rally 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 breeding 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 organization 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