460 



GENETICS 



law of Regression furnished the stimulus for the experimental 

 work of Johannsen, a Danish botanist. Johannsen conceived the 

 idea that if offspring of parents who were above or below the aver- 

 age were also above or below the average, it might be possible to 

 shift the average by continued selection of parents from among 

 the unusual groups. Working with beans in which self-fertiliza- 

 tion occurred, he chose the heaviest seeds from which to raise his 

 next generation. The progeny seeds varied around an average of 

 35 eg. on some plants to an average of 60 eg. on others, all of the 



Fig. 240. — Diagram illustrating the fact that selection of variations that result 

 from environmental conditions does not shift the average. Above, the size 

 variations of seeds from a single bean plant are shown. Below, the size varia- 

 tions in progeny from the largest and smallest of these seeds. 



(After Baur in "Grundlagen der Pflanzenziichtung.") 



plants having been produced by seeds weighing 80 eg. He next 

 tried similar experiments on the progeny of single plants that 

 were propagated by self-fertilization so that bipai'ental inheritance 

 was not a factor. Such progeny constitute a " pure line." Within 

 a pure line Johannsen found that he could not shift the average 

 by selecting the heaviest and the lightest seeds for parents (Fig. 

 240 and cf. Fig. 308, p. 559). 



In the work of practical breeders it is common to select the 

 animals or plants that have the most favorable characteristics as 

 the parents of the new generation. This selection must, how- 

 ever, be continued in each generation since it is not possible to 



