174 THE EVOLUTION OF SEX. 



§ 3. Occurrence of Farthe?ioge?iesis. — In these distinct sets of 

 animals — rotifers, crustaceans, and insects — parthenogenesis has 

 become a confirmed physiological habit. 



(a) Take first the curious little rotifers, or wheel-animalcules, which 

 abound both in fresh and salt water. They are usually placed in the 

 chaotic alliance of worm-types, and have long been famous for their alleged 

 power of surviving prolonged desiccation. With one or two exceptions 

 the males are markedly different from the females, and are usually small and 

 degenerate. In one group {Philodinadcz) the females have two ovaries, 

 while males have never been found. They have dwindled out of existence. 

 In the rest the females have one ovary, part of which has degenerated into 

 a yolk-gland, and small males occur. These are quite superfluous as mates, 

 however, for parthenogenesis prevails. Even when impregnation, which is 

 a peculiarly random process, occurs, the sperms appear to miss their mark, 

 and to perish in the body-cavity. The numbers keep up, notwithstanding, 

 so that we have here an entire class where parthenogenesis has firmly 

 established itself. 



(/') Among crustaceans, parthenogenesis is restricted to the lower orders, 

 viz., branchiopods and ostracods. In the former, it is exhibited by the 

 brine-shrimp Artetnza and the common fresh-water Aptis in one division; 

 by daphnids {e.g., Daphnia and Moina, common " water fleas ") in the 

 other. In ostracods, some species of the common Cypris are partheno- 

 genetic. If a female water-flea, say Daphnia, be isolated from birth, 

 she becomes the mother of an abundant progeny of females. Males and 

 sexual reproduction do however eventually return, and the same is 

 probably true of the majority. Among three thousand specimens of the 

 brine-shrimp only one male occurred; while Von Siebold repeatedly in- 

 vestigated every member of a colony of Apus, once over five thousand in 

 numlDcr, without finding a single male. At other times he found one per 

 cent,, while in certain unknown conditions (probably when food is 

 scarce and life generally unfavourable) the males may be developed in 

 crowds. 



In the daphnids, which have been so successfully studied by Weismann, 

 the facts are more complex. There are two kinds of eggs — winter and 

 summer ova. The former are large, thick shelled, capable of resisting 

 drought and the like, and of remaining long latent. They only develop if 

 fertilised, and always produce females. In every way they are highly 

 anabolic ova. The summer eggs, on the other hand, are smaller, and thinner 

 in the shell. They can develop without fertilisation, and that is indeed 

 in some cases physically impossible. Males are produced from summer eggs 

 alone. They usually appear in autumn, when life is becoming harder, or 

 the conditions more katabolic. 



In the little cyprids the reproductive relations are very varied. Thus in 

 Cypris ovzmi and Notodromiis ??ioiiachtis the males are abundant all the year 

 round, and parthenogenesis is unknown. In other species, e.g., Candona 

 Candida, the males are still frequent, but parthenogenesis nevertheless 

 occurs. Lastly, parthenogenesis prevails in some cases, like Cypris fusca 

 and C. piibera, and the males are rare, appearing usually in spring. 



(<:) In insects, as we have seen, the degrees of parthenogenesis are very 

 varied ; so too is the systematic position of the forms in which normal 

 parthenogenesis occurs. Two butterflies {Psyche helix and Solenobia, 



