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 differences in the reproductive system. In all
other respects, excluding fertility, there is a close general
resemblance between hybrids and mongrels."--Pp. 276-8.

We fully agree with the general tenor of this weighty passage; but
forcible as are these arguments, and little as the value of fertility
or infertility as a test of species may be, it must not be forgotten
that the really important fact, so far as the inquiry into the origin of