134 G. P. FRETS 



independence of the characteristics (better hereditary factors or genes) is 

 accepted, — and this is of supreme importance — in consequence of segrega- 

 tion, we assume then the purety of gametes in crossing. 



Whilst Mendel studied heredity by crossing, Johannsen occupied himself 

 with pure lines (reinen Linien). According to Johannsen a pure line con- 

 sists of individuals who descend from one fertilized individual. 



Johannsen examined the significance of the curve of Quetelet. He 

 experimented with beans. He grouped the beans into weight-classes. The 

 bean-material thus grouped can best be represented by the curve of Quetelet, 

 and if we compare the average weight of the progeny for the various weight- 

 classes with those of the original material, we see that regression in the 

 sense of Galton is present. 



Johannsen asked himself, on what the regression was based and sought 

 the explanation in unequal hereditary value of the variants in each group of 

 the original material. 



The proof Johannsen supplied by weighing separately variants of each 

 group, sowing them and collecting them separately. 



This he did for the groups with the smallest and with the largest beans. 

 Johannsen found that the average weight of the beans of the progeny of 

 beans of one class was not the same; there were some pronounced deviations. 

 It appeared thus that in one and the same class several hereditary varia- 

 tions were present. In a class of minus variants there are some minus 

 variants which belong to hereditary variations deviating less from the 

 average and the presence in each class of some hereditary variations not 

 belonging to the class brings about the regression. 



If this explanation of the regression is correct, it cannot be possible that, 

 by the cultivation from minus-, respectively from plus-variants, belonging 

 to a pure line, a selection can be attained; or what is the same thing, the 

 regression must be total, i.e., the average weight of beans for each class 

 must be equal to the average weight of the whole material. 



Johannsen carried out this investigation and the result indeed confirmed 

 this idea. This is an important result. It shows that the regularity of the 

 curve of Quetelet will not say that the material which it represents does not 

 contain any hereditary differences and this finding explains the regression 

 of Galton; this is attributed to the presence of hereditary differences in the 

 material. 



This wide-reaching result has led to the sharp discrimination between 

 hereditary and non-hereditary variations. Non-hereditary variations are 

 called, according to E. Baur, also modifications (or fluctuations). All that 

 is attained by external circumstances, thus by influences of environ- 



