74 TJie Different Modes of Origin of nezv Speeies. 



usually ranged as elementary species of larger species, 

 but sometimes regarded by the best authorities as "good" 

 species. The group most closely related to our experi- 

 mental series is that of Oenothera biennis or the subgenus 

 Onagra ;^ more remotely connected are the groups of 

 Hieraciuni, Rosa, etc., or of Draba verna, Viola tricolor 

 etc. Such groups appear to us as the relics of past periods 

 of mutation. The new forms which arise from such 

 periodical mutations are to be regarded as homonomous 

 subdivisions of the older species or as elementary species. 



It is natural in such periods not only that new specific 

 characters should arise, but that old established latent 

 ones should reappear more easily than at other times ; 

 and among the mutations of Oenothera Laniarckiana our 

 O. nanella is undoubtedly analogous to typical horticul- 

 tural dwarf varieties, and 0. laevifolia to those systematic 

 varieties which arise by the loss of a character. 



These latter, however, and similar retro- and degres- 

 sive changes are an entirely different matter. The essen- 

 tial condition for their production is always present, and 

 all that is needed is the external stimulus to induce the 

 mutation. This, it appears, need not occur periodically, 

 nor affect several characters at the same time. New horti- 

 cultural varieties appear at irregular intervals of time, 

 and here and there in the area of cultivation of the spe- 

 cies. But it is equally certain that we have to do in such 

 such cases exclusively, or almost exclusively, with retro- 

 gressive and degressive changes.^ Analogy and paral- 

 lelism are universal, and their effects sometimes go so 

 far that the characters of the species fall into the back- 

 ground. Double flowers look -so much alike that one 



^See Vol. I, p. 439; and §31, p. 490. 



^I am excluding from this consideraton the effects of crossing. 



