REVIEW AND: DISCUSSION. 137 
represented that a large contingent of Sonoran species had its origin in 
this immediate region, some of them possibly on the very ground of the 
Laboratory domain. Other species, relatively few in number, belong to 
genera which exhibit their greatest morphological differentiation and 
most numerous representation in the northwestern and western United 
States. The recent history of alfilaria shows with the greatest clearness 
one of the routes by which certain plants indigenous to California, or 
previously settled there, have reached us. 
Other plants have a history interwoven with that of the geology of 
western America, and accordingly it can only be stated in general terms. 
The outlines as sketched by Bray (1900) indicate that changes of ele- 
vation of the continental axis, together with climatic changes: dating 
from late Tertiary times, are the occasion of the wide and more or less 
discontinuous range of closely related species, which now grow on the 
plains of the southwestern United States and those of Chile and Argen- 
tina. At a relatively early day, possibly not later than the Pliocene, 
there seems to have been a continuous highway, since closed, from south- 
ern Mexico, along the Isthmus of Panama, southward to Argentina and 
Patagonia, by which the creosote-bush, various cacti, yuccas, certain 
composites and members of other groups represented on the Laboratory 
domain at the present day, made their southward journeys. In the 
northern hemisphere the later advance of the great ice-sheet in the Pleis- 
tocene drove southward many plants, some of which, as Anemone and 
Delphinvum, are found to-day on Tumamoc Hill, but in general they 
have made the mountains their pathway and also their home, and their 
minor movements can not well be traced. 
Of representatives of genera belonging to both hemispheres, including 
miscellaneous introduced species, it can only be said that they have 
entered the Laboratory domain by the most various routes and agencies. 
Each of these presents a separate history of its own, which in a few cases 
is well-known, while in others it remains undeciphered. 
The region in which the Desert Laboratory is situated has not only 
been the recipient of plants of other regions, but it has contributed its 
quota to other floras. Examples of this, in addition to those already 
given, are some of the cacti which have nearly or quite gained the British 
boundary in their northward movements. 
