or even to fertility under
certain conditions in excess. That their fertility, besides being eminently
susceptible to favourable and unfavourable conditions, is innately
variable. That it is by no means always the same in degree in the first
cross and in the hybrids produced {260} from this cross. That the fertility
of hybrids is not related to the degree in which they resemble in external
appearance either parent. And lastly, that the facility of making a first
cross between any two species is not always governed by their systematic
affinity or degree of resemblance to each other. This latter statement is
clearly proved by reciprocal crosses between the same two species, for
according as the one species or the other is used as the father or the
mother, there is generally some difference, and occasionally the widest
possible difference, in the facility of effecting an union. The hybrids,
moreover, produced from reciprocal crosses often differ in fertility.

Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely different
in degree, when various species are crossed, all of which we must suppose
it would be equally important to keep from blending together? Why should
the degree of sterility be innately variable in the individuals of the same
species? Why should some species cross with facility, and yet produce very
sterile hybrids; and other species cross with extreme difficulty, and yet
produce fairly fertile hybrids? Why should there often be so great a
difference in the result of a reciprocal cross between the same two
species? Why, it may even be asked, has the production of hybrids been
permitted? to grant to species the special power of producing hybrids, and
then to stop their further propagation by different degrees of sterility,
not strictly related to the facility of the first union between their
parents, seems to be a strange arrangement.

The foregoing rules and facts, on the other hand, {261} appear to me
clearly to indicate that the sterility both of first crosses and of hybrids
is simply incidental or dependent on unknown differences, chiefly in the
reproductive systems, of the species which are crossed. The differences
being of so peculiar and limited a nature, that, in reciprocal crosses
between two species the male sexual element of the one will often fr