Part 5

The evolution of animal life was paralleled by the rise of an equal diversity of forms in the vegetable world. From the Monera—organisms so lowly that it cannot be determined whether they are plant or animal—life diverged in opposite directions, producing, on the one hand, the plant, on the other, the animal. Organisms that could feed on inorganic matter assumed a stationary character and became the ancestors of the plants. Creatures that required organic food developed powers of locomotion to facilitate the search for sustenance, and of these were born the varied forms of earth’s animal population. From the first microscopic plants, vegetable life proceeded to seaweed, to mosses, to ferns, to the pine and the yew, to the vines, the fruit-bearing and flowering plants that crowd the forests and beautify the gardens of the world.

The truth of this wonderful story of evolution, of the progressive improvement of living forms with the advance of time, is proved by several lines of evidence. One of these lines of evidence is found in the fossil remains of the life of earlier times. Fossils are relics of forms that lived in the long ago. Dying, they were buried in sediment by the hand of time, and to-day they are found in rock formations, where they have been preserved from the destructive power of the ages. Now, if living things have developed in an increasing variety of forms as time has gone on, and if the stratified rocks of the earth’s crust, as they have grown from age to age, have kept a record of these living things, these testimonies of the rocks must prove the gradual unfoldment of the world’s life. Such precisely is the case. The earlier rocks are poor in fossils, because in the earlier ages the world was poor in life; but in the succeeding rocks, there is a larger deposit of fossils; and the rocks of every progressively later age show an ever increasing wealth of fossil remains.

(Pithecanthropus Erectus.)

Restored by Dr. Eugene Dubois, and the skull of a modern European.]

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.

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.

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.

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.

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.

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.

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.

Removed by Operation by Dr. Granville Harrison, in 1901. “A great number of such cases,” says Haeckel, “are given by Max Bartells in his essay on ‘Tailed Men,’ 1884.”]

Every human being carries the rudiment of an ancestral tail, the coccyx, at the base of his spine. In the human embryo, the very muscles for wagging this tail are still found. In adult man, these muscles are represented, as a rule, by bands of fibrous tissue. Occasionally, however, the dissecting surgeon finds these muscles well developed in the body of a man or woman. Science says that evolution alone can explain this lingering relic of a tail, with its attendant muscles, in the human body.

This brings us to another line of evidence, which proves the truth of evolution—the presence of rudimentary organs. Fig. 38 shows the vermiform appendix in the orang, in man, and in the babe before birth. It is much larger in the ape than in man. Man has inherited it from his ape-like ancestors, and while it serves no purpose in his body, its infection is the source of the frequently fatal disease, appendicitis.

Then there are the rudimentary muscles of the ear (Fig. 39). In our remote progenitors these muscles were developed, and with them they could move their ears. Monkeys move their ears, though not with the facility characteristic of horses and dogs. Occasionally a man is found who can move his ears as we move the skin of the forehead. But in the anthropoid apes and in most men these muscles are inoperative. Through disuse they have become rudimentary. When the ancestors of these apes and men began to assume the erect posture, they also began to turn their heads, instead of their ears, in the direction of sound. As the erect posture was improved, the turning head answered with increasing loyalty the call of the sound waves, and after innumerable ages of comparative rest and disuse, the ear muscles dwindled into their present impotence. Here, again, evolution alone explains the facts.

Moreover, the whole external ear is a rudimentary organ—a structure that has outlived its function. The male breasts point to a time when the father as well as the mother suckled the young. In some men the breasts are as well developed as in women and supplied with milk, and in many known instances babes have been suckled at these male breasts. Again, in some women and occasionally in men, two or more pairs of breasts appear—a fact which plainly shows that man has descended through humbler forms of life. Still another rudimentary organ is the nictitating membrane—the little fleshy pad at the inner corner of the eye—the relic of a third eyelid that our ancestors in the dim past flashed across the eyeball as the turtle and the eagle do to-day.

Man has evolved from creatures that went on all fours and lived in trees, and in consequence of his upright posture and changed habits of life, some structures which are active in the apes are rudimentary in the human body. A shoulder muscle that is a source of strength to the apes in climbing, lingers in man as a mere fibrous remnant that has dwindled through disuse. Another muscle which runs through the wrist into the palm of the hand, another which extends from the calf of the leg to the sole of the foot, another which passes from the shoulder to the neck and was once used to lift the collar bone—these muscles, which are well developed in the true apes, and less developed in the anthropoids, are rudimentary in man. These rudimentary organs and muscles are links in the chain of man’s descent and they point unerringly to the source of his origin.

The long heel and the poorly developed calf of the anthropoid ape remind us that he has but recently, so to speak, acquired the upright posture. In the negroid races also the long heel and diminutive calf are notable characteristics; while in the white and yellow races the heel is short and the calf well formed and muscular. Once more, the obvious conclusion is that the black man’s long heel and slender calf represent a more primitive development; while the leg and heel of the white man and the Mongolian show the work of evolution farther advanced.