HEREDITY IN THE PROTOZOA 285 



If these ideas are carried over to the Protozoa 

 we miglit anticipate that daughter individuals 

 would retain tlie characteristics of the parent cell 

 from which they came, and in general this is true. 

 Yet there are also records where selection of sister- 

 individuals may form the starting point for separate 

 lines that differ in definite ways. Our problem then 

 is to determine, if possible, what mechanism is 

 concerned in such results, how such differences arise, 

 and in what sense they may be said to be inherited. 

 The following evidence throws some light on these 

 questions. 



i { Jennings finds that in any mass culture, or in 

 any pond, there are generally present several races 

 of Paramecium besides the two standard types of 

 P. caudatum and P. aurelia. By breeding from 

 single individuals he separated from a certain mixed 

 culture at least eight lines differing mainly in size 

 (Fig, 65). Later Jennings and Hargitt have sepa- 

 rated still other races of Paramecium. Within such 

 races there is some fluctuating variability due to 

 environment, age, etc.; but any one of the indi- 

 viduals, whether large or small, will give rise to a 

 population that shows the same variability about a 

 mean as was shown by the race from which the 

 individual had been chosen. 



Later Middleton (19L5) working in Jennings' 

 laboratory studied the same problems in a different 

 infusorian, Stylonichia pustulata, that produces 

 fission lines in the same way as does i:)aramecium. 

 Starting with a single individual he subjected the 



