children the limit between the hairy scalp and the naked forehead is
sometimes not well defined; so that here we seem to have a trifling
case of reversion to a progenitor, in whom the forehead had not as yet
become quite naked.
It is well known that the hair on our arms tends to converge from above
and below to a point at the elbow. This curious arrangement, so unlike
that in most of the lower mammals, is common to the gorilla,
chimpanzee, orang, some species of Hylobates, and even to some few
American monkeys. But in Hylobates agilis the hair on the fore-arm is
directed downwards or towards the wrist in the ordinary manner; and in
H. lar it is nearly erect, with only a very slight forward inclination;
so that in this latter species it is in a transitional state. It can
hardly be doubted that with most mammals the thickness of the hair on
the back and its direction, is adapted to throw off the rain; even the
transverse hairs on the fore-legs of a dog may serve for this end when
he is coiled up asleep. Mr. Wallace, who has carefully studied the
habits of the orang, remarks that the convergence of the hair towards
the elbow on the arms of the orang may be explained as serving to throw
off the rain, for this animal during rainy weather sits with its arms
bent, and with the hands clasped round a branch or over its head.
According to Livingstone, the gorilla also sits in pelting rain with
his hands over his head. (8. Quoted by Reade, The African Sketch
Book, vol i. 1873, p. 152.) If the above explanation is correct, as
seems probable, the direction of the hair on our own arms offers a
curious record of our former state; for no one supposes that it is now
of any use in throwing off the rain; nor, in our present erect
condition, is it properly directed for this purpose.
It would, however, be rash to trust too much to the principle of
adaptation in regard to the direction of the hair in man or his early
progenitors; for it is impossible to study the figures given by
Eschricht of the arrangement of the hair on the human foetus (this
being the same as in the adult) and not agree with this excellent
observer that other and more complex causes have intervened. The points
of convergence seem to stand in some relation to those points in the
embryo which are last closed in during development. There appears,
also, to exist some relation between the arrangement of the hair on the
limbs, and the course of the medullary arteries. (9. On the hair in
Hylobates, see Natural History of Mammals, by C.L. Martin, 1841, p.
168