But after all, the
cause more probably lies in some imperfection in the original act of
impregnation, causing the embryo to be imperfectly developed, rather than
in the conditions to which it is subsequently exposed.
In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is somewhat different. I have more than
once alluded to a large body of facts showing that, when animals and plants
are removed from their natural conditions, they are extremely liable to
have their reproductive systems seriously affected. This, in fact, is the
great bar to the domestication of animals. Between the sterility thus
superinduced and that of hybrids, there are many points of similarity. In
both cases the sterility is independent of general health, and is often
accompanied by excess of size or great luxuriance. In both cases the
sterility occurs in various degrees; in both, the male element is the most
liable to be affected; but sometimes the female more than the male. In
both, the tendency goes to a certain extent with systematic affinity, for
whole groups of animals and plants are rendered impotent by the same
unnatural conditions; and whole groups of species tend to produce sterile
hybrids. On the other hand, one species in a group will sometimes resist
great changes of conditions with unimpaired fertility; and certain species
in a group will produce unusually fertile hybrids.
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