It is not surprising that the difficulty in crossing any two species, and
the sterility of their hybrid offspring, should in most cases correspond,
even if due to distinct causes: for both depend on the amount of
difference between the species which are crossed. Nor is it surprising
that the facility of effecting a first cross, and the fertility of the
hybrids thus produced, and the capacity of being grafted together--though
this latter capacity evidently depends on widely different
circumstances--should all run, to a certain extent, parallel with the
systematic affinity of the forms subjected to experiment; for systematic
affinity includes resemblances of all kinds.
First crosses between forms known to be varieties, or sufficiently alike to
be considered as varieties, and their mongrel offspring, are very
generally, but not, as is so often stated, invariably fertile. Nor is this
almost universal and perfect fertility surprising, when it is remembered
how liable we are to argue in a circle with respect to varieties in a state
of nature; and when we remember that the greater number of varieties have
been produced under domestication by the selection of mere external
differences, and that they have not been long exposed to uniform conditions
of life. It should also be especially kept in mind, that long-continued
domestication tends to eliminate sterility, and is therefore little likely
to induce this same quality.
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