Chap. VII. 
SUMMAEY. 
243 
to show that the mental qualities of our domestic ani- < 
mals vary, and that the variations are inherited. Still 
more briefly I have attempted to show that instincts 
vary slightly in a state of nature. No one will dispute 
that instincts are of the highest importance to each ani¬ 
mal. Therefore I can see no difficulty, under changing 
conditions of life, in natural selection accumulating 
slight modifications of instinct to any extent, in any 
useful direction. In some cases habit or use and dis¬ 
use have probably come into play. I do not pretend 
that the facts given in this chapter strengthen in any 
great degree my theory; but none of the cases of diffi¬ 
culty, to the ‘ best of my judgment, annihilate it. On 
the other hand, the fact that instincts are not always 
absolutely perfect and are liable to mistakes;—that no 
instinct has been produced for the exclusive good of 
other animals, but that each animal takes advantage of 
the instincts of others ;—that the canon in natural his¬ 
tory, of Natura non facit saltum,” is applicable to in¬ 
stincts as well as to corporeal structure, and is plainly 
explicable on the foregoing views, but is otherwise inex¬ 
plicable,—all tend to corroborate the theory of natural 
selection. 
This theory is, also, strengthened by some few other 
facts in regard to instincts; as by that common case of 
closely allied, but certainly distinct, species, when inhabit¬ 
ing distant parts of the world and living under consi¬ 
derably different conditions of life, yet often retaining 
nearly the same instincts. For instance, we can under¬ 
stand on the principle of inheritance, how it is that the 
thrush of South America lines its nest with mud, in the 
same peculiar manner as does our British thrush : how 
it is that the male wrens (Troglodytes) of North Ame¬ 
rica, build cock-nests,” to roost in, like the males of 
our distinct Kitty-wrens,—a habit wholly unlike that of 
M 2 
