344 THE LIMITS OF NATURAL SELECTION 



The Use of the Hairy Covering of Mammalia. 



Let us now consider another point in man's organi- 

 zation, 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 cli- 

 mate, 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 down- 

 wards, 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 corre- 

 sponds 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 fore-arm 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 marsupials to the quadru- 



