Chap. VIII. 
HYBKIDISM. 
245 
CHAPTEE VIIL 
Hybkidism. 
Distinction between the sterility of first crosses and of hybrids— 
Sterility various in degree, not universal, affected by close inter¬ 
breeding, removed by domestication—Laws governing the sterility 
of hybrids — Sterility not a special endowment, but incidental 
on other differences — Causes of the sterility of first crosses and 
of hybrids — Parallelism between the effects of changed con¬ 
ditions of life and crossing — Fertility of varieties when crossed 
and of their mongrel offspring not universal — Hybrids and 
mongrels compared independently of their fertility — Summary. 
The view generally entertained by naturalists is that 
species, when intercrossed, have been specially endowed 
with the quality of sterility, in order to prevent the con¬ 
fusion of all organic Torms. This view certainly seems 
at first probable, for species within the same country 
could hardly have kept distinct had they been capable 
of crossing freely. The importance of the fact that hy¬ 
brids are very generally sterile, has, I think, been much 
underrated by some late writers. On the theory of na¬ 
tural selection the case is especially important, inas¬ 
much as the sterility of hybrids could not possibly be of 
any advantage to them, and therefore could not have 
been acquired by the continued preservation of succes¬ 
sive profitable degrees of sterility. I hope, however, to 
be able to show that sterility is not a specially acquired 
or endowed quality, but is incidental on other acquired 
differences. 
In treating this subject, two classes of facts, to a large 
extent fundamentally different, have generally been con¬ 
founded together; namely, the sterility of two species 
