360 THE AMEBICAN NATURALIST [Vol. LI 



This conclusion, though in opposition to a great bulk of 

 the experimental evidence gathered by some of the fore- 

 most biologists, Johannsen, Maupas, Morgan, Castle, Jen- 

 nings and many others, is supported by some of the re- 

 sults obtained by Barber (1907), Calkins and Gregory 

 (1913), Middleton (1915) and Jennings (1916). 



Summary 



In a race of didinia originating from a single individual 

 there suddenly appeared a heritable variation in the rate 

 of fission. This variation occurred 721 generations after 

 conjugation and 197 generations after encystment. 



Two strains were isolated from this race and kept 

 under observation for 315 days. During this time the 

 lines in one strain produced an average of 838 + gen- 

 erations (2f per day) and those of the other 634 + gen- 

 erations (2 per day). 



