tely variable in individuals of the same species, and
is eminently susceptible 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 hybrid produced from this cross.

In the same manner as in grafting trees, the capacity of one species
or variety to take on another, is incidental on generally unknown
differences 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 them 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 them becoming inarched in our forests.

The sterility of first crosses between pure species, which have their
reproductive systems perfect, seems to depend on several circumstances;
in some cases largely on the early death of the embryo. The sterility of
hybrids, which have their reproductive systems imperfect, and which
have had this system and their whole organisation disturbed by being
compounded of two distinct species, seems closely allied to that
sterility which so frequently affects pure species, when their natural
conditions of life have been disturbed. This view is supported by a
parallelism of another kind;--namely, that the crossing of forms only
slightly different is favourable to the vigour and fertility of their
offspring; and that slight changes in the conditions of life are
apparently favourable to the vigour and fertility of all organic beings.
It is not surprising that the degree of difficulty in uniting two
species, and the degree of sterility of their hybrid-offspring should
generally correspond, though due to distinct causes; for both depend
on the amount of difference of some kind between the species which are
crossed. Nor is it surprising that the facility of effecting a first
cross, the fertility of the hybrids produced, and the capacity of being
grafted together--though this latter capacity evidently depends on
widely diffe