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 possibly have originated; cases, in which no
intermediate gradations are known to exist; cases of instinct of
apparently 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 parent, and
must therefore believe that they have been acquired by independent
acts of 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 my 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 very great difficulty in this being effected
by natural selection.
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