Chap. VIII. 
FEKTILITY OF MONGEELS. 
267 
parallelism extends to an allied yet very different class 
of facts. It is an old and almost universal belief, 
founded, I think, on a considerable body of evidence, 
that slight changes in the conditions of life are bene¬ 
ficial to all living things. We see this acted on by 
farmers and gardeners in their frequent exchanges of 
seed, tubers, &c., from one soil or climate to another, 
and back again. During the convalescence of animals, 
we plainly see that great benefit is derived from almost 
any change in the habits of life. Again, both with 
plants and animals, there is abundant evidence, that a 
cross between very distinct individuals of the same spe¬ 
cies, that is between members of different strains or 
sub-breeds, gives vigour and fertility to the offspring. 
I believe, indeed, from the facts alluded to in our fourth 
chapter, that a certain amount of crossing is indispens¬ 
able even with hermaphrodites; and that close inter¬ 
breeding continued during several generations between 
the nearest relations, especially if these be kept under 
the same conditions of life, always induces weakness 
and sterility in the progeny. 
Hence it seems that, on the one hand, slight changes 
in the conditions of life benefit all organic beings, and 
on the other hand, that slight crosses, that is crosses 
between the males and females of the same species 
which have varied and become slightly different, give 
vigour and fertility to the offspring. But we have 
seen that greater changes, or changes of a particular 
nature, often render organic beings in some degree 
sterile; and that greater crosses, that is crosses between 
males and females which have become widely or spe¬ 
cifically different, produce hybrids which are generally 
sterile in some degree. I cannot persuade myself that 
this parallelism is an accident or an illusion. Both 
series of facts seem to be connected together by some 
N 2 
