olivar, 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. This view has always seemed to me
to be unsupported by evidence, while it is also quite inadequate to
account for the facts. The only direct evidence, as set forth with his
usual fairness by Mr. Darwin himself, is opposed to his views. Several
entomologists assured him that, in moths, the females evince not the
least choice of their partners; and Dr. Wallace of Colchester, who has
largely bred the fine Bombyx cynthia, confirmed this statement. Among
butterflies, several males often pursue one female, and Mr. Darwin says,
that, unless the female exerts a choice the pairing must be left to
chance. But, surely, it may be the most vigorous or most persevering
male that is chosen, not necessarily one more brightly or differently
coloured, and this will be true "natural selection." Butterflies have
been noticed to prefer some coloured flowers to others; but that does
not prove, or even render probable, any preference for the colour
itself, but only for flowers of certain colours, on account of the more
agreeable or more abundant nectar obtained from them. Dr. Schulte called
Mr. Darwin's attention to the fact, that in the Diadema bolina the
brilliant blue colour surrounding the white spots is only visible when
we look towards the insect's head, and this is true of many of the
iridescent colours of butterflies, and probably depends upon the
direction of the striae on the scales. It is suggested, however, that
this display of colour will be seen by the female as the male is
approaching her, and that it has been developed by sexual
selection.[1