378 HEREDITY AND ENVIRONMENT 



ers have shown that selection creates nothing 

 new but merely isolates mutants or distinct 

 hereditary lines which are already present in a 

 mixed population. Johannsen found that from 

 a single species of beans he was able, by keep- 

 ing the progeny of each individual bean sepa- 

 rate from the others, to get 19 different "pure 

 lines" of beans, each differing in certain re- 

 spects from every other line. He further found 

 that when extremely large or small individuals 

 from any pure line were selected and bred 

 from, none of the progeny showed that char- 

 acter in a still more extreme degree but all 

 merely fluctuated within the original extremes 

 of that line. In short, he concludes that selec- 

 tion within a pure line is absolutely without 

 effect in modifying any character in the off- 

 spring of that line. 



Jennings found that different races or pure 

 lines of Paramecium differ in size, structure 

 and rate of division, and that these differences 

 are "as rigid as iron." With respect to aver- 

 age length of body he was able to isolate eight 

 pure lines which constantly differed more or 

 less from one another. Within each of these 



