laws governing the
sterility of first crosses and of hybrids. Our chief object will be to
see whether or not these laws indicate that species have been specially
endowed with this quality, in order to prevent their crossing and
blending together in utter confusion. The following conclusions are
drawn up chiefly from Gartner's admirable work on the hybridisation
of plants. I have taken much pains to ascertain how far they 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;
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 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 faci