ese two powers do not at
all necessarily go together. There are certain hybrids which, instead
of having, as is usual, an intermediate character between their two
parents, always closely resemble one of them; and such hybrids, though
externally so like one of their pure parent-species, are with rare
exceptions extremely sterile. So again among hybrids which are usually
intermediate in structure between their parents, exceptional and
abnormal individuals sometimes are born, which closely resemble one of
their pure parents; and these hybrids are almost always utterly sterile,
even when the other hybrids raised from seed from the same capsule have
a considerable degree of fertility. These facts show how completely the
fertility of a hybrid may be independent of its external resemblance to
either pure parent.

Considering the several rules now given, which govern the fertility
of first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, 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 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 the difference in the result of
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 inn