68 Plant Genetics 



use to represent the fluctuating individuals about a 

 specific type. 



This conception of cumulative factors, therefore, 

 has far-reaching significance. For a long time biologists 

 have recognized individual variation within the species. 

 Darwin depended upon it as the basis of his theory of 

 natural selection as the origin of species; in fact, ever 

 since Darwin's Origin of species, individual variation 

 has been fundamental in our conceptions. To account 

 for this universally recognized phenomenon, Darwin 

 proposed his transportation hypothesis as a possible 

 explanation, which, as will be recalled, did not long 

 survive. Weismann offered in explanation his germinal 

 selection, which was soon discarded because it was beyond 

 the possibility of experimental testing. Aside from 

 these two attempts to explain individual variation no 

 other comprehensive scheme had been presented. Bi- 

 ologists had simply recognized the fact of individual 

 variation without any conception of the mechanism. 

 They knew that individual variation existed but had 

 even stopped asking why it existed. 



The importance of this new theory, therefore, is 

 obvious. It is an ingenious explanation of the inherit- 

 ance of quantitative characters and of the existence of 

 individual variations. Furthermore, the theory has 

 not been developed through meditation, but has its 

 basis in scientific experiments. It is imaginative to a 

 certain extent, of course, as is every other valuable 

 theory, but unlike most such theories it has a substantial 

 foundation, namely, Mendel's law. 



The importance of the possible role of cumulative 

 factors in explaining individual variation, which in 



