MENDELISM AND SELECTION 



the principle of the " pure line." The pro- 

 geny of a single self-fertilized homozygous 

 bean plant constitute a pure line. They are 

 all alike, so far as the hereditary transmission 

 of size is concerned, for they are all derived 

 from like gametes. The differences in size 

 which occur among them are due to differences 

 in nutrition, not to germinal differences, and 

 they are not transmitted. But in a mixed 

 population of beans, such as is represented by 

 a field crop, differences of size occur which 

 are due to heredity as well as those which are 

 due to the environment. In the case of the 

 former, selection naturally has effect; in the 

 case of the latter, it does not. 



Jennings has obtained similar results in his 

 studies of paramecium, a one-celled animal 

 which multiplies asexually by dividing into two 

 similar parts. It lives in stagnant water and 

 may be reared in great numbers in a hay-infu- 

 sion, for it multiplies with great rapidity, divid- 

 ing two or three times within twenty-four hours. 

 The variations in size which occur in parame- 

 cium are shown in Fig. 40. 



When from an ordinary culture of parame- 

 113 



