Finally, considering all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility, both
in first crosses and in hybrids, is an extremely general result; but that
it cannot, under our present state of knowledge, be considered as
absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We will now consider a little more in detail the 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.
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