ough natural selection; and as
this sterility follows the same general laws in the vegetable as in the
animal kingdom, it is improbable, though apparently possible, that with
plants crossed species should have been rendered sterile by a different
process. From this consideration, and remembering that species which have
never co-existed in the same country, and which therefore could not have
received any advantage from having been rendered mutually infertile, yet
are generally sterile when crossed; and bearing in mind that in reciprocal
crosses between the same two species there is sometimes the widest
difference in their sterility, we must give up the belief that natural
selection has come into play.

As species have not been rendered mutually infertile through the
accumulative action of natural selection, and as we may safely conclude,
from the previous as well as from other and more general considerations,
that they have not been endowed through an act of creation with this
quality, we must infer that it has arisen incidentally during their slow
formation in connection with other and unknown changes in their
organisation. By a quality arising incidentally, I refer to such cases as
different species of animals and plants being differently affected by
poisons to which they are not naturally exposed; and this difference in
susceptibility is clearly incidental on other and unknown differences in
their organisation. So again the capacity in different kinds of trees to be
grafted on each other, or on a third species, differs much, and is of no
advantage to these trees, but is incidental on structural or functional
differences in their woody tissues. We need not feel surprise at sterility
incidentally resulting from crosses between distinct species,--the modified
descendants of a common progenitor,--when we bear in mind how easily the
reproductive system is affected by various causes--often by extremely
slight changes in the conditions of life, by too close interbreeding, and
by other agencies. It is well to bear in mind such cases, as that of the
_Passiflora alata_, which recovered its self-fertility from {189} being
grafted on a distinct species--the cases of plants which normally or
abnormally are self-impotent, but can readily be fertilised by the pollen
of a distinct species--and lastly the cases of individual domesticated
animals which evince towards each other sexual incompatibility.

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