Mr. Lubbock made drawings for me with the camera lucida of the jaws
which I had dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, by acting
on the fertile parents, could form a species which should regularly
produce neuters, either all of large size with one form of jaw, or all
of small size with jaws having a widely different structure; or lastly,
and this is our climax of difficulty, one set of workers of one size
and structure, and simultaneously another set of workers of a different
size and structure; a graduated series having been first formed, as in
the case of the driver ant, and then the extreme forms, from being the
most useful to the community, having been produced in greater and
greater numbers through the natural 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 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
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