A PROBLEM IN HUMAN EVOLUTION. 253 



to suppose that no other cause save that which we are considering can 

 ever produce hairlessness. It will be enough if we can show that the 

 cause actually under examination does with reasonable certainty bring- 

 about such an effect. 



If, then, the portion of animals which generally comes in contact 

 with the ground or other external bodies acquires in this manner a 

 hairless condition shown alike in hands, feet, tail, and belly what 

 will be the result upon animals which are gradually acquiring the 

 erect position ? Of this we can obtain an almost complete series by 

 looking first at the beaver, which rests upon its scaly tail alone ; then 

 at the baboons, which rest upon the naked callosities on their haunches ; 

 thirdly, at the gorilla ; and, last of all, at mankind. 



The gorilla, according to Professor Gervais, is the only mammal 

 which agrees with man in having the hair thinner on the back, where 

 it is partly rubbed off, than on the lower surface. This is a most im- 

 portant approach to a marked human peculiarity, and is well worthy 

 of investigation. " I have myself come upon fresh traces of a goril- 

 la's bed on several occasions," says Du Chaillu, " and could see that 

 the male had seated himself with his back against a tree-trunk. In 

 fact, on the back of the male gorilla there is generally a patch on 

 which the hair is worn thin from this position, while the nest-building 

 Troglodytes calvus, or bald-headed nshiego, which constantly sleeps 

 under its leafy shelter on a tree-branch, has this bare place on its side, 

 and in quite a different way. . . . When I surprised a pair of gorillas," 

 he observes elsewhere, " the male was generally sitting down on a rock 

 or against a tree." Once more, in a third passage he writes : " In both 

 male and female the hair is found worn off the back ; but this is only 

 found in very old females. This is occasioned, I suppose, by their 

 resting at night against trees, at whose base they sleep." And, when 

 we inquire into the difference between the sexes thus disclosed, we 

 learn that the female and young generally sleep in trees, while the 

 male places, himself in the position above described against the trunk. 



The gorilla has only very partially acquired the erect position, and 

 probably sits but little in the attitudes common to man. But if a 

 developing anthropoid ape were to grow more and more upright in his 

 carriage, and to lie more and more upon his back and sides, we might 

 naturally expect that the hair upon those portions of his body would 

 grow thinner and thinner, and that the usual characteristics of the 

 mammalia as to dorsal and sternal pilosity would be completely 

 reversed. This is just what has probably happened in the case of 

 man. In proportion as he grew more erect, he must have lain less 

 and less upon his stomach, and more and more upon his back or sides. 

 For fully developed man, with the peculiar set of his neck, face, and 

 limbs, it is almost impossible to rest upon his stomach. On the other 

 hand, all savage races lie far more upon their backs than even Euro- 

 peans with their sofas, couches, and easy-chairs ; for the natural posi- 



