two individuals who share
them unite; these will produce offspring bearing similar
characteristics, and, if successive generations restrict themselves to
similar unions, a distinct race will then be formed. But perpetual
intermixture will cause all characters acquired through particular
circumstances to disappear. If it were not for the distances which
separate the races of men, such intermixture would quickly obliterate
all national distinctions.

_IV.--The Conclusion_

Here, then, is the conclusion to which we have come. It is a fact that
every genus and species of animal has its characteristic habits combined
with an organisation perfectly in harmony with them. From the
consideration of this fact one of two conclusions must follow, and that
though neither of them can be proved.

(1) The conclusion admitted hitherto--that nature (or its Author) in
creating the animals has foreseen all the possible sets of circumstances
in which they would have to live, has given to each species a constant
organisation, and has shaped its parts in a determined and invariable
way so that every species is compelled to live in the districts and the
climates where it is actually formed, and to keep the habits by which it
is actually known.

(2) My own conclusion--that nature has produced in succession all the
animal species, beginning with the more imperfect, or the simpler, and
ending with the more perfect; that in so doing it has gradually
complicated their organisation; and that of these animals, dispersed
over the habitable globe, every species has acquired, under the
influence of the circumstances amid which it is found, the habits and
modifications of form which we associate with it.

To prove that the second of these hypotheses is unfounded, it will be
necessary, first, to prove that the surface of the globe never varies in
character, in exposure, situation, whether elevated or sheltered,
climate, etc.; and, secondly, to prove that no part of the animal world
undergoes, even in the course of long periods of time, any modification
through change of circumstances, or as a consequence of a changed manner
of life and action.

Now, a single fact which establishes that an animal, after a long period
of domestication, differs from the wild stock from which it derives, and
that among the various domesticated members of a species may be found
differences no less marked between individuals which, have been
subjected to one use and