472 



Plant Physiology 



pure line cannot change the averages, or shift the mode. 

 These results are most suggestive and important, and 

 the principle has been confirmed by Jennings and others ; 

 but many additional data will be required before this 

 type of behavior is recognized as of general significance. 



TABLE SHOWING EXTENT OF VAKIATION 



287. Mutations. A sudden variation which is fully 

 transmissible has been called by De Vries a mutation. It 

 is a discontinuous variation or hereditary saltation. 

 Mutation phenomena have become prominent in variation 

 studies since the publication by De Vries of his Mutation 

 Theory. Already the term has been loosely employed, but 

 in general it has the special significance above indicated. 

 De Vries advanced the view that all evolutionary progress 

 is based upon the occurrence of mutations. His conclu- 



