ions in important points of structure between the different castes
of neuters in the same species, that I gladly availed myself of Mr. F.
Smith's offer of numerous specimens from the same nest of the driver
ant (Anomma) of West Africa. The reader will perhaps best appreciate
the amount of difference in these workers, by my giving not the actual
measurements, but a strictly accurate illustration: the difference was
the same as if we were to see a set of workmen building a house of whom
many were five feet four inches high, and many sixteen feet high; but
we must suppose that the larger workmen had heads four instead of three
times as big as those of the smaller men, and jaws nearly five times
as big. The jaws, moreover, of the working ants of the several sizes
differed wonderfully in shape, and in the form and number of the teeth.
But the important fact for us is, that though the workers can be grouped
into castes of different sizes, yet they graduate insensibly into each
other, as does the widely-different structure of their jaws. I speak
confidently on this latter point, as 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 m