the males and females of the same species of butterfly are in several
cases known (26. H.W. Bates, The Naturalist on the Amazons, vol. ii.
1863, p. 228. A.R. Wallace, in Transactions, Linnean Society, vol.
xxv. 1865, p. 10.) to inhabit different stations, the former commonly
basking in the sunshine, the latter haunting gloomy forests. It is
therefore possible that different conditions of life may have acted
directly on the two sexes; but this is not probable (27. On this whole
subject see The Variation of Animals and Plants under Domestication,
1868, vol. ii. chap. xxiii.) as in the adult state they are exposed to
different conditions during a very short period; and the larvae of both
are exposed to the same conditions. Mr. Wallace believes that the
difference between the sexes is due not so much to the males having
been modified, as to the females having in all or almost all cases
acquired dull colours for the sake of protection. It seems to me, on
the contrary, far more probable that it is the males which have been
chiefly modified through sexual selection, the females having been
comparatively little changed. We can thus understand how it is that the
females of allied species generally resemble one another so much more
closely than do the males. They thus shew us approximately the
primordial colouring of the parent-species of the group to which they
belong. They have, however, almost always been somewhat modified by the
transfer to them of some of the successive variations, through the
accumulation of which the males were rendered beautiful. But I do not
wish to deny that the females alone of some species may have been
specially modified for protection. In most cases the males and females
of distinct species will have been exposed during their prolonged
larval state to different conditions, and may have been thus affected;
though with the males any slight change of colour thus caused will
generally have been masked by the brilliant tints gained through sexual
selection. When we treat of Birds, I shall have to discuss the whole
question, as to how far the differences in colour between the sexes are
due to the males having been modified through sexual selection for
ornamental purposes, or to the females having been modified through
natural selection for the sake of protection, so that I will here say
but little on the subject.
In all the cases in which the more common form of equal inheritance by
both sexes has prevailed, the selection of bright-coloured males would
tend to make the females bright-coloured; and the selection of
dull-coloured females would tend to make the males dull. If both
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