The World's Greatest Books [Volumes 1-15, 17-20]

We take first the nervous system. In the lowest animals, as in the early stages of the embryo, there are no nerve-cells. In the embryo the nerve-cells develop from the outer, or skin layer, of cells. This, though strange as regards the human nervous system, is a correct preservation of the primitive seat of the nerves. It was the surface of the animal that needed to be sensitive in the primitive organism. Later, when definite connecting nerves were formed, only special points in the surface, protected by coverings which did not interfere with the sensitiveness, needed to be exposed, and the nerves transmitted the impressions to the central brain.

This development is found in the animal world to-day. In such animals as the hydra we find the first crude beginning of unorganised nerve-cells. In the jelly-fish we find nerve-cells clustered into definite sensitive organs. In the lower worms we have the beginning of organs of smell and vision. They are at first merely blind, sensitive pits in the skin, as in the embryo. The ear has a peculiar origin. Up to the fish level there is no power of hearing. There is merely a little stone rolling in a sensitive bed, to warn the animal of its movement from side to side. In the higher animals this evolves into the ear.

The glands of the skin (sweat, fat, tears, etc.) appear at first as blunt, simple ingrowths. The hair first appears in tufts, representing the scales, from underneath which they were probably evolved. The thin coat of hair on the human body to-day is an ancestral inheritance. This is well shown by the direction of the hairs on the arm. As on the ape's arm, both on the upper and lower arm, they grow toward the elbow. The ape finds this useful in rain, using his arms like a thatched roof, and on our arm this can only be a reminiscence of the habits of an ape ancestor.

We have seen how the spinal cord first appears as a tube in the axis of the back, and the cartilaginous column closes round it. All bone appears first as membrane, then cartilage, and finally ossifies. This is the order both in past evolution and in present embryonic development. The brain is at first a bulbous expansion of the spinal nerve-cord. It is at first simple, but gradually, both in the scale of nature and in the embryo, divides into five parts. One of these parts, the cerebrum, is mainly connected with mental life. We find it increasing in size, in proportion to the animal's intelligence, until in man it comes to cover the whole of the brain. When we remove it from the head of the mammal, without killing the animal, we find all mental life suspended, and the whole vitality used in vegetative functions.

In the evolution of the bony system we find the same correspondence of embryology and evolution. The main column is at first a rod of cartilage. In time the separate cubes appear which are to form the vertebræ of the flexible column. The skull develops in the same way. Just as the brain is a specially modified part of the nerve-rod, the skull is only a modified part of the vertebral column. The bones that compose it are modified vertebræ, as Goethe long ago suspected. The skull of the shark gives us a hint of the way in which the modification took place, and the formation of the skull in the embryo confirms it.

That adult man is devoid of that prolongation of the vertebral column which we call a tail is not a distinctive peculiarity. The higher apes are equally without it. We find, however, that the human embryo has a long tail, much longer than the legs, when they are developing. At times, moreover, children are born with tails—perfect tails, with nerves and muscles, which they move briskly under emotion, and these have to be amputated. The development of the limb from the fin offers no serious difficulty to the osteologist. All the higher animals descend from a five-toed ancestor. The whale has taken again to the water, and reconverted its limb into a paddle. The bones of the front feet still remain under the flesh. Animals of the horse type have had the central toe strengthened, for running purposes, at the expense of the rest. The serpent has lost its limbs from disuse, but in the python a rudimentary limb-bone is still preserved.

The alimentary system, blood-vessel system, and reproductive system have been evolved gradually in the same way. The stomach is at first the whole cavity in the animal. Later it becomes a straight, simple tube, strengthened by a gullet in front. The liver is an outgrowth from this tube; the stomach proper is a bulbous expansion of its central part, later provided with a valve. The kidneys are at first simple channels in the skin for drainage, then closed tubes, which branch out more and more, and then gather into our compact kidneys. We thus see that the building up of the human body from a single cell is a substantial epitome of the long story of evolution, which occupied many millions of years. We find man bearing in his body to-day traces of organs which were useful to a remote ancestor, but of no advantage, and often a source of mischief to himself. We learn that the origin of man, instead of being placed a few thousand years ago, must be traced back to the point where, hundreds of thousands of years ago, he diverged from his ape-cousins, though he retains to-day the plainest traces of that relationship. Body and mind—for the development of mind follows with the utmost precision on the development of brain—he is the culmination of a long process of development. His spirit is a form of energy inseparably bound up with the substance of his body. His evolution has been controlled by the same "eternal, iron laws" as the development of any other body—the laws of heredity and adaptation.