206 PHYSICAL BASIS OF HEREDITY 



would have appeared only after the beans had become 

 homozygous through repeated inbreeding. But Johann-' 

 sen, starting with homozygous beans, was able to obtain 

 his extremely important results, because if selection could 

 bring about any change it would have to be due to a 

 change in the genes themselves. Here, by means of a 

 crucial experiment, he exposed an error that had been 

 accepted by selectionists from 1859 to 1903. It would have 

 been difficult, almost impossible, to give this demonstra- 

 tion on any plant or animal in which self-fertilization or 

 asexual reproduction was not the rule ; for, if the material 

 had been heterozygous either for the main factors for a 

 character, or for modifying factors for that character, 

 selection in one or another direction would be expected 

 through recombination of factors to change the original 

 frequency distribution. It is true that any stock, even 

 such as reproduces by males and females, may be made 

 homozygous by inbreeding brother and sister for ten or 

 more generations, but even such stock would have to be 

 constantly watched for mutation. 



Johannsen defined a pure line as a race or family of 

 individuals descended through an unbroken series of self- 

 fertilizations from an ancestor homozygous in all its 

 genes. By making this definition precise he made clear 

 the essential point of his demonstration. Now that his 

 point is made, it seems no longer necessary or even desir- 

 able, I think, to narrow the definition of a pure line to 

 races that self-fertilize, since this is only one form of 

 inbreeding, resulting in the production of homozygous 

 individuals. By extending the definition of a pure line 

 to all forms whose genes are the same in all individuals 

 (whether the pairs are homozygous or not), the definition 

 covers all cases of parthenogenesis that do not undergo 

 reduction, and all cases propagating by non-sexual means, 

 for, in these cases the same complex of genes is present 

 in successive generations. 



Many plants are propagated by offshoots, stolons, 



