194 Reproduction in Simocephalus vetulus 



is being increased by the progress of the " cycle " or by the cumulative 

 effect of an unfavourable environment. Hence the existence of labile 

 periods is as readily explained on the one hypothesis as on the other. 



7. There is no justification for retaining the hypothesis of an 

 inherent reproductive cycle — that is to say, the hypothesis that the 

 number of generations or lapse of time since the last fertilised egg 

 influences, as such, the production of sexual or degenerate forms. For 

 the production of these forms is under certain conditions not influenced 

 even by the lapse of an enormous number of parthenogenetic generations, 

 while their production certainly is influenced by environment in other 

 cases. The residuum of cases being equally well explicable on either 

 hypothesis (cycle or environment) it is most reasonable to suppose that 

 the factor that was effective in the one case (environment) was the one 

 that was effective in the other, and conversely, that the ineffective factor 

 of the one case ("reproductive cycle") was ineffective in the other. 



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