94 HEREDITY AND VARIATION 



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 Variation. — 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 Linnsean 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 modi- 

 fications ; De Vries calls them " elementary species." Thus 

 there are about two hundred " elementary species " of the com- 

 mon Crucifer, Draba verna, and a few " elementary species " of 

 the common European heartsease (Viola tricolor), and so on. 



" The systematic species," De Vries says, " are the practical 

 units of the systematists and florists, and all friends of wild nature 

 should do their utmost to preserve them as Linnaeus has proposed 

 them. These units, however, are not really existing entities ; 

 they have as little claim to be regarded as such as the genera and 

 families have. The real units are the elementary species ; their 

 limits often apparently overlap, and can only in rare cases be 

 determined on the sole ground of field-observations. Pedigree- 

 culture is the method required, and any form which remains 

 constant and distinct from its allies in the garden is to be con- 

 sidered as an elementary species " (1905, p. 12). 



Elementary species are considered to have originated from 

 their parent form in a progressive way ; they have succeeded in 

 attaining something quite new for themselves. 



ReTrograde Varieties. — De Vries applies this term to those 

 numerous forms which have thrown off some peculiarity charac- 

 teristic of their ancestors. Like elementary species they may arise 



