al selection: cases
in which we cannot see how an instinct could possibly have originated;
cases in which no intermediate gradations are known to exist; cases of
instinct of such trifling importance that they could hardly have been
acted upon 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 cannot believe that they were independently acquired
through natural selection. I will not here enter on those 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
neuters, or sterile females in insect communities; for these neuters
often differ widely in instinct and structure from both the males and
the fertile females, and yet, from being sterile, they cannot propagate
their kind." (p. 289) He is candid enough to say, in conclusion, "I do
not pretend that the facts given in this chapter (on instinct)
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it." (p. 297) When it
is remembered that his theory is, that slight variations occurring in an
individual advantageous to it (not to its associates), in the struggle
for life, is perpetuated by inheritance, it is no wonder that the case
of sterile ants gave him so much trouble. Accidental sterility is not
favorable to the individual, and its being made permanent by
inheritance, is out of the question, for the sterile have no
descendants. Yet these sterile females are not degenerations, they are
in general larger and more robust than their associates.

We have thus seen that, according to Mr. Darwin, all the infinite
variety of structure in plants and animals is due to the law of natural
selection. "On the principle of natural selection with divergence of
character," he says, "it does not seem incredible that, from some such
low and intermediate form, both animals and plants have been developed,
and if we admit this, we must likewise admit that all the organized
beings which have ever lived on this earth may be descended from some
one primordial form." (p. 573) We have seen also that he does not
confine his theory to organic structure, but applies it to all the
instincts and all the forms of intelligence manifested by irrational
creatures. Nor does he stop th