SCORPIONS, SPIDERS AND CRABS 263 



able whether the origin of this sickle-claw can be referred 

 to sexual selection, for without this clamping-organ 

 copulation in most Daphnids would not be possible. It 

 was thus not as an advantage which one male had over 

 another that the clamping-sickle evolved, but rather as 

 a necessary acquisition of the whole family, which must 

 have developed in all the species at the same time as 

 the other peculiarities, and notably those of the shell. 

 The competition of the males among themselves is thus 

 in this case simply an expression of the struggle for 

 existence on the part of the species as such, and it is not 

 a question merely of a character which makes it easier 

 for the males to gain possession of the females, but of 

 one which had necessarily to arise lest the species should 

 become extinct. In other words, in this case Natural 

 Selection and Sexual Selection coincide. 



" The case of the antennae of Moina, which have been 

 modified into grasping organs is quite different ; these 

 owe their origin, not to natural selection, but to sexual 

 selection, for antennae of that kind are not indispensable 

 to the existence of the species, as we can see from the 

 closely related genera, Daphnia and SimocephaluSy where 

 the males have quite short, stump-like antennae, furnished 

 with olfactory filaments not much more numerous than 

 the females possess. Just as these supernumerary 

 olfactory filaments were produced by 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 blunted, 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 characteristics. 

 Neither of them is of advantage to the species as such. 



