urbed 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 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
species goes, is, that there are such things in Nature as groups of
animals and of plants, whose members are incapable of fertile