This is almost certainly
the case with the dog, and probably with the hog, the ox, and the sheep;
yet the various breeds are now all perfectly fertile, although we have
every reason to suppose that there would be some degree of infertility
if the several aboriginal species were crossed together for the first
time.
_Parallelism between Crossing and Change of Conditions._
In the whole series of these phenomena, from the beneficial effects of
the crossing of different stocks and the evil effects of close
interbreeding, up to the partial or complete sterility induced by
crosses between species belonging to different genera, we have, as Mr.
Darwin points out, a curious parallelism with the effects produced by
change of physical conditions. It is well known that slight changes in
the conditions of life are beneficial to all living things. Plants, if
constantly grown in one soil and locality from their own seeds, are
greatly benefited by the importation of seed from some other locality.
The same thing happens with animals; and the benefit we ourselves
experience from "change of air" is an illustration of the same
phenomenon.
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