i 9 4 THE POPULAR SCIENCE MONTHLY 



selected beans were not so extreme, however, as their parents but re- 

 gressed toward the average character of the parent race. This was 

 nothing new. Galton had discussed the matter a decade before and had 

 interpreted the regression as due to the " pull toward mediocrity " ex- 

 erted by former ancestors that must have been on the average mediocre. 

 Johannsen was not satisfied with this interpretation and in order to in- 

 vestigate the subject more thoroughly introduced the individual pedi- 

 gree culture method, or pure line method as he spoke of it, into his 

 work. All of his plants under experiment were self-fertilized for suc- 

 cessive generations, so that all of his future bean progeny were descend- 

 ants of single individuals from the original commercial variety. Each 

 pure line he found to fluctuate around a typical size just as the com- 

 mercial variety had done. Some types were exactly the same as the 

 original mixed type, but others fluctuated around averages that would 

 have been considered more or less extreme variations in the original. 

 He then grew extreme variants from each of his pure lines and made 

 the discovery that no progress at all was made by repeated selections of 

 this kind. The progeny of the high extremes and the progeny of the 

 low extremes each were found to fluctuate around the same pure line 

 average. It was quite evident then that in the first place he had been 

 dealing with a mixed race. This mixture consisted of sub-races each with 

 a heritable difference in the character size. These heritable variations, 

 however, were obscured by size fluctuations produced by differences in 

 moisture, sunlight and fertilizer received by the different individual 

 plants. There was even a difference in the size of individual beans on 

 the same plant, due probably to location of some pods in places on the 

 plant more desirable than others for the utilization of the plant's sol- 

 uble foods waiting to be stored in the seeds. These differences due to 

 immediate environment were not inherited. They behaved exactly as 

 the acquired characters of an animal. This made the role of selection 

 clear. The only improvement that selection can achieve is to isolate a 

 substrain if such a substrain or substrains exist in the variety under 

 experiment. When this substrain has been isolated, selection has abso- 

 lutely no effect, and even if continued for countless generations will 

 have no effect until nature produces one of the heritable changes which 

 are so much rarer than the fluctuations produced by environment. It 

 is also evident that the older idea that improvements made by continued 

 selection; — i. e., gradual isolation of a type — are inconstant, is wrong. 

 The explanation is that since non-inherited fluctuations obscure the 

 heritable variations, only a pure line method can absolutely isolate a 

 pure strain; and in the German method of mass selection with poor 

 control against mediocre pollen, the chances were overwhelmingly in 

 favor of the selected type recrossing with the more commonly culti- 

 vated and poorer type from which it came. 



