anufactured instruments, a perfect
division of labour could be effected with them only by the workers being
sterile; for had they been fertile, they would have intercrossed, and
their instincts and structure would have become blended. And nature
has, as I believe, effected this admirable division of labour in the
communities of ants, by the means of natural selection. But I am bound
to confess, that, with all my faith in this principle, I should never
have anticipated that natural selection could have been efficient in so
high a degree, had not the case of these neuter insects convinced me
of the fact. I have, therefore, discussed this case, at some little
but wholly insufficient length, in order to show the power of natural
selection, and likewise because this is by far the most serious special
difficulty, which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any amount
of modification in structure can be effected by the accumulation of
numerous, slight, and as we must call them accidental, variations, which
are in any manner profitable, without exercise or habit having 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 tak