llowing 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 cause of each slight variation or
individual difference; and we are immediately made conscious of this
by reflecting on the differences between the breeds of our domesticated
animals in different countries, more especially in the less civilized
countries, where there has been but little methodical selection. Animals
kept by savages in different countries often have to struggle for
their own subsistence, and are exposed to a certain extent to natural
selection, and individuals with slightly different constitutions would
succeed best under different climates. With cattle susceptibility to the
attacks of flies is correlated with colour, as is the liability to be
poisoned by certain plants; so that even colour would be thus subjected
to the action of natural selection. Some 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 the head would
probably be affected. The shape, also, of the pelvis might affect
by pressure the shape of certain parts of the young in the womb. The
laborious breathing necessary in high regions tends, as we have
good reason to believe, to increase the size of the chest; and again
correlation would come into play. The effects of lessened exercise,
together with abundant food, on the whole organisation is probably still
more important, and this, as H. von Nathusius has lately shown in
his excellent Treatise, is apparently one chief cause of the great
modification which the breeds