'Chap. XI. BUTTERFLIES AND MOTHS. 391 
male having transferred his colours to the female, 
or to the male having retained, or perhaps reco- 
vered, the primordial colours of the genus to which the 
species belongs. It also deserves notice that in those 
groups in which the sexes present any difference of 
colour, the females usually resemble the males to a cer- 
tain extent, so that when the males are beautiful to an 
extraordinary degree, the females almost invariably ex- 
hibit some degree of beauty. From the numerous 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, whatever they may 
be, which have determined the brilliant colouring of the 
males alone of some species, and of both sexes in a more 
or less equal degree of other species, have generally 
been the same. 
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 4 has shewn 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 
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 
4 1 The Naturalist on the Amazons,’ vol. i. 1863, p. 19. 
