Chap. VIII. 
LAWS OF STEKILITY. 
255 
and in hybrids, is an extremely general result; but that 
it cannot, under our present state of knowledge, be con¬ 
sidered as absolutely universal. 
Laws governing the Sterility of first Crosses and of Hy¬ 
brids ,—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 con¬ 
sidering how scanty our knowledge is in regard to hy¬ 
brid animals, I have been surprised to find how gene¬ 
rally the same rules apply to both kingdoms. 
It has been already remarked, that the degree of fer¬ 
tility, 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 differ¬ 
ent 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 
