46 The Selection Theory 



merely to give notice of the presence of individuals of the other sex, 

 but it soon became an excitant, and as the individuals which caused 

 the greatest degree of excitement were preferred, it reached as high 

 a pitch of perfection as was possible to it. I shall confine myself here 

 to the comparatively recently discovered fragrance of butterflies. 

 Since Fritz Miiller found out that certain Brazilian butterflies 

 gave off" fragrance " like a flower," we have become acquainted with 

 many such cases, and we now know that in all lands, not only many 

 diurnal Lepidoptera but nocturnal ones also give off a delicate odour, 

 which is agreeable even to man. The ethereal oil to which this 

 fragrance is due is secreted by the skin-cells, usually of the wing, as 

 I showed soon after the discovery of the scent-scales. This is the 

 case in the males ; the females have no special scent-scales recog- 

 nisable as such by their form, but they must, nevertheless, give off 

 an extremely delicate fragrance, although our imperfect organ of 

 smell cannot perceive it, for the males become aware of the presence 

 of a female, even at night, from a long distance off, and gather round 

 her. We may therefore conclude, that both sexes have long given 

 forth a very delicate perfume, which announced their presence to 

 others of the same species, and that in many species (not in all) these 

 small beginnings became, in the males, particularly strong scent-scales 

 of characteristic form (lute, brush, or lyre-shaped). At first these 

 scales were scattered over the surface of the wing, but gradually they 

 concentrated themselves, and formed broad, velvety bands, or strong, 

 prominent brushes, and they attained their highest pitch of evolution 

 when they became enclosed within pits or folds of the skin, which 

 could be opened to let the delicious fragrance stream forth suddenly 

 towards the female. Thus in this case also we see that characters, 

 the original use of which was to bring the sexes together, and so to 

 maintain the species, have been evolved in the males into means for 

 exciting the female. And we can hardly doubt, that the females are 

 most readily enticed to yield to the butterfly that sends out the 

 strongest fragrance, that is to say, that excites them to the highest 

 degree. It is a pity that our organs of smell are not fine enough 

 to examine the fragrance of male Lepidoptera in general, and to 

 compare it with other perfumes which attract these insects 1 . As far 

 as we can perceive them they resemble the fragrance of flowers, but 

 there are Lepidoptera whose scent suggests musk. A smell of musk 

 is also given oft' by several plants : it is a sexual excitant in the 

 musk-deer, the musk-sheep, and the crocodile. 



As far as we know, then, it is perfumes similar to those of flowers 

 that the male Lepidoptera give off in order to entice their mates, 

 and this is a further indication that animals, like plants, can to a 



1 See Poulton, Essays on Evolution, 1908, pp. 316, 317. 



