HERITABLE DIFFERENCES 61 



heredity lies in the fact that, contrary to Darwin's 

 belief, it is apparently mutations, and not fluctuations 

 or "modifications,'* that make up heritable variations. 

 If this supposition proves to be true, mutations furnish 

 the essential material in the study of heredity. Conse- 

 quently, whatever knowledge we may gain of them has 

 a direct relation to the entire problem of genetics. 



