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, 

 80 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 its gradual intensijication 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 Hpecics wliich brood on an exposed nest. In the parrots one can 

 often observe that the general brilliant colouring of the male is found 



1 Tlie Evolution Theory, London, 190i, i. p. 219. 



