Elementary Species in Nature 47 



many dozens of elementary forms, each of which 

 remains constant and unchanged in successive 

 generations, even if cultivated in the same gar- 

 den and under similar external conditions. 



Leaving the violets, we may take the vernal 

 whitlow-grass or Draba verna for a second illus- 

 tration. This little annual cruciferous plant is 

 common in the fields of many parts of the 

 United States, though originally introduced 

 from Europe. It has small basal rosettes which 

 develop during summer and winter, and pro- 

 duce numerous leafless flowering stems early in 

 the spring. It is a native of central Europe 

 and western Asia, and may be considered as one 

 of the most common plants, occurring anywhere 

 in immense numbers on sandy soils. Jordan 

 was the first to point out that it is not the same 

 throughout its entire range. Although a hasty 

 survey does not reveal differences, they show 

 themselves on closer inspection. De Bary, 

 Thuret, Rosen and many others confirmed this 

 result, and repeated the pedigree-cultures of 

 Jordan. Every type is constant and remains 

 unchanged in successive generations. The an- 

 thers open in the flower-buds and pollinate the 

 stigmas before the expansion of the flowers, 

 thus assuring self-fertilization. Moreover, 

 these inconspicuous little flowers are only spar- 

 ingly visited by insects. Dozens of subspecies 



