nd
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. 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 {199} 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