals and walruses and sea elephants form still another group displaying
similar bodily characters, but differing more widely from the "cat theme"
in these differences. They are all true carnivora, but in the course of
their evolution they have progressively changed so as to be adapted to
life in the water where they find their prey. The bones of the limbs are
the same in number and arrangement as in the cat's limb, but the seal's
anterior appendage or "arm" has altered in numerous ways so as to become
an efficient flexible paddle, while the hind limbs have shifted
posteriorly, very much as screw propellers have evolved in the history of
steam vessels. How the members of the seal tribe have changed in their
descent from purely terrestrial ancestors is partly explained by such
intermediate animals as the otter. This form is adapted by its slender
body and partly webbed feet to a semi-aquatic life; it seems to have
halted at a point beyond which all of the seals have passed in their
evolution.
Each one of these tribes by itself provides conclusive evidence of
evolution, for it is most reasonable to regard the "theme" in every case
as a product of common inheritance, while the variations of any theme are
best understood as the results of adaptive changes in various directions.
But the examples have disclosed a larger relation and a principle of wider
scope, as indeed the assignment of all these tribes to the single natural
group of the _carnivora_ implies. These tribes are put together because
comparative anatomy finds that the common characters of all cats are
fundamentally like those of all dogs and bears and seals, and in these
common qualities the carnivora differ from all other mammalia. Does this
mean that the branches which bear respectively the various members of the
several tribes are outgrowths of a single limb of the evolving animal
tree? Science does not hesitate to give an affirmative answer, because, as
in the case of the similar but varying domestic cats, no other explanation
of tribal resemblance in structure seems so reasonable and natural.