There are, in fact, many indications of a regular succession of tints in
which colour development has occurred in the various groups of
butterflies, from an original grayish or brownish neutral tint. Thus in
the "Aeneas" group of Papilios we have the patch on the upper wings
yellowish in P. triopas, olivaceous in P. bolivar, bronzy-gray with a
white spot in P. erlaces, more greenish and buff in P. iphidamas,
gradually changing to the fine blue of P. brissonius, and the
magnificent green of P. sesostris. In like manner, the intense crimson
spots of the lower wings can be traced step by step from a yellow or
buff tint, which is one of the most widespread colours in the whole
order. The greater purity and intensity of colour seem to be usually
associated with more pointed wings, indicating greater vigour and more
rapid flight.
_Sexual Selection as a supposed Cause of Colour Development._
Mr. Darwin, as is well known, imputed most of the brilliant colours and
varied patterns of butterflies' wings to sexual selection--that is, to a
constant preference, by female butterflies, for the more brilliant
males; the colours thus produced being sometimes transmitted to the
males alone, sometimes to both sexes.
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