The earlier rocks contain fossils of the lower forms of life, like
seaweed and simple creatures in shells; in succeeding rocks come the
fishes—the first living things with a backbone—primitive forms being
followed by more elaborate types; in later strata are found the fossil
remains of the amphibians; following these, in rocks higher up, are the
relics of the reptiles; then come the reptile-birds, and in still later
rocks appear the primitive mammals; onward and upward rises the spiral
of life, and in yet later rocks occur placental mammals—early horses,
marmosets and lemurs; higher still, the strata imprison fossil cats,
pigs, elephants, antelopes and apes; and over the fossil remains of all
other creatures, in the rock-crust of the earth, as is required by the
facts of biological unfoldment, lie the fossilized forms of man-like
apes and men. Thus geology confirms the conclusions of biology.
[Illustration: FIG. 31.—A RESTORATION OF THE NEANDERTHAL MAN.
This picture is a retouched photograph taken of a model made by
Guernsey Mitchell according to instructions of Prof. Henry A. Ward of
Chicago. Reprinted with the permission of The Open Court Publishing
Co.]
In Fig. 32 we have a series of fossil heads from different periods in
the ascending geological order, illustrating the gradual development
of the elephant’s tusks and trunk. See how the tusk, beginning as an
upper tooth in the Eocene period when the elephant was no larger than
a rabbit, gradually lengthens as the periods pass, until we get, after
millions of years, the great curved tusk of the elephant of to-day.
The growth of the trunk kept pace, of course, with the extension of
the tusks. On the left side may be seen the gradual and corresponding
modification of the elephant’s teeth.
The horse, the noblest of the quadrupeds, has been evolved from a
small five-toed animal. The Eohippus, the “dawn horse,” whose fossil
remains have been found in the lower Eocene rocks, was no bigger than
a small fox (Fig. 33). It had four toes and a splint on each front
foot, and three toes and two splints on each hind foot—which proves
that its ancestors had five toes on each foot. The “dawn horse” was a
browsing animal. Restorations of this little horse, and of the larger
three-toed horse that followed him, are now familiar figures in natural
history museums. Evolution, you see, is not a figment of somebody’s
imagination; it is, rather, the compelling, common sense interpretation
of the facts of nature.
[Illustration]
[Illustration]
[Illustration: FIG. 32.—EVOLUTION OF THE PROBOSCIDEA.
On the right, a series of skulls; on the left, last lower molar.
Reprinted with the permission of the author and publishers from
Prof. William B. Scott’s “A History of Land Mammals in the Western
Hemisphere.” The Macmillan Co., New York.]
All modern horned animals have been evolved from ancestors that were
without horns. The progenitors of the deer, the elk, the caribou,
the moose, before the Miocene period, were entirely destitute of
horns. Then the first horns appeared, each as a simple prong. As the
succeeding ages passed away, the horns gradually increased in size and
took on more prongs. The fossil antlers, found in the successive rock
formations from the Miocene period onward, show the gradual evolution
of the horns that adorn the heads of living animals (Fig. 34).
But this is only half the story. The history of an individual creature
is called ontogeny. The history of a race of creatures is called
phylogeny. Now, it is a biological law that ontogeny is always a
recapitulation of phylogeny; that is to say, that every creature in
the course of its development, especially before birth, passes through
the various stages permanently occupied by its ancestors. We see this
law beautifully illustrated in the growth of the antlers of one living
deer (Fig. 35). During the first year, or thereabout, the deer develops
one plain horn, and each year thereafter another prong is added, until
after a series of years, the antlers are full grown and full-pronged.
In other words, each and every deer, in the development of his horns,
repeats the long experience of his race in acquiring horns.
[Illustration: FIG. 33.—EOHIPPUS, THE “DAWN HORSE,” FROM THE LOWER
EOCENE ROCKS.
Restored from a skeleton in the American Museum of Natural History,
New York. Reprinted with the permission of the author and publishers
from Prof. William B. Scott’s “A History of Land Mammals in the Western
Hemisphere.” The Macmillan Co., New York.]
Now, this process of recapitulation applies to man as well as to all
other creatures. This brings us to another line of evidence which
proves the truth of evolution—embryology, or development before birth.
Dogs, cattle, sheep, elephants, horses, apes, men and all other mammals
had the same origin. Therefore, the early stages in the evolutionary
process were the same for all these creatures. That is to say, the
course of life’s development, before diverging toward these respective
forms, followed for a time a general line of advance.
The truth of this is strikingly illustrated in Fig. 36. Here we have
the embryos of four mammals in different stages of development—the
dog, the bat, the rabbit and man. These embryos prove that all these
creatures have come through the same line of development; that they
all had the same remote ancestors. The distant ancestors of all
these creatures were fishes in the sea, and here, in the embryo of
the dog, the bat, the rabbit and man, we have, still persisting, the
gill-clefts of the fish (top row). These gill-slits, which all mammals,
before birth, still inherit from the fishes through which ascending
life passed in the morning of the world, occasionally persist in
man throughout the embryonic period and appear in the child’s neck
as fissures pointing towards the throat, through which milk, when
swallowed, passes to the outside of the neck. Only on the theory that
all mammalian life, including man, has evolved through the fish stage
does the persistence of these gill-clefts become intelligible.
[Illustration: FIG. 34.—FOSSIL ANTLERS.]
The embryonic development of man and the other mammals demonstrates
their remote relationship and their gradual evolution. The dog, the
sheep, the ox, the horse, the ape and man and every other mammal begins
life as a single cell. That cell grows and divides into two; the two
enlarge and divide into four; the four divide into eight. Dividing
in this way, the cells come to form a cluster resembling a mulberry.
This is the morula stage. The cells now form a hollow sphere, one
cell in thickness, and the sphere fills with fluid—the blastula. Now
the sphere, like a punctured India rubber ball, falls in upon itself,
assuming a cup shape with double walls. This is the gastrula stage,
when the embryo resembles a worm in the figure of a horseshoe. A third
layer of cells is now formed between the other two, and from these
three layers of cells are gradually unfolded all the complex parts of
the body. From the outer cells arise the skin, the hair, the lenses of
the eyes, the nervous system, the membranes of the mouth and nose, and
the enamel of the teeth; from the inner cells arise the lining of the
larynx, the trachea and the lungs, the intestines, the liver and the
thyroid glands; from the middle layer of cells are formed the skeleton,
the inner structure of the teeth, the muscles, the blood-vessels and
the blood itself, the membranes of the heart and lungs, the kidneys and
the reproductive organs.
[Illustration: FIG. 35.—ANTLERS OF ONE DEER AT DIFFERENT STAGES OF
THEIR DEVELOPMENT.]
For a time the human embryo is a simple trunk, without skull or spine,
without arms or legs, with only a pulsating tube for a heart. Then
the rod of the amphioxus appears running down the back; then the
growing embryo resembles a fish, with gill-slits in its neck and with
a two-chambered fish heart; then comes the reptile stage when the
babe has a three-chambered heart and other features of his reptilian
ancestors; then the heart becomes four-chambered and the babe, passing
through other transformations, reaches the finished human form.
In all these details of growth, the development of man is paralleled
by the development of every other mammal. And certainly only one
conclusion may be drawn from the fact that for a time in the embryonic
life of the rabbit, the sheep, the pig, the dog, the ape and man, these
creatures are formed and look so nearly alike that even the man of
science, unaware of their identity, cannot distinguish one from the
other.
The significant structural similarity of nearly all living creatures
points unmistakably to evolution. For example, the limbs of amphibians,
reptiles, birds and mammals consists of one long bone above, then two
shorter bones below, then two transverse rows of bones forming the
ankle, with the foot ending in five toes. In the foot of the crocodile,
in the flipper of the seal, in the paddle of the whale and in the foot
of the dog may be seen the general plan of the human hand. So the wing
of the bat or bird, the forelimb of the lizard or elephant and the
little shovel-like leg and foot of the mole present the same number of
bones in essentially the same structural arrangement as do the arm and
hand of man. It is the different uses to which these organs have been
put that have determined their various developments.
[Illustration]
[Illustration]
[Illustration: FIG. 36.—EMBRYOS OF FOUR MAMMALS.