48 The Selection Theory 
are many weighty arguments in favour of it, and these form a body 
of presumptive evidence so strong that it almost amounts to 
certainty. 
In the first place, there is the analogy with other secondary sexual 
characters. If the song of birds and the chirping of the cricket have 
been evolved through sexual selection, if the penetrating odours of 
male animals,—the crocodile, the musk-deer, the beaver, the carni- 
vores, and, finally, the flower-like fragrances of the butterflies have 
been evolved to their present pitch in this way, why should decorative 
colours have arisen in some other way? Why should the eye be less 
sensitive to specifically male colours and other visible signs enticing 
to the female, than the olfactory sense to specifically male odours, 
or the sense of hearing to specifically male sounds? Moreover, the 
decorative feathers of birds are almost always spread out and dis- 
played before the female during courtship. I have elsewhere? pointed 
out that decorative colouring and sweet-scentedness may replace one 
another in Lepidoptera as well as in flowers, for just as some modestly 
coloured flowers (mignonette and violet) have often a strong perfume, 
while strikingly coloured ones are sometimes quite devoid of fragrance, 
so we find that the most beautiful and gaily-coloured of our native 
Lepidoptera, the species of Vanessa, have no scent-scales, while these 
are often markedly developed in grey nocturnal Lepidoptera. Both 
attractions may, however, be combined in butterflies, just as in flowers. 
Of course, we cannot explain why both means of attraction should 
exist in one genus, and only one of them in another, since we do not 
know the minutest details of the conditions of life of the genera 
concerned. But from the sporadic distribution of scent-scales in 
Lepidoptera, and from their occurrence or absence in nearly related 
species, we may conclude that fragrance is a relatively modern 
acquirement, more recent than brilliant colouring. 
One thing in particular that stamps decorative colouring as a 
product of selection is ¢és gradual intensification by the addition 
of new spots, which we can quite well observe, because in many 
cases the colours have been first acquired by the males, and later 
transmitted to the females by inheritance. The scent-scales are 
never thus transmitted, probably for the same reason that the deco- 
rative colours of many birds are often not transmitted to the females: 
because with these they would be exposed to too great elimination 
by enemies. Wallace was the first to point out that in species with 
concealed nests the beautiful feathers of the male occurred in the 
female also, as in the parrots, for instance, but this is not the case 
in species which brood on an exposed nest. In the parrots one can 
often observe that the general brilliant colouring of the male is found 
1 The Evolution Theory, London, 1904, 1. p. 219. 
