192 Rejrroduction in Simocephalus vetulus 



tendency to sexuality or degeneration appears even after an enormous 

 number of parthenogenetic generations, it seems necessary, on the 

 principle of accepting the simplest hypothesis which will fit the facts, 

 to conclude that the sexual cycle (obligatory parthenogenesis — labile 

 period — obligatory or preponderating sexuality, often accompanied under 

 experimental conditions by " degeneration ") is, when present, due 

 entirely to the cumulative effect of an unfavourable environment or to 

 an actual though often unsuspected change from a favourable to an 

 unfavourable environment. 



This view does not, of course, diminish the physiological interest of 

 the change from the parthenogenetic to the sexual mode of reproduction. 

 Indeed the tendency to replace asexual by sexual reproduction under 

 certain conditions (often unfavourable ones) is a phenomenon of the 

 deepest significance, but it seems to be no more due to an inherent life 

 cycle than the increasing hunger, ending in degeneration and death 

 unless the conditions are changed, which accompanies the withholding 

 of food from an organism, is due to an inherent physiological cycle. 



The ascertainment of the exact conditions under which asexual is 

 replaced by sexual reproduction, and the precise advantage conferred 

 by the latter, is indeed a most important task. The external conditions 

 under which parthenogenesis will continue indefinitely are certainly 

 different for different species. They may be rarely realised in nature, 

 and still more rarely for any prolonged period, natural conditions being 

 subject to seasonal and other changes. For many species of the related 

 Ostracoda however these conditions do seem to obtain in nature. On 

 the other hand the other extreme is not impossible, that for some species 

 in which asexual reproduction is one of the normal modes of reproduc- 

 tion, there are no conditions under which asexual reproduction can go 

 on for more than a limited time. In other words, that there is no 

 environment which does not act in a way prejudicial to parthenogenetic 

 reproduction. If such species do exist, it might be legitimate to speak 

 of their sexual cycle, but in that case it would be necessary to remember 

 that the " cycle " is not due to anything of general import but merely 

 to the peculiar relations of that particular species to the environment. 



It appears that a similar change of view is taking place in regard to 

 the life cycle of the Infusoria. For many years the orthodox view has 

 been that originated by Maupas as the result of his classical experi- 

 ments. According to this view the life history is a cyclical one, the 

 near descendants of the exconjugate being in a state of immaturity, 

 which gradually yields, as asexual multiplication proceeds, to puberty, 



