on, the
front limbs and even the head would probably be affected. 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.

The foregoing remarks lead me to say a few words on the protest lately
made by some naturalists, against the utilitarian doctrine that every
detail of structure has been produced for the good of its possessor.
They believe that very many structures have been created for beauty in
the eyes of man, or for mere variety. This doctrine, if true, would be
absolutely fatal to my theory. Yet I fully admit that many structures
are of no direct use to their possessors. Physical conditions probably
have had some little effect on structure, quite independently of any
good thus gained. Correlation of growth has no doubt played a most
important part, and a useful modification of one part will often have
entailed on other parts diversified chan