Page:Popular Science Monthly Volume 15.djvu/263

Rh derived from the skins or wool of other animals, for the natural apparel which the species has so unaccountably lost, it does not seem surprising that even Mr. Wallace should be staggered by the difficulty, and should fall back upon an essentially supernatural explanation.

The great key to the whole problem lies, it would seem, in the fact thus forced upon our attention, that the back of man forms the specially hairless region of his body. Hence we must conclude that it is in all probability the first part which became entirely denuded of hair. Is there any analogy elsewhere which will enable us to explain the original loss of covering in this the normally hairiest portion of the typical mammalian body? The erect position of man appears immediately to suggest the required analogy in the most hairless region of other mammals.

Almost all animals except man habitually lie upon the under surface of the body. Hence arises a conspicuous difference between the back and the lower side. This difference is seen even in lizards, crocodiles, and other reptiles, among which, as a rule, the tegumentary modifications of the under surface are much less extended and less highly differentiated than those of the upper. It is seen among birds, which usually have the plumage far less copious on the breast than on the back. But it is most especially noticeable in mammals, which have frequently the under side almost entirely bare of hair, while the back is covered with a copious crop. Now, it would seem as though this scantiness of natural clothing on the under side were due to long continued pressure against the ground, causing the hair to be worn away, and being hereditarily transmitted in its effects to descendants. We are, therefore, led to inquire whether all parts of the mammalian body which come into frequent contact with other objects are specially liable to lose their hair.

The answer seems to be an easy one. The soles of the feet in all mammals are quite hairless where they touch the ground. The palms of the hands in the quadrumana present the same phenomenon. The knees of those species which frequently kneel, such as camels and other ruminants, are apt to become bare and hard-skinned. The callosities of the Old-World monkeys, which sit upon their haunches, are other cases in point; but they do not occur among the more strictly arboreal quadrumana of the American Continent, nor among the lemurs, for the habits of these two classes in this respect are more similar to those of ordinary mammals. On the other hand, the New World monkeys possess a prehensile tail, with which they frequently swing from bough to bough or lower themselves to the ground, and in these creatures, says Cuvier, "la partie prenante de la queue est nue en dessous." Wherever we find a similar organ, no matter how widely different may be the structure and genealogy of the animals which possess it, we always find the prehensile portion free from hair. This is the case with the marsupial tarsipes, with many rodents, and