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 disuse 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 difficulty, 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 history, of "Natura non facit saltum,"
is applicable to instincts as well as to corporeal structure, and is
plainly explicable on the foregoing views, but is otherwise
inexplicable,--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 inhabiting distant parts of the world and living
under considerably different conditions of life, yet often retaining nearly
the same instincts. For instance, we can understand 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 America, build "cock-nests," to roost
in, like the males of our distinct Kitty-wrens,--a habit wholly unlike that
of {244} any other known bird. Finally, it may not be a logical deduction,
but to my imagination it is far more satisfactory to look at such instincts
as the young cuckoo ejecting its foster-brothers,--ants making slaves,--the
larvae of ichneumonidae feeding within the live bodies of caterpillars,--not
as specially endowed or created instincts, but as small consequences of one
general law, leading to the advancement of all organic beings, namely,
multiply, vary, let the strongest live and the weakest die.

* * * * *

{245}

CHAPTER VIII.

HYBRIDISM.

Distinction between the sterility of first crosses and of
hybrids--Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication--Laws governing the sterility
of hybrids--Sterility not a special endowment, but incidental on other
differences--Cause