The Descent of Man

ternal structure or to ordinary differences in
constitution. One important element in the sterility
of crossed species apparently lies in one
or both having been long habituated to fixed
conditions; for we know that changed conditions
have a special influence on the reproductive
system, and we have good reason to believe
(as before remarked) that the fluctuating
conditions of domestication tend to eliminate
that sterility which is so general with species, in
a natural state, when crossed. It has elsewhere
been shewn by me (ibid. vol. ii. p. 185, and 'Origin
of Species,' 5th edit. p. 317), that the sterility
of crossed species has not been acquired
through natural selection: we can see that when
two forms have already been rendered very
sterile, it is scarcely possible that their sterility
should be augmented by the preservation or
survival of the more and more sterile individuals;
for, as the sterility increases, fewer and
fewer offspring will be produced from which to
breed, and at last only single individuals will