THE FACTORIAL HYPOTHESIS 213 



invariability of the factors involved. When, how- 

 ever, these factors were sorted out so that strains 

 became homozygous, some variability probably due 

 to evironic differences still remained. That is, in 

 addition to the variation due to recombination it 

 has been found that even in pure races "unit char- 

 acters" vary. Why, then, it may be asked, do not 

 the factors that produce them vary also? 



Johannsen's work on material of a kind suitable 

 to give a definite answer to this question and by 

 methods that have not been questioned, has brought 

 out clearly certain facts only vaguely stated before. 

 In a population of beans he found that each bean 

 gave rise by self-fertilization to what he called a pure 

 line. Each of the original beans proved to be homo- 

 zygous for all of the factors involved. This was 

 probably due to self-fertilization through many genera- 

 tions, a process that automatically produces homo- 

 zygous lines. The weights of the descendants of any 

 given bean gave a curve of frequency which was 

 different from that of the whole population (Fig. 62) . 

 Within the group derived from one bean, however, it 

 was found that any bean, whether heavier or lighter 

 than the others, gave a curve exactly like the curve 

 of the line from which it came. Evidently then the 

 size differences within these pure lines are not inherited. 

 They must be due to the environment of the plant, or 

 to the position of the bean in the pod, etc.; in other 

 words to conditions that are extrinsic to the germ 

 plasm. Here is a demonstration that the factors 

 do not vary, but give identical results in successive 



