110 



GENETICS 



in color around the original average of the entire 

 series. This process of selecting each time an ex- 

 treme pair of dark parents was continued for twelve 

 generations, as shown in Figure 36, without in any 

 way increasing the percentage of brunette potato 

 beetles in the progeny. 



Thus in a pure line formed by the breeding of two 

 individuals alike with respect to color, the selection 

 of an extreme variant was quite without effect in 

 modifying the color of the progeny. 



7. JENNINGS' WORK ON PARAMECIUM 



An instance of the third category of pure lines 

 is furnished by Jennings' remarkable work on the 

 protozoan Paramecium, which was published in 1909. 

 Jennings carried on his experiments quite independ- 



206 2OO 



FIG. 37. Eight pure races of Paramecium. The actual mean length of 

 each race is given in micra below the corresponding outline. Magni- 

 fied about 230 diameters. After Jennings. 



ently of Johanssen, but he nevertheless arrived at 

 the same general conclusion, namely, that selection 

 within a pure line is without effect. 



Jennings found that Paramecia differ from each 



