mal cases, even
to an excess of fertility, beyond that which the plant's own pollen will
produce. So in hybrids themselves, there are some which never have
produced, and probably never would produce, even {256} with the pollen of
either pure parent, a single fertile seed: but in some of these cases a
first trace of fertility may be detected, by the pollen of one of the pure
parent-species causing the flower of the hybrid to wither earlier than it
otherwise would have done; and the early withering of the flower is well
known to be a sign of incipient fertilisation. From this extreme degree of
sterility we have self-fertilised hybrids producing a greater and greater
number of seeds up to perfect fertility.

Hybrids from two species which are very difficult to cross, and which
rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced--two classes of facts which are
generally confounded together--is by no means strict. There are many cases,
in which two pure species can be united with unusual facility, and produce
numerous hybrid-offspring, yet these hybrids are remarkably sterile. On the
other hand, there are species which can be crossed very rarely, or with
extreme difficulty, but the hybrids, when at last produced, are very
fertile. Even within the limits of the same genus, for instance in
Dianthus, these two opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is the fertility of pure species.
But the degree of fertility is likewise innately variable; for it is not
always the same when the same two species are crossed under the same
circumstances, but depends in part upon the constitution of the individuals
which happen to have been chosen for the experiment. So it is with hybrids,
for their degree of fertility is often found to differ greatly in the
several individuals raised from seed out of the same capsule and exposed to
exactly the same conditions. {257}

By the term systematic affinity is meant, the resemblance between species
in structure and in constitution, more especially in the structure of parts
which are of high physiological importance and which differ little in the
allied species. Now the fertility of first crosses between species, and of
the hybrids produced from them, is largely governed by their systematic
af