imate unions
of dimorphic and trimorphic plants and of their illegitimate progeny,
perhaps render it probable that some unknown bond in all cases connects
the degree of fertility of first unions with that of their offspring.
The consideration of these facts on dimorphism, as well as of the
results of reciprocal crosses, clearly leads to the conclusion that
the primary cause of the sterility of crossed species is confined to
differences in their sexual elements. But why, in the case of distinct
species, the sexual elements should so generally have become more or
less modified, leading to their mutual infertility, we do not know; but
it seems to stand in some close relation to species having been exposed
for long periods of time to nearly uniform conditions of life.

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.
Independently of the question of fertility, in all other respects there
is the closest general resemblance between hybrids and mongrels, in
their variability, in their power of absorbing each other by repeated
crosses, and in their inheritance of ch