From this
absolute zero of fertility, the pollen of different species applied to the
stigma of some one species of the same genus, yields a perfect gradation in
the number of seeds produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases, even to an
excess of fertility, beyond that which the plant's own pollen produces. So
in hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of the pure parents, 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.
The hybrids raised 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, as in the genus Verbascum, can be united
with unusual facility, and produce numerous hybrid offspring, yet these
hybrids are remarkably sterile.
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