"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 the 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 hybrids produced from it, 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 which are
subjected to experiment; for systematic affinity attempts to express all
kinds of resemblance between all species.

"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 quite universally, fertile. Nor is this nearly general
and perfect fertility surprising, when we remember 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 not of