The reader may think that I am grouping together very dissimilar structures in this chapter. The eye, the ear, the nose, the hair, the skin, the nails, the teeth, sweat, milk and sebaceous glands, although very different in appearance and function, have one character in common ; all are developed from the covering or epidermis of the embryo. We have been in the habit from childhood of thinking of the skin as merely a covering or clothing for the body. When, however, we study the skin or surface covering of even the lowest animal we find it is triple in its function : it is a covering, a sense organ, and a gland. We have already seen how a strip of the embryonic covering is modified and enclosed to form the beginning of the spinal cord and brain. Along the gums a fold of the skin grows down to provide the enamel crowns of the teeth. The teeth are really incrustations formed round skin papillae. During the fourth and fifth months sprouts of the epithelial cells of the surface layer grow into the true skin and form sweat glands ; sebaceous glands which are developed at the roots of hairs and supply a natural oil for the hair and skin are formed in a similar manner. The milk glands we have also seen arise along a ridge on the skin ; they appear to have been evolved out of sweat glands. On the tips of the digits the epithelial cells of the skin are modified to form nails. The inner ear, which gives rise to a most intricate arrangement of parts for the transformation of sound waves into nerve impulses arises as a minute pocket of skin which sinks into the head until its mouth on the surface is closed. In some fishes the opening persists, the fish retaining in adult life an arrangement which is seen only in the embryo of higher vertebrate animals. The olfactory membrane also arises as a pair of skin pockets, which become enclosed by the parts which form the nose. Some of the epithelial cells of the olfactory membrane are so constituted as to be affected by odoriferous substances floating in the air. In the tongue certain cells are sensitized for taste; in the skin others are modified for touch. The lens of the eye, which focuses images on the retina, also arises as an epithelial ingrowth from the skin. It is even liable to the horny or corneous change which occurs in skin epithelium; when the lens becomes old, it often becomes corneous and opaque, giving rise to the condition of cataract. Thus it will be seen that the skin gives origin to a most complex series of structures, but we must suppose, from the facts of development just narrated, that man is the descendant of a stock which had a skin, simple in structure but complex in function, which served as a nerve system, gland system, as well as a body covering.
The anthropoid apes which in all structural features come nearest to man, have their bodies covered with hair, all except the soles of the feet and palms of the hand. We notice, however, that on the cheeks and forehead the hairs are short and not so apparent. How is it, then, that man is almost hairless and is thus so different to the anthropoids ? We infer that man comes of a stock which was as hairy as the anthropoids. Hair roots are developed all over his body, except on the palms and soles. At the seventh and eighth months of fcetal life the body is clad with a fine downy covering—the lanugo. In the adult, especially in some individuals, the body becomes covered with rough hair. In some races, such as the Ainus of Japan, the hairy covering of the body is very apparent. In other races, such as the African negro, the skin is almost hairless. Occasionally we see human freaks in which the face and head, or even the whole body, becomes as thickly covered with hair as a terrier. Indeed, when we examine carefully the arrangement or " lie " of the hair on the upper extremities of man we see that in the upper arm it is directed downwards to the elbow while on the extensor surface of the forearm it slopes up to the elbow. In the upper extremity of the anthropoid apes the hair is also arranged in this manner. We cannot explain all these facts unless we suppose that modern man has come of a hairy stock with the hair arranged exactly as we see it in anthropoids. We do not as yet know how the hairless condition came about, but there are several lines of suggestive enquiry. Professor Elliot Smith has observed that there is a correlation between the evolution of the human brain and the disappearance of a hairy covering. The brain is linked up by sensory nerves with every papilla of the skin : the more nude and thin the skin the greater its sensitiveness and the greater the number of messages sent to the brain. It is, therefore, possible that the nude condition of our skin and the size of our brain are connected in some way. We have still to explain why and how the hair has disappeared. We have already seen that many points in the structure of man have been evolved by a retention of a fcetal condition. Now fcetal anthropoids are hairless until the third month before birth ; if the fcetal condition then were to be retained or prolonged a hairless condition would result. The growth of the hair, too, is regulated by a mechanism which we do not know fully, but the secretion of the thyroid has a distinct influence on the nourishment of the skin and the growth of the hair. The deposition and absorption of fat is also influenced by thyroid secretion. The healthy human baby differs from that of the anthropoid in being plump, owing to the thick layer of fat which lies beneath the skin ; it is also hairless. The two conditions are probably correlated, and the mechanism which brings the change about is probably a function of the thyroid. We have every reason to suppose that civilization—or rather an ample command of food all the year round— was acquired by man at a much earlier period in the history of the earth than we have hitherto thought. The human foetus would be much more richly nourished when such conditions came into existence. The glands of internal secretion, such as the thyroid, would probably be affected. It is possible then that in man's hairless condition we see one of the first effects of civilization on the human race.