The
naked skin on the head of a vulture is generally looked at as a direct
adaptation for wallowing in putridity; and so it may be, or it may
possibly be due to the direct action of putrid matter; but we should
be very cautious in drawing any such inference, when we see that the
skin on the head of the clean-feeding male turkey is likewise naked.
The sutures in the skulls of young mammals have been advanced as a
beautiful adaptation for aiding parturition, and no doubt they
facilitate, or may be indispensable for this act; but as sutures occur
in the skulls of young birds and reptiles, which have only to escape
from a broken egg, we may infer that this structure has arisen from
the laws of growth, and has been taken advantage of in the parturition
of the higher animals.
We are profoundly ignorant of the causes producing slight and
unimportant variations; and we are immediately made conscious of this
by reflecting on the differences in the breeds of our domesticated
animals in different countries,--more especially in the less civilized
countries where there has been but little artificial selection.
Careful observers are convinced that a damp climate affects the growth
of the hair, and that with the hair the horns are correlated. Mountain
breeds always differ from lowland breeds; and a mountainous country
would probably affect the hind limbs from exercising them more, and
possibly even the form of the pelvis; and then by the law of
homologous variation, the front limbs and even the head would probably
be affected.
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