LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We will now consider a little more in detail the circumstances and
rules governing the sterility of first crosses and of hybrids. Our chief
object will be to see whether or not the rules indicate that species
have specially been endowed with this quality, in order to prevent their
crossing and blending together in utter confusion. The following rules
and conclusions are chiefly drawn up from Gartner's admirable work on
the hybridisation of plants. I have taken much pains to ascertain how
far the rules apply to animals, and considering how scanty our knowledge
is in regard to hybrid animals, I have been surprised to find how
generally the same rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be shown to
exist; but only the barest outline of the facts can here be given. When
pollen from a plant of one family is placed on the stigma of a plant of
a distinct family, it exerts no more influence than so much inorganic
dust. From this absolute zero of fertility, the pollen of different
species of the same genus applied to the stigma of some one species,
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 will produce. So in hybrids themselves,
there are some which never have produced, and probably never would
produce, even 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 c