This is
chiefly owing to the forms which are crossed having been exposed to
somewhat different conditions of life; for I have ascertained by a
labourious series of experiments that if all the individuals of the same
variety be subjected during several generations to the same conditions, the
good derived from crossing is often much diminished or wholly disappears.
This is one side of the case. On the other side, we know that species
which have long been exposed to nearly uniform conditions, when they are
subjected under confinement to new and greatly changed conditions, either
perish, or if they survive, are rendered sterile, though retaining perfect
health. This does not occur, or only in a very slight degree, with our
domesticated productions, which have long been exposed to fluctuating
conditions. Hence when we find that hybrids produced by a cross between
two distinct species are few in number, owing to their perishing soon after
conception or at a very early age, or if surviving that they are rendered
more or less sterile, it seems highly probable that this result is due to
their having been in fact subjected to a great change in their conditions
of life, from being compounded of two distinct organisations. He who will
explain in a definite manner why, for instance, an elephant or a fox will
not breed under confinement in its native country, whilst the domestic pig
or dog will breed freely under the most diversified conditions, will at the
same time be able to give a definite answer to the question why two
distinct species, when crossed, as well as their hybrid offspring, are
generally rendered more or less sterile, while two domesticated varieties
when crossed and their mongrel offspring are perfectly fertile.
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