igos.] JENNINGS— HEREDITY IN PROTOZOA. 485 



first experiments consisted of attempts to break the two lines derived 

 respectively from D and c into other races of different sizes by selec- 

 tion and breeding of individuals of different sizes. This led inci- 

 dentally, as w^e have seen, to the study of the effects of growth and 

 environment on size; it was found that the observable differences 

 between dift"erent members of either race were due to these factors, 

 so that selection of such members did not lead to the establishment 

 of races of different sizes. The results of a large amount of time- 

 consuming work along this line, done before the investigation of 

 growth and environmental action, were throughout negative." 



As a result of this work, I was disposed toward the belief that 

 the characteristic sizes of D and c represent conditions of stability, 

 which have properly been distinguished as two species, and that races 

 of other sizes were not to be found or produced. 



But the work thus far has, of course, been based on " pure lines," 

 in the sense in which that expression is used by Johannsen (1903, 

 1906). The lines D and c are each derived from a single individual, 

 reproducing asexually, so that no admixture from outside has entered 

 them during the experiments. Now, while it appears difficult or 

 impossible to produce other races zvithin these pure lines, there 

 remains, of course, the possibility that still other lines exist in nature. 

 Can we find in a " wild " culture, by proper selection of differing 

 individuals, still other races of differing size? This was the question 

 next investigated. 



I. Selection for Different Races in a Wild Culture. 

 (o) Races Isolated from Cultures Not Conjugating. 

 Attempts to separate out other races than those represented by 

 D {" caudatum form") and c (" aurelia form") were first made 

 with a wild culture which I called 01. This culture developed in 

 decaying vegetation from a marsh. It contained two well marked 

 sets of individuals: (i) very large individuals, corresponding in 

 many respects to the D line, but with a mean length on January 3, 

 1908, of 238.280 microns; these we will designate E; (2) smaller 



^ To the experiments on selection within a pure line we return in a later 

 section. 



