SEXUAL SELECTION 227 



sexual selection, and not by the ordinary natural selection, because 

 those males with the more acute sense of smell had an advantage over 

 those in which it was blunter, so the males of the genus Moina which 

 could grasp most securely had an advantage over those that gripped 

 less firmly, and thus arose these two different kinds of male charac- 

 teristics. Neither of them is of advantage to the species as such, but 

 only to the males in their competition for the possession of the 

 females. 



But, where the production of a novel character in the male is 

 concerned, natural selection cannot proceed in a different manner 

 from sexual selection ; the process of selection is exactly the same : 

 the better equipped males sui-vive, the less well-equipped die without 

 begetting offspring ; the difference lies only in the fact that in the one 

 case the improvement is in the species as such, in the other case only 

 in one sex without the existence of the species being thereby made 

 more secure. Such cases are instructive, because they make a denial 

 of the process of sexual selection quite impossible if that of species- 

 selection is admitted. If processes of selection are operative at all as 

 factors in transformation, they must act even where the advantage is 

 not to the species but only ' intra-sexual,' and the one process must 

 often run into the other, so that it is often quite impossible to draw 

 an exact line of demarcation between them. 



Numerous secondary sexual differences probably depend purely 

 on species selection, that is to say, they include an improvement of 

 the species in relation to the struggle for existence. We may find a 

 case in point in the dwarf-like smallness of the males in many 

 parasitic crustaceans, in some worms, in many Rotifers, and in the 

 Cirripedes. It can hardly have been of advantage for the individual 

 male to be smaller than his fellows, but it was of advantage for the 

 species to produce as many males as possible in order to ensure a 

 meeting with the females, and, as the enormous production of males 

 made it advantageous for the species that as little material as possible 

 should be used in their individual production, we can readily under- 

 stand the minuteness of the males, and in some cases, as in the Rotifers 

 a.nd Bonellia, their poor equipment, lack of nutritive organs, and 

 ephemeral existence. The marine worm, Bonellia viridis, whose 

 female may be a foot in length, is not the only case in which a 

 microscopically small, male lives like a parasite inside the female. 

 Among the round-worms, too, there is a species called Trichosomum 

 crassicauda, discovered by Leuckart in the rat, the dwarf males of 

 which live in the reproductive organs of the female. All these are 

 arrangements for securing the propagation of the species, which 



p a 



