282 DEATH 



great detail as long ago as 1858, and was led to 

 the conclusion that it was a true sexual act, com- 

 parable to fertilisation in the higher animals. Bal- 

 biani's views were for a long time discredited, 

 mainly through the unwillingness of zoologists to 

 admit the possibility of sexual reproduction occur- 

 ring in unicellular animals. More recent researches 

 however and in particular those of Maupas, have 

 shown that while Balbiani was not absolutely right 

 with regard to all the details of the process, yet 

 that he was correct in his interpretation of the 

 conjunction of Infusoria as a true sexual act. 



The problem which Maupas set himself to solve 

 was to determine the relation between the two 

 modes of reproduction in Infusoria, to ascertain the 

 conditions under which asexual and sexual repro- 

 duction respectively occur, and to find out what 

 causes lead to the substitution at particular times 

 of one process for the other. To do this it was 

 necessary to isolate an individual Infusorian, to 

 place it under known conditions as to temperature 

 and food supply, and then to follow the fate of the 

 successive generations of offspring to which it gave 

 rise by fission. 



The investigation was an extraordinarily labo- 

 rious one. Continuous observations for over five 

 months were necessary ; the production and fate 

 of from 200 to 300 successive generations had to 

 be followed accurately, and in some cases the 

 observations extended over more than 600 genera- 

 tions. The most suitable temperature at which to 

 conduct the experiments, and the kind of food best 



