\VYMAX REED GREEN. 



d. for three generations, the only instances in which both 

 males and females did not appear in some of the broods were 

 those in which they were very few-, i.e., in which the isolated fe- 

 males died early. The same was true of sexual females. They 

 always appeared in some of the broods unless they were too few. 

 Pure broods of sexual females were very rare and they have never 

 predominated in any of my cultures no matter how severe the 

 conditions were. Though I have not carried out a definite ex- 

 periment to determine how many generations a culture may be 

 carried through the sexual females alone (except in isolation ex- 

 periment 3, where distinct lines were carried to the third genera- 

 tion) I have not the slightest reason to suppose that they may 

 not be carried on indefinitely, since each female, whether sexual 

 or asexual, gives rise to both sexual and asexual females, provided 

 only that the food supply and other cultural conditions have 

 proper attention. I am convinced that much of the confusion 

 in the literature on the relation of sexuality to the senescence of 

 cultures is due simply to the extreme prolificacy of the Daphnians 

 and the complex phenomena resulting therefrom. Sexual phase 

 and asexual phase may well be applied to individuals, and asexual 

 phase to cultures, but in Simocephalus vet id us cultures never be- 

 come wholly sexual, only partially so, since in the most sexual of 

 cultures the offspring of a given female are always mostly asexual, 

 and the sexual females pass most of their lives in the asexual state. 

 Finally let us recall that at the extremely modest rate of produc- 

 tion of ephippial eggs assumed in our calculations above, th.it 

 starting with 10 females we should have 23,400 ephippia at the 

 end of the second generation, whereas, as a matter of fact the 

 most sexual of cultures of Simocephalus vetulus produces only a 

 few during a course of many months. Hence it is folly to con- 

 sider the onset of sexuality causally related to senescence of rul- 

 tures in this species. 



6. Does Simocephalus vetulus depend upon external factors to 

 call into expression maleness, sexuality in the females, and 

 parthenogenesis? 



With respect to this point the results of Grosvenor and Smith 



1913), Banta (1914), and Agar (i9i4b) are particularly instruc- 



They succeeded in completely inhibiting sexual form.-- in 



