Evolution 347 



It was the merit of de Vries^ to have pointed out that 

 fluctuating variations are not hereditary and hence 

 could not have played the role assigned to them by 

 Darwin, while discontinuous variations as they appear 

 in the so-called ** sports" or mutations are inherited. 

 This was an important step in the history of the theory 

 of evolution. It did not touch the foundation of Darwin's 

 work, namely the substitution of the idea of an acci- 

 dental evolution for that of a purposeful creation; it 

 only modified the conception of the possible mechanism 

 of evolution. According to de Vries, there are special 

 species or groups of species which are in a state of muta- 

 tion. He considers the evening primrose on which he 

 made his observations as one of these forms. Morgan 

 and his pupils have observed over 130 mutations in a 

 fly Drosophila, From our present H mi ted knowledge we 

 must admit the possibility that the tendency toward the 

 production of mutants is not equally strong in different 

 forms. Whether this part of de Vries's idea is or is not 

 correct there can be no doubt that variations occur which 

 consist in the loss and apparently, though in rarer cases, 

 in the gain or a modification of a Mendelian factor. If we 

 wish to visualize the basis of such a change we may do so 

 by imagining well-defined chemical constituents in one or 

 more of the chromomeres undergoing a chemical change. 



»de Vries, H., The Mutation Theory, translated by Farmer, J. B., 

 and Darbishire, A. D., Chicago, 1909. Species and Varieties. Chicago, 

 1906. Gruppenweise Artbildung. Berlin, 1913. 



