226 THE EVOLUTION THEORY 



on mere theory, for I was once able, by a happy chance, to observe for 

 a considerable time, under the microscope, a female to whose shell 

 two males were clinging, each trying to push the other off. Never- 

 theless it seems to me very questionable 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 ease 

 simply an expression of the struggle for existence on the part of the 



Fie. 58. Moina paradoxa, feniale. The letters of Fig. 57 apply 

 mutatis mutandis, bi-r, brood-pouch, ov, ovary, sr, margin of shell. 



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 ease 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, Baphnia and Simocephalus, 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 



