LIFE CYCLE OF SIMOCEPIIALUS VETULUS. 53 



there is autonomous variation in this respect, which places the 

 whole question beyond the pale of possible experimental proof. 

 The careful experiments of Papanicolau (1911) merit more con- 

 sideration than is admissible in this paper. The views of Issa- 

 kowitsch (1907) and Scharfenberg (1911) agree with those of 

 Woltereck and Papanicolau in that they all assume certain in- 

 herent tendencies to sexuality which cannot be completely over- 

 come by any kind of environmental conditions. 



2. The alternative view is that outer causes, such as chemical 

 substances, hunger, temperature, kind of food, etc., are largely 

 or entirely responsible for the varying degrees of sexual and 

 parthenogenetic reproduction. Evidence for this heterodox 

 view had begun to accumulate at least as early as 1875, when 

 Kurz noted a correlation between the drying up of the water 

 and the appearance of sexual forms in Cladocera. Two years 

 later Schmankewitsch (1877) gave as his opinion that the efficient 

 cause is the increasing salt concentration due to evaporation. 

 Among the unfavorable conditions mentioned by Kerherve 

 (1892, p. 236) poor nourishment is particularly emphasized as 

 being responsible for the appearance of both males and sexual 

 females. Ostwold (1094) found temperature singularly effective 

 as a cause of sexuality in Daphnids. By varying the temperature 

 he produced at the same time all of the forms that are found in 

 nature at different seasons. Langhans (1909, p. 291) says that 

 Weismann's theory of a fixed generation cycle will not bear 

 critical examination. 



In regard to other forms whose life cycle is in general similar 

 to that of Cladocera, there is much diversity of opinion, although 

 evidence against a fixed internal "cycle" is rapidly accumulating. 



Finally we must include under this second category the very 

 significant and definite conclusions of Grosvenor and Smith 

 (1913) working with Moina rectirostris, of Banta (1914) working 

 with Daphnia pulex, and of Agar (1914) working with Simoceph- 

 alus vetuhis, each of whom has conclusively demonstrated that, 

 for the form experimented on, there are certain environmental 

 complexes which will indefinitely inhibit the appearance of males 

 and sexual states in any of the females. It is of interest to note 

 that in some instances the conditions which will thus prevent 

 the full expression of the species is quite narrowly prescribed. 



