ural selection of the parents which generated them; until none with an
intermediate structure were produced.

Thus, as I believe, the wonderful fact of two distinctly defined castes of
sterile workers existing in the same nest, both widely different from each
other and from {242} their parents, has originated. We can see how useful
their production may have been to a social community of insects, on the
same principle that the division of labour is useful to civilised man. As
ants work by inherited instincts and by inherited organs or tools, and not
by acquired knowledge and manufactured 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 affect 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 {243} 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