344 THE LIMITS OF NATURAL SELECTION 
The Use of the Hairy Covering of Mamunalia. 
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- 
