2 DISTRIBUTION AND MOVEMEN'’S OF DESERT PLAN‘S. 
example, growing luxuriantly in close proximity to the most pronounced 
types of xerophytes. Thus, as regards historic, edaphic, and climatic 
factors, the tract selected offers exceptional advantages for such a study 
as is here undertaken. 
The fact that Tumamoc Hill owes its origin to volcanic agencies, inter- 
mittently 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 accept 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 empha- 
sizing the efficiency of existing agencies as factors in their present actual 
distribution, without invoking other causes to explain phenomena trace- 
able 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 been 
taking place, broadly speaking, since early Pleistocene times. 
It can hardly be doubted that in this region, during the period from 
the Tertiary to the present, two distinct floras of different origin have 
existed side by side as they do to-day; the one including the desert plants 
of the arid plains and lower elevations, the other the mesophytes of the 
mountains. As just stated, the species belonging to the former have 
apparently originated where they now live, or at least have undergone 
migrations of very limited extent compared with those which many of 
the latter have made as representatives of plants which in earlier days 
migrated southward from arctic regions, and which still, at the present 
time, exhibit features of the old Eurasian stock. ‘There is, of course, 
more or less intermingling of the elements of these very diverse floras, 
where climatic and soil conditions make this possible, but one has only 
