1 62 THE EVOLUTION OF SEX. 



brood of seven to ten larva? literally feed upon her to the death. They 

 finally leave the corpse and begin life for themselves, only, however, 

 to fall themselves victims to a similar fate. The process may thus go 

 on for several generations, during which the ova, or pseudova as some 

 would insist upon calling them, become smaller and smaller. Event- 

 ually the larvae become too constitutionally poor to be precociously 

 parthenogenetic, and develop into adult midges — male and female, the 

 latter producing, however, only a few eggs. 



In another dipterous insect known as Chironomns, the ova begin to 

 be produced at a very early stage, are laid just at the time when the 

 larval life ends, and develop parthenogenetically. According to 

 Jaworowski, by the rupture of the ovarian membrane the ova fall into 

 the body-cavity, where the abundant nutritive stimulus takes the place 

 of fertilization. Juvenile parthenogenesis is also said by Von Siebold 

 to occur among the Strepsiptera, little insects which infest bees. 



{g) Total Parthenogenesis. — Lastly, in some of the minute aquatic 

 crustaceans and in many rotifers no males have ever been found. 

 There is every probability that the parthenogenesis is thus total; and 

 as the numbers are abundant, it has apparently been established with- 

 out detriment to, at least, the continuance of the species. 



III. Occurrence of Parthenogenesis. — 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 ot 

 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 

 {Philodinadcr) 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 yelk-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. 



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

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

 brine-shrimp Artemia and the common fresh-water Apus in one division ; by 

 daphnids (for example, Daphnia and Moina, common "water-fleas") in the 

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

 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 Yon Siebold repeatedly investigated every- member of a colony of Apus, 



