142 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
The experimental work of Dr. Livingston on evaporation has gone far 
to provide an efficient means of testing the ecological equivalence of 
habitats, and this work goes to show the value and practicality of maps 
of distribution based on the principle of such equivalency, which would 
serve, as nothing else could, as a rational basis for various horticultural 
undertakings, that at present are in an almost wholly empirical stage. 
(10) The general continuity of geological history since the Tertiary 
indicates a relatively long period within which plants of the Laboratory 
domain have one by one, or at any rate by no mass movements, become 
established in their places. There is reason to believe that throughout 
this period the processes now going on before our eyes have been in 
progress. ‘The present flora, therefore, may be assumed to be merely 
the final stage, thus far, of just such a series of events as are now observ- 
able. Invasions have taken place, competition has ensued, certain spe- 
cies have become established as prominent constituents of the various 
associations, while others have died out or taken subordinate places. 
Species now growing side by side have reached Tumamoc Hill from 
widely separated regions and at intervals of possibly thousands of years 
apart. The small area within its limits has received representatives of 
genera that have shared in the great migrations south and north along 
the Cordilleras, but through the time that has elasped since these greater 
movements it has also received, by entirely ordinary means, the plants 
that have come and are still coming to it. The general history is not 
essentially affected by the fact that portions of Tumamoc Hill are of 
quite recent origin. These parts give evidence of having received their 
~ flora in the main from the immediate neighborhood, although certain 
genera have come from points at some little distance, without settling 
in the less favorable intervening territory. 
If, then, as the evidence seems to indicate, the same geological agencies 
have been operating in the same way from the Tertiary down to the 
present, and generally arid conditions have prevailed, it is safe to say 
that during that long period the domain of the Desert Laboratory has 
suffered no greater, if as great, extremes of climate as prevail to-day in 
places no farther apart than Tucson and the Santa Catalina Mountains. 
It seems likely that to-day, within areas only a few miles apart, we may 
see floras more diverse in character than the late Tertiary and present- 
day floras of the Laboratory domain. As a corollary it may be assumed 
that, neglecting minor divisions, there have existed in this region, at least 
from the late Tertiary, two widely different floras, that of the plains and 
that of the mountains, the former of essentially desert character, the latter 
of mesophytic species, and that during this period, without losing at any 
time their distinctive features, they have, by precisely the same agencies 
as are operative to-day, received accessions, lost or passed on waning 
species, and otherwise suffered gradual modification. 
