220 THE EVOLUTION THEORY 



If the odoriferous organs we have been discussing had merely 

 been a means of attraction, serving to announce the proximity of 

 a member of the species, then they should have occurred, not in the 

 males but in the females, for these are sought out by the males, 

 not conversely. The males are able to track their desired mates 

 from great distances, and many remarkable examples of this are 

 known, some of them indeed sounding almost fabulous. The females 

 must therefore also exhale a fragrance, and perhaps continually, but 

 it is much more delicate, carries extraordinarily far, and is quite 

 imperceptible to our weak sense of smell. It is possible that it streams 

 out from all the scales covering the wings and body, for, as I long 

 ago pointed out, all the scales retain a connexion with the living cells 

 of the skin, however minute these may be, and it is therefore quite 

 possible tha.t the cells produce scent imperceptible by us, and let it 

 exhale through the ordinary scales, since the male scent-scales owe 

 their ethereal oil to the large gland-like cells of the hypodermis on 

 which they are placed. 



Here we see very clearly the difference between ordinary natural 

 selection and sexual selection. The male odoriferous organs depend 

 on the latter, for they do not serve for the maintenance of the species, 

 but are of advantage in the courting competitions among the males 

 for the possession of the females, while the assumed fragrant cells of 

 the females must depend on natural selection, since they are of general 

 importance for the mutual discovery of the sexes, which would other- 

 wise be in most cases impossible. This hypothetical ' species scent,' as 

 we may call it, is first of all useful in securing the existence of the 

 species, and must therefore be referred to natural selection. The other, 

 the ' male scent,' might be, and actually is, wanting in many species, 

 although it may be necessary to reproduction in cases where it has 

 become a male specific character, and could not be absent from any 

 male without dooming him to sterility. 



That the ' species scent ' really exists admits of no doubt, although 

 we may be unable to perceive it. Entomologists have long been in 

 the habit of catching the males of the rarer Lepidoptera, especially of 

 the nocturnal forms, by freely exposing a captive female. Some 

 years ago I kept for some time in my study, with a view to certain 

 experiments, females of the eyed hawk-moth (Smerinthus ocellatus), 

 and placed them at first, without any special intention, in a gauze- 

 covered vessel near the open window. The very next morning several 

 males had gathered and were sitting on the window-sill, or on the 

 wall of the room close to the vessel, and by continuing the experiment 

 I caught, in the course of nine nights, no fewer than forty-two males 



