The illustrated story of evolution

by Marshall J. Gauvin

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.

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.

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.