152 A. FRANKLIN SHULL 



DISCUSSION 



Those who have studied parthenogenesis and sexual repro- 

 duction in Hydatina have drawn very different conclusions regard- 

 ing the causes of the transition from one mode of reproduction 

 to the other. Some have been impressed with the influence of 

 external conditions in causing or preventing the inauguration 

 of the sexual mode, others have held that internal factors are 

 the chief if not the sole agents in this transition. Among the 

 former were Maupas ('91) and Nussbaum ('97), to whom, tem- 

 perature and starvation, respectively, seemed sufliicient to account 

 for all the phenomena they observed. Punnett ('06), on the other 

 hand, discarded both these external agents, and suggested that 

 the factor which determined the proportion of male-producers 

 (sexual females) is wholly internal, and is fixed at the time of fer- 

 tilization. Whitney ('07), in his earlier paper, remained nearly 

 neutral, merely stating that he found no evidence of external in- 

 fluence. He likewise found no evidence for the one internal agent, 

 zygotic constitution, proposed by Punnett. But in a later paper 

 (Whitney, '10) he implicitly ranges himself on the side of those 

 who hold external factors accountable for all variations in the 

 percentage of sexual forms. In my own article (ShuU, '10b) all 

 the evidence presented was in favor of external agents, though the 

 possibility of internal factors was admitted. 



The evidence presented in this and the earlier paper can hardly 

 fail, I believe, to convince one that both external and internal 

 agents are involved in the production of the life cycle. It has 

 been shown that starvation, of itself, is probably not one of the 

 external agents that have an influence on the proportion of male- 

 producers; but any attempt to bring about starvation by experi- 

 ment may make it seem to have an influence, because food can 

 not be introduced in diminished amount without introducing 

 diminished quantities of other things. We have found that differ- 

 ences in temperature may have an influence, as shown in this 

 paper, but it is practically certain that this influence is an indirect 

 one. If the effect of temperature were direct, it would be expected 

 that its influence upon the proportion of male-producers would be 



