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 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.
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