ning the facility of first crosses are
incidental on unknown differences in their reproductive systems. These
differences in both cases follow, to a certain extent, as might
have been expected, systematic affinity, by which term every kind of
resemblance and dissimilarity between organic beings is attempted to be
expressed. The facts by no means seem to indicate that the greater or
lesser difficulty of either grafting or crossing various species
has been a special endowment; although in the case of crossing, the
difficulty is as important for the endurance and stability of specific
forms as in the case of grafting it is unimportant for their welfare.

ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

At one time it appeared to me probable, as it has to others, that
the sterility of first crosses and of hybrids might have been slowly
acquired through the natural selection of slightly lessened degrees of
fertility, which, like any other variation, spontaneously appeared in
certain individuals of one variety when crossed with those of another
variety. For it would clearly be advantageous to two varieties or
incipient species if they could be kept from blending, on the same
principle that, when man is selecting at the same time two varieties, it
is necessary that he should keep them separate. In the first place,
it may be remarked that species inhabiting distinct regions are often
sterile when crossed; now it could clearly have been of no advantage
to such separated species to have been rendered mutually sterile,
and consequently this could not have been effected through natural
selection; but it may perhaps be argued, that, if a species was rendered
sterile with some one compatriot, sterility with other species would
follow as a necessary contingency. In the second place, it is almost as
much opposed to the theory of natural selection as to that of special
creation, that in reciprocal crosses the male element of one form should
have been rendered utterly impotent on a second form, while at the same
time the male element of this second form is enabled freely to fertilise
the first form; for this peculiar state of the reproductive system could
hardly have been advantageous to either species.

In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty
will be found to lie in the existence of many graduated steps, from
slightl