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. Dog, Bat, Rabbit, Man—at Three Different Stages of Development.] Comparative anatomy traces the arms and legs of man through various land forms back to the limbs of fishes. And the human heart and lungs, the liver, kidneys and stomach, the eyes and ears, the nose and mouth, have been fashioned from the organs of primitive creatures through successive modifications during countless ages. The larynx, which makes possible the human voice, appears in diverse stages of development in the Amphibia. Man’s remote ape-like progenitors had tails, and for a time in its embryonic development, the human babe has a tail longer than his legs (Fig. 36, middle row). Moreover, in the annals of medical science, there are records of many otherwise well-formed children born with tails. Professor Haeckel, in his “Evolution of Man,” shows photographs of a six-months-old tailed boy (Fig. 37). The presence of this tail is another rare instance of the vaulting power of heredity—a case in which Nature recalls a phase of her distant life. [Illustration: FIG. 37.—TAIL OF A SIX-MONTHS-OLD BOY.