come into play. For no amount of exercise, or habit, or volition, in
the utterly sterile members of a community could possibly have affected
the structure or instincts of the fertile members, which alone leave
descendants. I am surprised that no one has advanced this demonstrative
case of neuter insects, against the well-known doctrine of Lamarck.
_Summary_. I have endeavoured briefly in this chapter to show that the
mental qualities of our domestic animals 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 animal. 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 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 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 larv of
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