It is certain that with species the cause lies
exclusively in differences in their sexual constitution. Now the varying
conditions to which domesticated animals and cultivated plants have been
subjected, have had so little tendency towards modifying the reproductive
system in a manner leading to mutual sterility, that we have good 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.
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