groups the females of the several species resemble each other much more
closely than do the males. In some cases, however, to which I shall
hereafter allude, the females are coloured more splendidly than the
males. In the second place, these details have been given to bring
clearly before the mind that within the same genus, the two sexes
frequently present every gradation from no difference in colour, to so
great a difference that it was long before the two were placed by
entomologists in the same genus. In the third place, we have seen that
when the sexes nearly resemble each other, this appears due either to
the male having transferred his colours to the female, or to the male
having retained, or perhaps recovered, the primordial colours of the
group. It also deserves notice that in those groups in which the sexes
differ, the females usually somewhat resemble the males, so that when
the males are beautiful to an extraordinary degree, the females almost
invariably exhibit some degree of beauty. From the many cases of
gradation in the amount of difference between the sexes, and from the
prevalence of the same general type of coloration throughout the whole
of the same group, we may conclude that the causes have generally been
the same which have determined the brilliant colouring of the males
alone of some species, and of both sexes of other species.
As so many gorgeous butterflies inhabit the tropics, it has often been
supposed that they owe their colours to the great heat and moisture of
these zones; but Mr. Bates (6. The Naturalist on the Amazons, vol. i.
1863, p. 19.) has shown by the comparison of various closely-allied
groups of insects from the temperate and tropical regions, that this
view cannot be maintained; and the evidence becomes conclusive when
brilliantly-coloured males and plain-coloured females of the same
species inhabit the same district, feed on the same food, and follow
exactly the same habits of life. Even when the sexes resemble each
other, we can hardly believe that their brilliant and
beautifully-arranged colours are the purposeless result of the nature
of the tissues and of the action of the surrounding conditions.
With animals of all kinds, whenever colour has been modified for some
special purpose, this has been, as far as we can judge, either for
direct or indirect protection, or as an attraction between the sexes.
With many species of butterflies the upper surfaces of the wings are
obscure; and this in all probability leads to their escaping
observation and danger. But butterflies would be particularly liable to
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