Control of Heredity: Eugenics 267 



and that thereafter selection serves only to maintain the charac- 

 ter at its high level but not to advance it. The probable explana- 

 tion of this fact has been found only in recent years. The re- 

 searches of deVries, Johannsen, Jennings, Tower, Pearl, Mor- 

 gan and others have shown that in some cases at least selection 

 merely isolates mutants or distinct hereditary lines which are al- 

 ready present in a mixed population but that it does not "build 

 up" characters nor produce new mutations; in short it does not 

 create the variations on which it acts. 



Johannsen found that from a single species of beans he was 

 able, by keeping the progeny of each individual bean separate from 

 the others, to isolate 19 different "pure lines," each differing in 

 certain respects from every other line. These lines were not 

 created by selection but were merely sorted out of the general 

 species where they existed already. He further found that when 

 extremely large or small individuals from any pure line were se- 

 lected and propagated, none of the progeny showed that charac- 

 ter in a still more extreme degree but all merely fluctuated within 

 the original extremes of that line. He concludes therefore that 

 selection within a pure line is absolutely without effect in modify- 

 ing any character in the offspring of that line. 



Jennings found that different races of Paramecium differ in 

 size, structure and rate of division, and that these differences are 

 "as rigid as iron." With respect to average length of body he was 

 able to isolate eight lines which constantly differed more or less 

 from one another. Within each of these lines there was con- 

 siderable fluctuation in size, but he was unable by selecting ex- 

 tremes to increase these fluctuations, the progeny of any line al- 

 ways fluctuating about the mean of that line (Fig. 22). 



Similarly Tower found in his studies on the potato beetle that 

 he was unable to shift the mean or the extremes of any character 

 by selection of extreme forms of an inbred line. 



Pearl also made an extensive study of the records of breed- 

 ing experiments extending over many years in which the attempt 



