HEREDITY IN THE PROTOZOA 285 
If these ideas are carried over to the Protozoa 
we might anticipate that daughter individuals 
would retain the 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. 
:;fJennings 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 (1915) working in Jennings’ 
laboratory studied the same problems in a different 
infusorian, Stylonichia pustulata, that produces 
fission lines in the same way as does paramecium. 
Starting with a single individual he subjected the, 
