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 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 tools or weapons, 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.
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