94 HEREDITY AND VARIATION 



observed changes affect many organs and parts, and in no 

 definite direction ; and the mutabihty seems to be periodic, 

 not continuous. 



If cases Hke that of 0. lamarckiana are indicative of what 

 often occurs and has occurred in nature, then our view of the 

 evolution-process must be in several respects modified. 



It will be necessary to distinguish more sharply between 

 fluctuating variations and discontinuous mutations. If a new 

 elementary species may arise as it were ready-made, "at a single 

 advance," it is not necessary to hold to the formula that species 

 have arisen by the gradual accumulation (under selection) of 

 minute individual variations. As mutations occur in large 

 numbers and occur repeatedly and are very constant, the familiar 

 difficulties in regard to the swamping of novelties, the inap- 

 preciable value of incipient stages, the apparent non-utilitarian 

 character of some specific differences, and so on, will be greatly 

 lessened. The reader may be referred to Prof. T. H. Morgan's 

 Evolution and Adaptation (1903) for a valuable discussion 

 of the advantages of the Mutation Theory. 



De Yries's Analysis of Yariation. — In order to appreciate more 

 thoroughly the importance of the changes which De Vries has 

 necessitated in our evolutionary conceptions, we must briefly 

 refer to his analysis of the distinct phenomena which have been 

 too often unfortunately slumped under the title " Variations." 



"Elementary Species." — In many groups of organisms 

 which are usually called Linnaean species, there are several or 

 numerous " subspecies," or " varieties." They remain more or 

 less constant in their characters from generation to generation, 

 they breed true in artificial conditions, they are not local races 

 with similar modifications ; De Vries caUs them " elementary 

 species." Thus there are about two hundred " elementary 

 species " of the common Crucifer, Draha verna, and a few 

 " elementary species " of the common European heartsease 

 [Viola tricolor), and so on. 



