HEREDITY 



cium Jennings selects the largest and the small- 

 est individuals respectively, he finds that the 

 descendants of the one lot will be of larger size 

 than the other. This looks like an effect of selec- 

 tion upon racial size. But if selection is made not 

 within a mixed population but among the de- 

 scendants of a single individual, it is found 

 that the descendants of large individuals are 

 of no greater average size than those of small 

 individuals. 



The explanation of this fact is to be 

 found in the existence of what Johannsen 

 has called pure lines. Jennings has been able 

 to isolate eight distinct pure lines of parame- 

 cium differing in average size, as shown in 

 Fig. 40. The range of variation in size within 

 one of these races is great, but if one selects 

 extremely large or extremely small individuals 

 within the same pure line, i. e. among the 

 asexually produced descendants of the same 

 animal, no change in the average size of the 

 race is brought about. 



A very different result is obtained, however, 

 if one mixes together several pure lines and then 

 selects from the mixed race on the basis of size. 



114 



