194 NATURAL SELECTION ix 



The brain of prehistoric and of savage man seems to me to 

 prove the existence of some power distinct from that which 

 has guided the development of the lower animals through 

 their ever-varying forms of being. 



The Use of the Hairy Covering of Mammalia 

 Let us now consider another point in man's organisation, 

 the bearing of which has been almost entirely overlooked by 

 writers on both sides of this question. One of the most 

 general external characters of the terrestrial mammalia is 

 the hairy covering of the body, which, whenever the skin is 

 flexible, soft, and sensitive, forms a natural protection against 

 the severities of climate, and particularly against rain. That 

 this is its most important function is well shown by the 

 manner in which the hairs are disposed so as to carry off the 

 water, by being invariably directed downwards from the most 

 elevated parts of the body. Thus, on the under surface the 

 hair is always less plentiful, and, in many cases, the belly is 

 almost bare. The hair lies downwards, on the limbs of all 

 walking mammals, from the shoulder to the toes ; but in the 

 orang-utan it is directed from the shoulder to the elbow, and 

 again from the wrist to the elbow, in a reverse direction. 

 This corresponds to the habits of the animal, which, when 

 resting, holds its long arms upwards over its head, or clasping 

 a branch above it, so that the rain would flow down both the 

 arm and forearm to the long hair which meets at the elbow. 

 In accordance with this principle, the hair is always longer 

 or more dense along the spine or middle of the back 

 from the nape to the tail, often rising into a crest of hair 

 or bristles on the ridge of the back. This character prevails 

 through the entire series of the mammalia, from the mar- 

 supials to the quadrumana, and by this long persistence it 

 must have acquired such a powerful hereditary tendency 

 that we should expect it to reappear continually even 

 after it had been abolished by ages of the most rigid 

 selection; and we may feel sure that it never could have 

 been completely abolished under the law of natural selec- 

 tion, unless it had become so positively injurious as to 

 lead to the almost invariable extinction of individuals 

 possessing it. 



