LIFE CYCLE OF SIMOCEPIIALUS VETULUS. 55 



the change to sexual reproduction, together with the degenera- 

 tion that so often accompanies it under experimental conditions, 

 are wholly dependent upon the environment. Later experi- 

 mentation however (experiment 5), has demonstrated their 

 contention to be correct, at least for Simocephalus vet-nlns. 

 Though these last experiments are of a very different type from 

 those described by Agar (19146, Table I.), Grosvenor and Smith 

 (1913), and Banta (1914), who succeeded in carrying Daphnians 

 through many generations with no loss of vigor, they are corro- 

 borative of their results. That the success of these authors in 

 rearing purely parthenogenetic generations for an indefinite 

 time could not have been due to the accidental selection of lines 

 with a strong internal tendency to parthenogenesis is rendered 

 certain not only by the use of several distinct lines, but also by 

 parallel cultures subjected separately to unfavorable temperatures 

 and crowding, which gave a large proportion of males and sexual 

 females. The genera used were Simocephalus, Moina and 

 Daphnia. Banta carried Daphnia pulcx for 127 generations with 

 no loss of vigor. 



In groups other than Cladocera in which it has been found that 

 an indefinite number of generations can be reared parthe- 

 nogenetically, it seems that it has always been possible to bring 

 on sexuality by a proper change in the environment, which would 

 indicate that pure lines with respect to parthenogenesis do not 

 exist. It now seems altogether likely that bisexuality may be 

 indefinitely inhibited in many more of the lower animals than 

 experimenters have been aware of heretofore. The conditions 

 under which this is possible have been found in some instances to 

 be narrowly circumscribed, as e.g., certain Cladocera require the 

 high temperature of 28 C., although the more recent authors 

 seem to believe that a considerable number of species of Cladocera 

 wi 1 ! be found which can be induced to reproduce by partheno- 

 genesis indefinitely if only the external conditions are properly 

 manipulated. It appears also that, whatever may be the ex- 

 planation, any set of nearly uniform conditions finally becomes 

 prejudicial to continued parthenogenesis. Parthenogenetic re- 

 production seems to be favored by conditions which are conducive 

 to rapid growth, though they may vary within certain limits. 



