grounds for admitting
the directly opposite doctrine of Pallas, namely, that such conditions
generally eliminate this tendency; so that the domesticated descendants
of species, which in their natural state probably would have been in
some degree sterile when crossed, become perfectly fertile together.
With plants, so far is cultivation from giving a tendency towards
sterility between distinct species, that in several well-authenticated
cases already alluded to, certain plants have been affected in an
opposite manner, for they have become self-impotent, while still
retaining the capacity of fertilising, and being fertilised by, other
species. If the Pallasian doctrine of the elimination of sterility
through long-continued domestication be admitted, and it can hardly
be rejected, it becomes in the highest degree improbable that similar
conditions long-continued should likewise induce this tendency; though
in certain cases, with species having a peculiar constitution, sterility
might occasionally be thus caused. Thus, as I believe, we can understand
why, with domesticated animals, varieties have not been produced
which are mutually sterile; and why with plants only a few such cases,
immediately to be given, have been observed.

The real difficulty in our present subject is not, as it appears to me,
why domestic varieties have not become mutually infertile when crossed,
but why this has so generally occurred with natural varieties, as soon
as they have been permanently modified in a sufficient degree to take
rank as species. We are far from precisely knowing the cause; nor is
this surprising, seeing how profoundly ignorant we are in regard to the
normal and abnormal action of the reproductive system. But we can
see that species, owing to their struggle for existence with numerous
competitors, will have been exposed during long periods of time to more
uniform conditions, than have domestic varieties; and this may well make
a wide difference in the result. For we know how commonly wild animals
and plants, when taken from their natural conditions and subjected
to captivity, are rendered sterile; and the reproductive functions of
organic beings which have always lived under natural conditions would
probably in like manner be eminently sensitive to the influence of an
unnatural cross. Domesticated productions, on the other hand, which,
as shown by the mere fact of their domestication, were not originally
highly sensitive to cha