, annihilate it. On the other
hand, the fact that instincts are not always absolutely perfect and are
liable to mistakes; that no instinct can be shown to have been produced
for the good of other animals, though animals take 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 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 tropical South America lines
its nest with mud, in the same peculiar manner as does our British
thrush; how it is that the Hornbills of Africa and India have the same
extraordinary instinct of plastering up and imprisoning the females in a
hole in a tree, with only a small hole left in the plaster through which
the males feed them and their young when hatched; how it is that the
male wrens (Troglodytes) of North America, build "cock-nests," to roost
in, like the males of our Kitty-wrens,--a habit wholly unlike that of
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.

CHAPTER IX. 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, not accumulated by natural selection--Causes of
the sterility of first crosses and of hybrids--Parallelism between the
effects of changed conditions of life and of crossing--Dimorphism and
trimor