32 The Plant World. 



slightly upwards of 3,000 feet, gives at this latitude, in connec- 

 tion with its topographical features and variety of soils, an en- 

 semble of conditions under which plants of very different habits 

 occur together; well-marked mesophytes, for example, growing 

 luxuriantly in close proximity to the most pronounced types 

 of xerophytes. Thus, as regards historic, edaphic, and cli- 

 matic factors, the tract selected offers exceptional advantages 

 for studies of local distribution. 



The fact that Tumamoc Hill owes its origin to volcanic 

 agencies, intermittently active from the Tertiary to within 

 Pleistocene history, fixes the period within which its flora has 

 become established. During the whole of this period, if we ac- 

 cept the views of present day geologists, generally arid conditions 

 have prevailed throughout this region, except at higher altitudes. 

 So far, then, as available evidence goes, the desert plants now 

 growing here originated in their present places or came to them 

 as desert plants; there is no evidence that they became such 

 after their arrival; and taking them as we find them, we are under 

 the necessity of emphasizing the efficiencv of existing agencies 

 as factors in their present actual distribution, without invoking 

 other causes to explain phenomena traceable to those now in 

 operation. 



It is a matter of congratulation that the existing flora has 

 been so little disturbed or modified by human agency. The few 

 introduced weeds, though in some cases conspicuous, have made 

 little real impression upon its character, and there is no evidence 

 that extermination of species has followed the occupation of 

 Tumamoc Hill by the prehistoric people the outlines of whose 

 dwellings are still distinctly traced upon its summit, nor even 

 through the ruthless work of modern quarrymen, by which its 

 sides are here and there marred. Changes due to human agency 

 have undoubtedly occurred, and are seen most plainly along the 

 various trails and wagon roads that have been constructed; but 

 there is no reason to suppose that thus far they are of more than 

 the most superficial character. Certain changes are beginning 

 to follow the fencing of the reservation, coincidently with the 

 exclusion of hunters and cattle; but the flora in all its essential 

 features, as it now exists, is presented to us as the final phase, 

 thus far, of the natural movements and adjustments that have 



