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. 
