learned, for it finds that adult animals array themselves at different
levels of a scale beginning at the bottom with the protozoa, continuing on
to the two-layered animals like _Hydra_ and jellyfish and sea-anemones,
and then extending upwards to the region of the more complicated
invertebrates and vertebrates. It was difficult perhaps to believe that
these successive grades of organic structure indicated an order of
evolution, because it seemed impossible that an animal so simple as a
protozoan could produce offspring with the complex organization of a frog
or a cat, even in long ages. But development delivers its evidence
relating to this matter with telling and impressive force. How can we
doubt the possibility of an evolution of higher animals from ancestors as
simple as _Hydra_ and _Amoeba_ when a frog and a cat, like all other
complicated organisms, begin individual existence as single cells, and
pass through gastrula stages? If we deny it, we contradict the evidence of
our senses, for the development is actually accomplished by the
transformation of a single cell into a double-walled sac, and of this into
different and more intricate organic mechanisms. The process _can_ take
place, for it _does_ take place. Not until the investigator becomes
familiar with a wide range of diverse animals and the peculiar qualities
of their similar early stages, can he estimate the tremendous weight of
the facts of comparative embryology. Were the statement iterated and
reiterated on every page and in every paragraph, there would be no undue
emphasis put upon the astounding fact that the apparently impassable gap
between a one-celled animal like _Amoeba_ and a mammal like a cat is
actually compassed during the development of the last-named organisms from
single cells. The occurrence of gill-slits in the embryos of lizards,
birds, and mammals now seems a small thing when compared with the
correspondences disclosed by the earliest stages of development. But in
spite of their complexity, all the changes of "growing up" are explained
and understood by the simple formula that the mode of individual
development owes its nature primarily to the hereditary influence of
earlier ancestors back to the original animals which were protozoa.
* * * * *
Embryology as a distinct division of zoölogy has grown out of studies of
classification and comparative anatomy. Its beginnings may be found in
medieval natural history, for as far back as 1651 Harvey had pointed out
that all living things originate from somewhat similar germs, the terse
dictum being "Ex ovo omnia." By the end of the eighteenth century many had
turned to the study of developing organisms, though their views by no
means agreed as to the way an adult was related to the egg. Some, like
Bonnet, held that the germ was a minute and complete replica of its
parent, which simply unfolded and enlarged like a bud to produce a similar
organism. Even if this were true, little would be gained, for it would
still remain unknown how the germinal miniature originated to be just what
it was conceived and assumed to be. Wolff was the originator of the view
that is now practically universal among naturalists, namely, that
development is a real process of transformation from simpler to more
complex conditions.
The subject of comparative embryology grew rapidly during the nineteenth
century as the field of comparative anatomy became better known, and when
naturalists became interested in animals, not only as specific types, but
also as the finished products of an intricate series of transformations.
When life-histories were more closely compared, the meaning of the
resemblances between early stages of diverse adult organisms was read by
the same method which in comparative anatomy finds that consanguinity is
expressed by resemblance. The great law of recapitulation, stated in one
form by Von Baer and more definitely by Haeckel in the terms employed in
the foregoing sections, was for a time too freely used and too rigidly
applied by naturalists whose enthusiasm clouded their judgment. A strong
reaction set in during the latter part of the nineteenth century, when
attention was directed to the anachronisms of the embryonic record and to
the alterations that are the results of larval or embryonic adaptation as
short cuts in development. Nevertheless, it is not seriously questioned, I
believe, that the main facts of a single life-history owe their nature to
the past evolution of the species to which a given animal belongs.
Nowadays the problems in this well-organized department are concerned not
only with more accurate accounts of the development of animals, but also
with the mechanics of development, with the relative value of external and
internal influences, and above all with the physical basis of inheritance.
It is clear that the factors that direct the development of a wood frog's
egg so that it becomes a wood-frog and not a tree-toad must lie in the egg
itself, as derivatives from the two parent organisms. Weismann and his
followers have proved that a peculiar substance in the nuclei of the egg
and its daughter-products contains the essential factors of development,
whatever these may be. Experiments dealing with the phenomena of heredity
in pure and mixed breeds have largely confirmed Weismann's doctrine, and
they have prepared the way for a deeper investigation of the marvelous
process of biological inheritance.
However much he may be interested in the details of embryological science,
the general student of natural history is more concerned with the bearing
of its primary laws upon the great problem of evolution. In the foregoing
brief review of the fundamental facts and principles of this subject, the
purpose has been to show how the phenomena of development are viewed by
men of science, and how they take their place in the doctrine of organic
evolution. And it has also been made plain that comparative anatomy and
comparative embryology support and supplement one another in countless
ways and places, although each in itself is a complete demonstration that
evolution is a real and a natural process.
III
THE EVIDENCE OF FOSSIL REMAINS
Few natural objects appeal to the interest and imagination of the student
with more force than the fragments of animals and plants released from the
rocks where they have been entombed for ages. Our lives are so brief that
it is impossible for us to comprehend the full duration of the slow
process which constructed the burial shrouds of these creatures of long
ago. We try to picture the earth and its inhabitants as they were when
lizards were the highest forms of animals, and we wonder how life was
lived in the dense forests of the coal age. Science can never learn all
about the ancient history of the earth and of the organisms of bygone
times; yet it has been able to accomplish much through its endeavors to
reconstruct the past, for its method is one by which sure results can
always be obtained whenever there are definite facts with which it can
work. In our present study of evolution we reach the point when we must
examine the testimony of the rocks, and the results and methods of that
department of knowledge called palæontology, which is concerned with
fossils and their interpretation.
The word "palæontology" means literally the "science of living things of
long ago." It deals directly with the remains of animals and plants found
as fossils, and it interprets them through its knowledge of the way modern
animals are constructed and of the changes the earth's crust has
undergone. A skull-like object may be found in a coal field and may come
into the hands of the palæontologist: from his acquaintance with the head
skeletons of recent types he will be able to assign the extinct creature
which possessed the skull to a definite place in the animal scale and to
understand its nearer or wider affinities with other animals of later
times and of earlier epochs. In doing these things palæontology employs
the methods of comparative anatomy with which we have now become familiar.
In the performance of its other tasks, however, palæontology must work
independently. It is necessary to know when a fossilized animal lived, not
that its time need be measured by an absolute number of a few thousands or
millions of years antedating our own era, for that is impossible. But the
important thing is to know its relative age, and whether it preceded or
followed other similar animals of its own group or of different divisions.
The rocks themselves must be understood, how they have been formed and how
they are related in mineralogical nature and in historical succession.
Palæontology also deals with a number of subjects that are not in
themselves biological, such as the combination of circumstances necessary
for the adequate preservation of fossil relics. In so far as it is
concerned with physical matters, as contrasted with strictly biological
data, it is one with geology. Indeed, the investigators in these two
departments must always work side by side and render mutual assistance to
one another in countless ways, for each division needs the results of the
other in order to accomplish its own distinct purposes. It must be evident
to every one that it is impossible to understand the meaning of fossils
and the place of the testimony of the rocks in the doctrine of evolution
without knowing much about the geological history of the earth and the
influences at work in the past. For these reasons palæontology differs
somewhat from the other divisions of zoölogy where direct observation
gives the materials for arrangement and study; in this case the individual
data, that is, the fossil fragments themselves, can be made available only
through a knowledge of their exact situations, of the reasons for their
occurrence in particular places in the rock series and of the way rocks
themselves are constructed and worked over by natural agencies. Our task
is therefore twofold: certain physical matters of a geological nature must
first be investigated before the biological facts can be described.
No doubt most people feel justified in believing that the whole doctrine
of evolution must stand or fall according to the cogency of the
palæontological evidences. Plain common sense says that the owners of
shelly or bony fragments found in the deeply-laid strata of the earth must
have lived countless years ago, and if the evolutionist asserts that
primitive organic forms of ancient times have produced changed descendants
of later times, it would seem that fossil evidence would be supremely and
overwhelmingly important. It is true, of course, that this evidence is
peculiarly significant, because in some ways it is more direct than that
of the other categories already outlined. But it must not be forgotten
that the doctrine is already securely founded upon the basic principles of
anatomy and embryology. Science must treat the data of this category by
different methods and must view them in different ways. Therefore we are
interested in palæontology because of the way it tells the story of
evolution in its own words, and because we are justified in expecting that
its account should include a description of some such order of events as
that revealed by the developing embryos of modern organisms and that
demonstrated by the comparative anatomy of the varied species of adult
animals.
It is true that palæontology gives direct testimony about the evolutionary
succession of animals in geologic time. But we now know that embryology is
even more direct in its proof that organic transformation is natural and
real; while at the same time there is a completeness in the full series of
developmental stages connecting the one-celled egg with the adult creature
that must be forever lacking in the case of the fossil sequence of
species. If paragraphs and pages are missing from the brief embryonic
recapitulation, whole chapters and volumes of the fossil series have been
lost for all time. The investigators whose task it has been to decipher
the story of the earth's evolution have had to meet numerous and
exasperating difficulties which do not confront the embryologist and
anatomist who study living materials. Nevertheless the library of
palæontological documents is one which has been founded for over a
century, and it has grown fast during recent decades, so that consistent
accounts may now be read of the great changes in organic life as the earth
has altered and grown older. And in all this record, there is not a single
line or word of fact that contradicts evolution. What definite evidence
there is tells uniformly in favor of the doctrine, for it is possible, in
the first place, to work out the order of succession of many of the great
groups of animals, and this order is found to be the same as that
established by the other bodies of evidence. Secondly, some fossil groups
are astonishingly complete, so that the ancient history of a form like the
horse can be written with something approaching fullness. Finally, the
remains of certain animals have been found so situated in geological ways,
and so constructed anatomically, that the zoölogist is justified in
denoting them "missing links," because they seem to have been intermediate
between groups that have diverged so widely during recent epochs as to
render their common ancestry scarcely credible.
With these general results in mind, we must now become acquainted with
such subjects as the interpretation of fossils, the causes for the
incompleteness of the series, the conditions for fossilization, the forces
of geological nature, and other matters that make the fossils themselves
intelligible as scientific evidence.
* * * * *
Many views have been entertained regarding the actual nature of the relics
of antiquity exhumed from the rocks or exposed upon the surface by the
wear and tear of natural agencies. In earliest times such things were
variously considered as curious freaks of geological formation, as sports
of nature, or as the remains of the slain left upon the battle-ground of
mythical Titans. Some of the Greeks supposed that fossils were parts of
animals formed in the bowels of the earth by a process of spontaneous
generation, which had died before they could make their way to the
surface. They were sometimes described as the bones of creatures stranded
upon the dry land by tidal waves, or by some such catastrophe as the
traditional flood of the scriptures. In medieval times, and even in our
own day, some people who have been opposed to the acceptance of any
portion of the doctrine of evolution have actually defended the view that
the things called fossils were never the shells or bones of animals living
in bygone times, but that they only simulate such things and have been
created as such together with the layers of rock from which they may have
been taken. If we employed the same arguments in dealing with the broken
fragments of vases and jewelry taken from the Egyptian tombs or from the
buried ruins of Pompeii, we would have to believe that such pieces were
created as fragments and that they were never portions of complete
objects, just because no one alive to-day has ever seen the perfect vessel
or bracelet fashioned so long ago. Common sense directs us to discard such
a fantastic interpretation in favor of the view that fossils are what they
seem to be--simply relics of creatures that lived when the earth was
younger.
Until this common sense view was adopted there was no science of
palæontology. Cuvier was the first great naturalist to devote particular
attention to the mainly unrelated and unverified facts that had been
discovered before his time. He was truly the originator of this branch of
zoölogy, for he brought together the observations of earlier men and
extended his own studies widely and surely, emphasizing particularly the
necessity for noting carefully the geological situation of a fossil in
rocks of an older or later period of formation. His great result was the
demonstration that many groups of animals existed in earlier ages that
seem to have no descendants of the same nature to-day, and also that many
or most of our modern groups are not represented in the earliest formed
sedimentary rocks, although these recent forms possess hard parts which
would surely be present somewhere in these levels if the animals actually
existed in those times. But the meaning of these facts escaped Cuvier's
mind. He was a believer in special creation, like Linnæus and all but a
few among his predecessors, and he explained the diversity of the faunas
of different geological times in what seems to us a very simple and naïve
way. In the beginning, he held, when the world was created, it was
furnished with a complete set of animals and plants. Then some great
upheaval of nature occurred which overwhelmed and destroyed all living
creatures. The Creator then, in Cuvier's view, proceeded to construct a
new series of animals and plants, which were not identical with those of
the former time, but were created according to the same general working
plans or architectural schemes employed before. Another cataclysm was
supposed to have occurred, which destroyed the second series of organisms
and laid a new covering of rocks over the earth's surface for a subsequent
period of relative quiet; and so the process was continued. By this
account, Cuvier endeavored to reconcile the doctrine of supernatural
creation and intervention with the obvious facts that organisms have
differed at various times in the earth's history. Although he saw that
animals of successive periods displayed similar structures, like the
skeleton of vertebrates, which testified to some connection, Cuvier could
not bring himself to believe that this connection was a genealogical one.
Mainly through the influence of the renowned English man of science,
Charles Lyell, the students of the earth came to the conclusion that its
manifold structures had developed by a slow and orderly process that was
entirely natural; for they found no evidence of any sudden and drastic
world-wide remodeling such as that postulated by the Cuvierian hypothesis
of catastrophe. The battle waged for many years; but now naturalists
believe that the forces, of nature, whose workings may be seen on all
sides at the present time, have reconstructed the continents and ocean
beds in the past in the same way that they work to-day. The long name of
"uniformitarianism" is given to Lyell's doctrine, which has exerted an
influence upon knowledge far outside the department of geology. Darwin
tells us how much he himself was impressed by it, and how it led him to
study the factors at work upon organic things to see if he could discern
evidence of a biological uniformitarianism, according to which the past
history of living things might be interpreted through an understanding of
their present lives.
* * * * *
What, now, are the reasons why the palæontological evidence is not
complete and why it cannot be? In the first place the seeker after fossil
remains finds about three fifths of the earth's surface under water so
that he cannot explore vast areas of the present ocean beds which were
formerly dry land and the homes of now extinct animals. Thus the field of
investigation is seriously restricted at the outset, but the naturalist
finds his work still more limited, in so far as much of the dry land
itself is not accessible. The perennial snows of the Arctic region render
it impossible to make a thorough search in the frigid zone, and there are
many portions of the temperate and torrid zones that are equally
unapproachable for other reasons. But even where exploration is possible,
the surface rocks are the only ones from which remains can be readily
obtained, for the layers formed in earlier ages are buried so deeply that
their contents must remain forever unknown in their entirety. Only a few
scratches upon the earth's hard crust have been made here and there, so it
is small wonder that the complete series of extinct organisms has not been
produced by the palæontologist.
A brief survey of the varied groups of animals themselves is sufficient to
bring to light many biological reasons which account for still more of the
vacant spaces in the palæontological record. We would hardly expect to
find remains of ancient microscopic animals like the protozoa, unless they
possessed shells or other skeletal structures which in their aggregate
might form masses like the chalk beds of Europe. Jellyfish and worms and
naked mollusks are examples of the numerous orders of lower animals having
no hard parts to be preserved, and so all or nearly all of the extinct
species belonging to these groups can never be known. But when an animal
like a clam dies its shell can resist the disintegrating effects of
bacteria and other organic and inorganic agencies which destroy the soft
parts, and when a form like a lobster or a crab, possessing a body
protected by closely joined shell segments, falls to the bottom of the
sea, the chances are that much of the animal's skeleton will be preserved.
Thus it is that corals, crustacea, insects, mollusks, and a few other
kinds of lower forms constitute the greater mass of invertebrate
palæontological materials because of their supporting structures of one
kind or another. Perhaps the skeletal remains of the vertebrates of the
past provide the student of fossils with his best facts, on account of the
resistant nature of the bones themselves, and because the backboned
animals are relatively modern; then, too, the rocks in which their remains
occur have not been so much altered by geological agencies, or buried so
deeply under the strata formed later. Of course only the hardest kinds of
shells would remain as such after their burial in materials destined to
turn into rock; in the majority of cases, an entombed bone is infiltrated
or replaced by various mineral substances so that in time little or
nothing of the original thing would remain, though a mold or a cast would
persist.
But even if an animal of the past possessed hard structures, it must have
satisfied certain limited conditions to have its remains prove serviceable
to students of to-day. A dead mammal must fall upon ground that has just
the right consistency to receive it; if the soil is too soft, its several
parts will be separated and scattered as readily as though it had fallen
upon hard ground where it would be torn to pieces by carnivorous animals.
The dead body must then be covered up by a blanket of silt or sand like
that which would be deposited as the result of a freshet. If a skeleton is
too greatly broken up or scattered, it may be difficult or even impossible
for its discoverer to piece together the various fragments and assemble
them in their original relations. Very few individuals have been so buried
and preserved as to meet the conditions for the formation of an ideal
fossil. To realize how little may be left of even the most abundant of
higher organisms, we have only to recall that less than a century ago
immense herds of bison and wild horses roamed the Western plains, but very
few of their skulls or other bones remain to be enclosed and fossilized in
future strata of rocks. When we appreciate all these difficulties, both
geological and biological, we begin to see clearly why the ancient lines
of descent cannot be known as we know the path and mode of embryonic
transformation. The wonder is not that the palæontological record is
incomplete, but that there is any coherent and decipherable record at all.
Yet in view of the many and varied obstacles that must be surmounted by
the investigator, and the adverse factors which reduce the available
evidence, the rapidly growing body of palæontological facts is amply
sufficient for the naturalist to use in formulating definite and
conclusive principles of evolution.
* * * * *
For the purposes of palæontology, the most essential data of geology are
those which indicate the relative ages of the strata that make up the hard
outer crust of the earth, for only through them can the order of animal
succession be ascertained. It does not matter exactly how old the earth
may be. While it is possible to determine the approximate length of time
required for the construction of sedimentary rocks like those which
natural agencies are producing to-day, there are few definite facts to
guide speculation as to the mode or duration of the process by which the
first hard crystalline surface of the earth was formed. But palæontology
does not care so much about the earliest geological happenings, for it is
concerned with the manifold animal forms that arose and evolved after life
appeared on the globe. Questions as to the way life arose, and as to the
earliest transformations of the materials by which the earth was first
formed are not within the scope of organic evolution, although they relate
to intensely interesting problems for the student of the process of cosmic
evolution.