No. 534] 



THE PURE LINE THEORY 



:;55 



Biologists will agree, I believe, that to test critically 

 the effectiveness of selection in the population and in the 

 pure line, the experimental material mnst be an appar- 

 ently homogeneous wild species or a garden variety the 

 individuals of which are not differentiated into sub- 

 races by characters other than those under considera- 

 tion. 20 Conclusions drawn from any experiments in 

 which these simple precautions are neglected seem of 

 doubtful value. 



From Professor Johannsen's first memoir, that of 

 1903, we have no reason to suspect that his material is 

 not, so far as the biologist can judge, homogeneous. 21 

 We are told nothing of any vegetative differences seen 

 during the two generations grown in 1901 and 1902. Ap- 

 parently all the numerous reviewers have considered his 

 material perfectly homogeneous except for differentia- 

 tion into genotypes with respect to seed characters. 



In his book, however, one notes with some surprise the 

 casual information ("Elemente," p. 311) that his Pure 

 Line I also has curiously bent seeds, a special "Ver- 

 halten" in germination and a "groben Habilus" in the 

 vegetative organs. Indeed Johannsen states that from 

 the form and method of germination, etc., of a seed — 

 even though a strong " minus Abweicher" — he can gen- 

 erally recognize an individual belonging to Line I. 



These points should have been made clear at the be- 

 ginning. If Professor Johannsen's lines really differ in 

 their vegetative characters, so, for instance, that they 

 can be distinguished as they grow in the field, it seems to 

 me that their significance for the efficiency of selection is 



