160 THE MAIN CURRENTS OF ZOOLOGY 



sudden variations that breed true, and thus give rise 

 to new forms, he calls mutations. This indicates the 

 source of the name applied to his theory. 



In his Die Mutationstheorie, published in 1901, he 

 argues for the recognition of mutations as the 

 universal source of the variations that lead to species- 

 formation. Although he evokes natural selection 

 for the perpetuation and improvement of variations, 

 and points out that his theory is not antagonistic 

 to that of natural selection, it is nevertheless di- 

 rectly at variance with Darwin's fundamental con- 

 ception, that slight individual variations "are prob- 

 ably the sole differences which are effective in the 

 production of new species" and that "as natural 

 selection acts solely by accumulating slight, succes- 

 sive, favorable variations, it can produce no great or 

 sudden modifications." 



The work of De Vries is a most important con- 

 tribution to the study of the origin of species, and is 

 indicative of the fact that many factors must be 

 taken into consideration when one attempts to 

 analyze the process of organic evolution. His ob- 

 servations widen the field of exploration. While he 

 has demonstrated that species may arise by muta- 

 tions, there is at the same time good evidence from 



