386 HEREDITY AND ENVIRONMENT 



of beans he was able, by keeping the progeny 

 of each individual bean separate from the oth- 

 ers, to isolate 19 different "pure lines," each 

 differing in certain respects from every other 

 line. These lines were not created by selec- 

 tion but were merely sorted out of the general 

 species where they existed already. He fur- 

 ther found that when extremely large or small 

 individuals from any pure line were selected 

 and propagated, none of the progeny showed 

 that character in a still more extreme degree 

 but all merely fluctuated within the original 

 extremes of that line. He concludes there- 

 fore that selection within a pure line is abso- 

 lutety without effect in modifying any char- 

 acter in the offspring 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 

 lines there was considerable fluctuation in size, 

 but he was unable by selecting extremes to 

 increase these fluctuations, the progeny of any 



