124 WHAT EVOLUTION IS 



6. THE MUTATION THEORY 



For almost forty years after the pro- 

 mulgation of natural selection, biolo- 

 gists were content to speculate on the 

 way in which plants and animals 

 might be changed through this prin- 

 ciple. Only as the methods of zo- 

 ology and of botany changed, from 

 the more purely observational to the 

 experimental, did evolutionary inves- 

 tigation receive a new impulse. This 

 change in evolutionary work may be 

 said to have been initiated, about the 

 beginning of the present century, in 

 the studies on heredity, carried out 

 more or less independently by Tscher- 

 mak, by Correns, and especially by 

 de Vries. One of the results of these 

 studies was the unearthing of the 

 long-neglected but highly important 

 publications of Mendel which had 



