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.