eed of cattle, always yielding oxen with extraordinarily
long horns, could be slowly formed by carefully watching which
individual bulls and cows, when matched, produced oxen with the longest
horns; and yet no one ox could ever have propagated its kind. Thus
I believe it has been with social insects: a slight modification of
structure, or instinct, correlated with the sterile condition of certain
members of the community, has been advantageous to the community:
consequently the fertile males and females of the same community
flourished, and transmitted to their fertile offspring a tendency to
produce sterile members having the same modification. And I believe
that this process has been repeated, until that prodigious amount of
difference between the fertile and sterile females of the same species
has been produced, which we see in many social insects.

But we have not as yet touched on the climax of the difficulty; namely,
the fact that the neuters of several ants differ, not only from the
fertile females and males, but from each other, sometimes to an almost
incredible degree, and are thus divided into two or even three castes.
The castes, moreover, do not generally graduate into each other, but are
perfectly well defined; being as distinct from each other, as are any
two species of the same genus, or rather as any two genera of the same
family. Thus in Eciton, there are working and soldier neuters, with jaws
and instincts extraordinarily different: in Cryptocerus, the workers of
one caste alone carry a wonderful sort of shield on their heads, the use
of which is quite unknown: in the Mexican Myrmecocystus, the workers of
one caste never leave the nest; they are fed by the workers of another
caste, and they have an enormously developed abdomen which secretes a
sort of honey, supplying the place of that excreted by the aphides, or
the domestic cattle as they may be called, which our European ants guard
or imprison.

It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate my theory. In the simpler
case of neuter insects all of one caste or of the same kind, which have
been rendered by natural selection, as I believe to be quite possible,
different from the fertile males and females,--in this case, we may
safely conclude from the analogy of ordinary variations, that each
successive, slight,