Inheritance of Quantitative Characters 87 



East worked with the quantitative characters in corn, and the 

 explanation is the same. 



In addition to the practical value of the concei)tion of cumu- 

 lative factors, the theoretical value is worth considering, for it 

 explains things that have been very vaguely understood. This 

 conception suggests that the origin of species by natural selection 

 in the way described by Darwin, a method which for some time 

 has been thought impossible, may actually be possible within limits. 



Of course natural selection in a certain sense has always been 

 accepted, almost as generally as the fact of evolution. The point 

 in dispute is as follows. Darwin used as the basis of natural 

 selection those small individual variations which we have come to 

 call fluctuations, the same kind of variations the old plant breeder 

 used in his artificial selection. Darwin claimed that such varia- 

 tions could be piled up until the result would be a new species. It 

 was in 1900 that De \'ries showed in convincing way that this 

 kind of variation never resulted in a new species; at best it only 

 developed a race which approached the boundary of the species 

 and never crossed it. Moreover, such a race would revert to 

 type rapidly as soon as some slight change in conditions set up a 

 new standard for selection. This argument, confirmed by experi- 

 ment, has been generally accepted. 



We now know that individual variations are not always mere 

 fluctuations or responses, but may be due to varying doses of 

 cumulative factors. A selection on this basis may very well result 

 in a new race that breeds true; and a race that breeds true is 

 De Vries' definition of a new species. To reestablish Darwin's 

 ideas on the origin of species is certainly an important considera- 

 tion. The situation illustrates how genetics and evolution are 

 tied up together, so that neither one of them can be appreciated 

 fully without some knowledge of the other. 



A few words may be said in reference to the reversion of an 

 old race to its original specific type. De \'ries outlined the 

 situation clearly, and his conclusions are generally accepted. It 

 is doubtful, however, whether it has ever been understood, since 

 no one has ever devised a reasonable mechanism for such a rever- 

 sion. The conception of cumulativ'e factors supplies this mecha- 

 nism. A new race, developed by natural or artificial selection 



