758 THE AMERICAN NATURALIST [Vol. XLIV 



part in it. "We are dealing here with the process in its simplest 

 form." It is far from demonstrated that crossings have had 

 nothing to do with the improvement of the sugar beet. The 

 consensus of opinion of most biologists at the present time is 

 that selection can accomplish nothing except the isolation of 

 the best strain or best Mendelian combination existing in a given 

 population. It is hardly fair, however, to attribute to de Vries 

 the opinions expressed ten years ago, for he would probably 

 hold to-day that the opinions expressed concerning the effect of 

 selection in ''The Mutation Theory" have been proved to be 

 incorrect. Such a position is really more in keeping with the 

 fundamental principles involved in his theory, and I have no 

 doubt that de Vries would fully admit that selection can not 

 affect fluctuating variability, or at least that all of the recent 

 evidence points in this direction. 



For the purpose of discussing de Vries 's fundamental 

 theorem we may classify the various types of variation as fol- 

 lows: (1) Fluctuation; (2) those due to Mendelian recombi- 

 nations; (3) those due to change in personnel of the chromo- 

 somes or other cell organs having a relation to ontogenetic 

 development; (4) those due to fundamental changes in what- 

 ever material is responsible for the metabolic activities which 

 result in development. 



Fortunately, at the time 4 'The Mutation Theory" was written 

 the general facts of Mendelian recombination were recognized 

 and are taken into account by de Vries, though, as previously 

 stated, he frequently confuses them with other types of varia- 

 tion. De Vries also recognizes fluctuation, which he describes 

 by the term "individual variability," and appraises it at its 

 true value, except, as stated above, that he credits selection with 

 the power of producing temporary modifications by means of it. 



The last two types of variability were not recognized when 

 "The Mutation Theory" was written, so that they are utterly 

 confused in this book. 



Before de Vries undertook his (Enothera studies he was al- 

 ready committed to a theory concerning the manner in which 

 evolutionary changes come about, and frankly states that his 

 work was undertaken in order to find confirmation of this 

 theory. Strangely enough, Darwin was responsible for the 

 fundamental idea underlying de Vries 's theory of mutation. 

 It will be remembered that in attempting to explain the sup- 



