50 Elementary Species 



species in his garden. In this region they are 

 crowded together and not rarely two or even 

 more quite distinct forms are observed to grow 

 side by side on the same spot. Farther away 

 from this center they are more widely dispersed, 

 each holding its own in its habitat. In all, Jor- 

 dan has distinguished about two hundred spe- 

 cies of Draba verna from Europe and western 

 Asia. Subsequent authors have added new 

 types to the already existing number from time 

 to time. 



The constancy of these elementary species is 

 directly proven by the experiments quoted 

 above, and moreover it may be deduced from the 

 uniformity of each type within its own domain. 

 These are so large that most of the localities are 

 practically isolated from one another, and must 

 have been so for centuries. If the types were 

 slowly changing such localities would often, 

 though of course not always, exhibit slighter 

 differences, and on the geographic limits of 

 neighboring species intermediates would be 

 found. Such, however, are not on record. 

 Hence the elementary species must be regarded 

 as old and constant types. 



The question naturally arises how these 

 groups of nearly allied forms may originally 

 have been produced. Granting a common ori- 

 gin for all of them, the changes may have been 



