A PROBLEM IN HUMAN EVOLUTION. 251 



derived from the skins or wool of other animals, for the natural ap- 

 parel 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 special- 

 ly 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 imme- 

 diately 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 modifica- 

 tions 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 cal- 

 losities 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 



