MUTATION 181 



as seems to be the case with human hair color. 

 In many instances of geographic variation a gra- 

 dation of climatic conditions causes a gradation 

 in the development of a unit character all the 

 way from invisibility to strong expression. 

 Doubtless many important discoveries are about 

 to be made in the field of graduated characters. 

 But from henceforth we must, I think, start in 

 our studies of unit characters from the stand- 

 point of their normal discontinuity. While we 

 remember the services of De Vries in insisting 

 on the normal discontinuity of unit characters, 

 we shall, in considering the idea of the unit char- 

 acter, recognize more clearly how great is the 

 debt of biological science to the insight of Charles 

 Darwin. 



