84 Plant Genetics 



worked with the quantitative characters in corn, and 

 the explanation is the same. 



In addition to the practical value of the conception 

 of cumulative factors, the theoretical value is worth 

 considering, for it explains things which have been very 

 vaguely understood. The understanding of the fixation 

 of hybrids just mentioned, and races produced by artifi- 

 cial selection, clears up our practical breeding methods, 

 and this is valuable information; but the conception also 

 shows that the origin of species by natural selection as 

 announced by Darwin is possible, a method which 

 for some time has been thought to be impossible. 



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 variations could be piled up until the result 

 would be a new species. It was in 1900 that De Vries 

 (i) showed in a convincing way that this kind of varia- 

 tion never resulted in a new species; at best it could only 

 develop 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 experiment, 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 



