We see this in the want
of any close correspondence between the degree of sterility and the
amount of external difference in the species which are crossed; and
still more clearly in the wide difference in the results of crossing
reciprocally the same two species;--that is, when species A is crossed
with pollen from B, and then B is crossed with pollen from A. Bearing in
mind what has just been said on the extreme sensitiveness and delicate
affinities of the reproductive system, why should we feel any surprise
at the sexual elements of those forms, which we call species, having
been differentiated in such a manner that they are incapable or only
feebly capable of acting on one another? We know that species have
generally lived under the same conditions, and have retained their own
proper characters, for a much longer period than varieties.
Long-continued domestication eliminates, as I have shown in my
'Variation under Domestication,' the mutual sterility which distinct
species lately taken from a state of nature almost always exhibit when
intercrossed; and we can thus understand the fact that the most
different domestic races of animals are not mutually sterile. But
whether this holds good with cultivated varieties of plants is not
known, though some facts indicate that it does. The elimination of
sterility through long-continued domestication may probably be
attributed to the varying conditions to which our domestic animals have
been subjected; and no doubt it is owing to this same cause that they
withstand great and sudden changes in their conditions of life with far
less loss of fertility than do natural species.
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