No. 531] THE GENOTYPE HYPOTHESIS 



161 



mean that small variations would center closely around 

 a mode, and large variations would occur witli a rela- 

 tive frequency inversely proportional to their size. The 

 point that I wish to emphasize, however, is that neither 

 Darwin nor De Vries recognized the proper distinction 

 between a mutation and a fluctuation. Darwin made no 

 distinction. De Vries, however, considered fluctuations 

 to be linear; that is, to be limited to increase and de- 

 crease in characters already present. He thought that 

 selection of such variations brought about changes in 

 the selected population due to the inheritance of the 

 fluctuations, but that the selected populations returned 

 to the mean of the general population after selection 

 ceased. Mutations, on the other hand, were gains or 

 losses of entire characters — qualitative changes — which 

 were transmitted completely, i. e., were constant, from the 

 beginning. De Vries did indeed state that mutations 

 could take place in any direction, which would involve the 

 idea of linear change or quantitative mutations; yet it 

 seems quite evident from his general attitude in 4 'Die 

 Mutationstheorie" that to his mind qualitative and 

 quantitative variations were quite distinct. 



Many practical breeders had long known, however, 

 that the selection of linear variations often produced 

 new races which were as constant as any races, provided 

 they were not exposed to crossing with individuals of 

 the general population from which the selected race had 

 come. Why this was true was unknown. It was felt 

 that there was a real distinction between certain varia- 

 tions, to which Darwin had not called attention; yet it 

 was felt that the De Vriesian idea was not wholly cor- 

 rect. It has been in making this distinction clear-cut and 

 definite that Johannsen has rendered his great service. 

 His elaborate extensions of the genotype conception of 

 heredity have cleared up many debated points, and 

 corroborative evidence has been received from so many 

 lines that it can hardly be doubted that the main points 

 of the hypothesis are correct. It may seem, therefore, 



