SEXUAL SELECTION 221 



of this species, which I had never believed to be so numerous in the 

 gardens of the town. The males of the nocturnal Lepidoptera 

 obviously possess an incredibly delicate organ of smell, and its bearers, 

 the antennffi, are usually larger and more complex in structure in the 

 male sex than in the female. 



Butterflies are by no means the only creatures that produce a pecu- 

 liar odour at the breeding season ; many other animals do the same, 

 though in their case it does not seem so pleasant to our sense of smell. 

 It is true that the scent of the musk-deer and that of the beaver 

 (Gastoreuvi), when much diluted, are agreeable to man, but others, like 

 the odours exhaled by stags or by beasts of prey, are very disagreeable 

 to us, though they have for the species that produce them the same 

 significance as the others, and are therefore to be referred to sexual 

 selection. 



Darwin referred all the different 'mechanisms for the 'production 

 of sounds, up to the song of birds, to sexual selection, but it is probable 

 that natural selection has also to do with this in many ways. It is 

 certainly only the males which produce the well-known song of the 

 Cicadas, crickets, grasshoppers and birds, and I do not see any reason 

 to doubt that this ' music ' affects the females by arousing sexual 

 excitement. To some extent, then, the rivalry among the males for 

 the possession of the females — that is to say, sexual selection — must 

 have produced these mechanisms of song; and how long-continued 

 and gradual the accumulations must have been which produced the song 

 of the thrush or of the nightingale from the chirping of the sparrow we 

 may learn from the innumerable species which, as regards beauty of 

 song, may be ranged between these two extremes. 



My assumption that natural selection has also been operative in 

 the case of the song of insects and birds is based on the fact that many 

 of our songsters live widely scattered, and that the characteristic note 

 must be a means by which the two sexes find each other; That they 

 should find each other is an indispensable condition for the main- 

 tenance of the species. Thus it is well known that each species has 

 a characteristic ' note ' or love-call, which the male utters during the 

 breeding season, and which is answered by the female. From this 

 simple love-call the modern song of many species must have developed 

 by means of sexual selection. 



It is remarkable that here again the various distinguishing 

 characters of the male seem to be often mutually restrictive or 

 mutually exclusive. The best singers among our birds are incon- 

 spicuously coloured, grey or brown-grey, and this can hardly be 

 regarded as due to chance, but as the outcome of a greater sensitiveness 



