518 Mutations 



have been shown to be in reality composite as- 

 semblages. Roses and brambles, hawkweeds 

 and willows are the best known examples. Vio- 

 lets and Draba verna, dandelions and helian- 

 themums and many other instances were dealt 

 with in previous lectures. Even wheat and bar- 

 ley and corn afford instances of large groups 

 of elementary species. Formerly mixed in 

 the fields, they became separated during the 

 last century, and now constitute constant races, 

 which, for brevitv's sake, are dealt with under 

 the name of varieties. 



In such groups of nearly allied forms the sin- 

 gle members must evidently be of common or- 

 igin. It is not necessary for them to have orig- 

 inated all in the same place or at the same time. 

 In some cases, as with Draba verna, the present 

 geographic distribution points to a common 

 birthplace, from whence the various forms may 

 about the same period have radiated in all direc- 

 tions. The violets on the other hand seem to 

 include widely diffused original forms, from 

 which branches have started at different times 

 and in different localities. 



The origin of such groups of allied forms 

 must therefore be the object of our research. 

 Perhaps we might find a whole group, perhaps 

 only part of it. In my opinion we have the 

 right to assume that if Draba and violets and 



