A. TRIDENTATA. 

 RELATIONSHIPS. 



141 



The considerable number of herbaceous species of Artemisia common to Eurasia and 

 North America justifies the assumption that it is a boreal genus which has extended far 

 southward. This migration brought it into regions with climates growing drier as a 

 consequence of the action of climatic cycles, and led to the evolution of two new ecological 

 groups of species. The one was characterized by the assumption of the shrub habit in 

 response to arid climates, while the other was marked by halophytic adaptation to local 

 saline areas, usually in the form of a half-shrub. It seems probable that Artemisia 

 tridentata was one of the first shrubby species to be developed in response to climatic 



F:o. 20. — Phylogenetic chart of the subspecies of Artemisia tridentata. 



aridity in the southwestern United States and in Mexico. During the dry phase fol- 

 lowing the Pleistocene it appears to have moved northward, occupying the southern 

 half of the Great Basin as the great dominant and extending northward into Idaho, 

 Wyoming, Oregon, and Washington to form a low savannah in the bunch-grass and 

 mixed prairies. With the beginning of the historical period, grazing gradually eliminated 

 the grasses, and the sagebrush increased correspondingly, until much of the savannah 

 was converted into a pure sagebrush community. 



The phylogenetic origin of A. tridentata is connected with the evolution of the section 

 to which it belongs, namely, Seriphidium. It has been shown in the introduction that 

 this is one of the most highly developed sections, that it is a derivative of the section 



