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 changes in their conditions of life, and
which can now generally resist with undiminished fertility repeated changes
of conditions, might be expected to produce varieties, which would be
little liable to have their reproductive powers injuriously affected by the
act of crossing with other varieties which had originated in a like manner.
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