The shape, also, of the pelvis might affect by pressure
the shape of the head of the young in the womb. The laborious
breathing necessary in high regions would, we have some reason to
believe, increase the size of the chest; and again correlation would
come into play. Animals kept by savages in different countries often
have to struggle for their own subsistence, and would be exposed to a
certain extent to natural selection, and individuals with slightly
different constitutions would succeed best under different climates;
and there is reason to believe that constitution and colour are
correlated. A good observer, also, states that in cattle
susceptibility to the attacks of flies is correlated with colour, as
is the liability to be poisoned by certain plants; so that colour
would be thus subjected to the action of natural selection. But we are
far too ignorant to speculate on the relative importance of the
several known and unknown laws of variation; and I have here alluded
to them only to show that, if we are unable to account for the
characteristic differences of our domestic breeds, which nevertheless
we generally admit to have arisen through ordinary generation, we
ought not to lay too much stress on our ignorance of the precise cause
of the slight analogous differences between species. I might have
adduced for this same purpose the differences between the races of
man, which are so strongly marked; I may add that some little light
can apparently be thrown on the origin of these differences, chiefly
through sexual selection of a particular kind, but without here
entering on copious details my reasoning would appear frivolous.
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