When, as in these
cases, one sex differs much from the other, and varies greatly itself,
it may be that individual variations will occasionally occur, having a
distant resemblance to groups which are the objects of mimicry, and
which it is therefore advantageous to resemble. Such a variety will have
a better chance of preservation; the individuals possessing it will be
multiplied; and their accidental likeness to the favoured group will be
rendered permanent by hereditary transmission, and each successive
variation which increases the resemblance being preserved, and all
variation departing from the favoured type having less chance of
preservation, there will in time result those singular cases of two or
more isolated and fixed forms bound together by that intimate
relationship which constitutes them the sexes of a single species. The
reason why the females are more subject to this kind of modification
than the males is probably that their slower flight when laden with
eggs, and their exposure to attack while in the act of depositing their
eggs upon leaves, render it especially advantageous for them to have
additional protection.
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