THE PRACTICE OF BREEDING 291 



sion among many students that in this case a new breed 

 had actually been created with new characters resulting 

 from selection of continuous variations. That the results 

 obtained in this and similar examples may te explained 

 on other and more probable grounds has been clearly 

 demonstrated by Johannsen. 1 This Danish botanist 

 working with garden beans found that the whole popula- 

 tion is made up of many races which he called " pure 

 lines." Selecting from populations composed of pure 

 lines, the breeder merely sorts out one of these races 

 and in time secures a pure line. This pure line or race 

 is not new. It has not resulted from gradual or continu- 

 ous variations but has simply been separated out from 

 a number of others. According to Johannsen, selec- 

 tion within a pure line during a number of generations 

 had no effect in improving the variety. The germ- 

 plasm of all individuals of a pure line tends to become 

 homogeneous. 



The mating of individuals from a pure line having 

 in their respective germ-plasms identical factors will 

 result in producing identical offspring and hence any 

 amount of selection will prove futile. Such pure line 

 individuals are much more likely to be found among plants 

 than among animals. Plants that are self-fertilizing may 

 be expected to develop typical pure lines and selection 

 within such pure lines will be of no avail. Among the 

 higher animals similar results undoubtedly occur, but 

 the difficulties of securing a strictly pure line are very 

 much greater owing to the necessity of mating two dis- 

 tinct individuals whose ancestry, while similar, cannot 

 possibly from the very nature of the case, be exactly 

 identical. 



1 Johannsen, "Elemente der exakten Erblichkeitslehre," 1909. 



