to neglect other materials
and to make its nest exclusively of inspissated saliva? And so in other
cases. It must, however, be admitted that in many instances we cannot
conjecture whether it was instinct or structure which first varied.

No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection--cases, in which we cannot see how
an instinct could have originated; cases, in which no intermediate
gradations are known to exist; cases of instincts of such trifling
importance, that they could hardly have been acted on by natural
selection; cases of instincts almost identically the same in animals
so remote in the scale of nature that we cannot account for their
similarity by inheritance from a common progenitor, and consequently
must believe that they were independently acquired through natural
selection. I will not here enter on these several cases, but will
confine myself to one special difficulty, which at first appeared to
me insuperable, and actually fatal to the whole theory. I allude to
the neuters or sterile females in insect communities: for these neuters
often differ widely in instinct and in structure from both the males
and fertile females, and yet, from being sterile, they cannot propagate
their kind.

The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much greater
than that of any other striking modification of structure; for it can
be shown that some insects and other articulate animals in a state of
nature occasionally become sterile; and if such insects had been social,
and it had been profitable to the community that a number should have
been annually born capable of work, but incapable of procreation, I can
see no especial difficulty in this having been effected through natural
selection. But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from both the males
and the fertile females in structure, as in the shape of the thorax, and
in being destitute of wings and sometimes of eyes, and in instinct. As
far as instinct alone is concerned, the wonderful difference in this
respect between the workers and the perfect females would have been
better exemplified by the hive-bee. If a working ant or other neuter
insect had been an ordinary animal, I should hav