SEXUAL SELECTION 219 



white felt which fills a folded-over portion of the edge of the 

 posterior wing. In many cases the perfume can be retained, and 

 then, by a sudden turning out of the wing-fold, be allowed to stream 

 forth. But there are a great many species of butterfly which do 

 not possess odoriferous scales, and they are often wanting in near 

 relatives of fragrant species ; they are obviously of very late origin, 

 and arose only after the majority of our modern species were already 

 differentiated. It often seems as if they bore a compensatory relation 

 to beauty of colour, somewhat in the same way as many modestly 

 coloured flowers develop a strong perfume, while, conversely, many 

 magnificently coloured fiowers have no scent at all. Although among 

 butterflies, as among flowers, there are species which possess both 

 beauty and fragrance, yet our most beautiful diurnal butterflies, 

 the Vanessas, the Apaturas, and Limenitis, possess no scent-scales ; 

 and many inconspicuous, that is, protectively coloured nocturnal 

 Lepidoptei-a, are strongly fragrant, like most night-flowers: I need 

 only mention the convolvulus hawk-moth (Sphinx convolvuli), whose 

 m.usk-like odour was known to entomologists long before the 

 discovery of scent-scales. 



It is, however, always only in the males that this odoriferous 

 apparatus is present. It must not be believed on this account that 

 this fragrance has the significance of a means of attraction comparable 

 to the perfume of the flowei-s which induces butterflies to visit them ; 

 indeed, we cannot assume that the odour carries to a distance, 

 for, as far as we can make out, it is perceptible only within 

 a very short radius, and this is indicated also by the manifold 

 arrangements of the odoriferous organs, which are all calculated to 

 retain the fragrance, and then — in the immediate neighbourhood of the 

 female — to let it suddenly stream forth. Obviously, this arrangement 

 can have no other significance than that of a sexual excitant ; its use 

 is to incline the female to the male, to fascinate her, just as do the 

 beautiful colours, in regard to which we must draw the same inference. 

 It is in this direction that the already mentioned relation of compensation 

 between beautiful colours and pleasant odours is particularly interesting, 

 for it confirms our interpretation of the decorative colours as a means 

 of sexual excitement. The most delicately fragrant or the most 

 beautifully coloured males were those which most excited the females, 

 and thus most easily attained to reproduction. The expression used 

 by Darwin, that the females ' choose,' must be taken metaphorically ; 

 they do not exercise a conscious choice, but they follow the male 

 which excites them most strongly. Thus there arises a process of 

 selection among these distinctively male characteristics. 



