354 



EVOLUTION AND GENETICS 



It must be used in connection with hybridization for the isolation 

 of desirable characters and character coml^inations in the homo- 

 zygous state, and if the heterozygous individuals are desired it 



Pure Line 



3 



^yy^^H^gfgyy 



may be used for the elimi- 

 nation of their homozygous 

 offspring. 



The Pure Line. Dar- 

 win's theory of natural 

 selection influenced thought 

 in such a way that for many 

 years selection, through 

 natural or artificial means, 

 was supposed to result in 

 actual modification of the 

 line selected. Galton's law 

 of filial regression also sug- 

 gested very strongly the 

 possibility of shifting the 

 general character of a group 

 of organisms by always 

 selecting extreme individ- 

 uals as the parents of the 

 next generation. The 

 Danish botanist, Johannsen, 

 tested the accuracy of this 

 view and in doing so dis- 

 covered the existence of 

 pure lines. 



Johannsen used for his 



Fig. 196.-D7agmm~showmg five p«reZwes experiments a cultivated 

 and a -population formed by their union, bean {Phaseolus vulgaris 

 The beans of each pure Hne are repre- nana). The weights of seeds 

 sen ted as assorted into inverted test tubes, 1,1 1 i i 

 making a curve of fluctuating variabihty. Planted were recorded and 

 Test tubes containing beans of the same the entire lot of seeds pro- 

 weight are placed in the same vertical duced by every plant was 



row. (From Walter, after Johannsen.) /. ,, , , ■, j 



carefully harvested and 



weighed. In general the largest beans produced the largest 



offspring, but Johannsen was impressed by two important 



facts. In the first place, the seeds produced by a single plant 



sometimes fluctuated about a mean quite different from that 



