The sterility is innately variable in
individuals of the same species, and is eminently susceptible to action of
favourable and unfavourable conditions. The degree of sterility does not
strictly follow systematic affinity, but is governed by several curious and
complex laws. It is generally different, and sometimes widely different in
reciprocal crosses between the same two species. It is not always equal in
degree in a first cross and in the hybrids produced from this cross.
In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally of an
unknown nature, in their vegetative systems, so in crossing, the greater or
less facility of one species to unite with another is incidental on unknown
differences in their reproductive systems. There is no more reason to
think that species have been specially endowed with various degrees of
sterility to prevent their crossing and blending in nature, than to think
that trees have been specially endowed with various and somewhat analogous
degrees of difficulty in being grafted together in order to prevent their
inarching in our forests.
The sterility of first crosses and of their hybrid progeny has not been
acquired through natural selection. In the case of first crosses it seems
to depend on several circumstances; in some instances in chief part on the
early death of the embryo.
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