220 BOTANICAL GAZETTE [SEPTEMBER 
All the species in the Colorado column have southeastern 
affinities; all those in the Mojave column have northeastern 
affinities. In the former, ten out of sixteen extend into Lower 
California; in the latter, none. 
The distribution of the desert flora, and fauna as well, is not 
known, as yet, with sufficient exactness to permit positive state- 
ments. But facts already accumulated indicate that in the 
Colorado subarea the Lower Sonoran flora, which extends over 
the entire desert area, is very slightly modified by any other. 
The Mojave subarea, on the contrary, shows a marked influence 
from the Great Basin life-area. The limit to which this extends 
appears to be defined by the Chuckawalla Mountains. 
While it is true that, within our territory, the general eleva- 
tion of the Mojave Desert considerably exceeds that of the 
Colorado Desert, a difference having an undoubted modifying 
effect on their floras, yet the precipitation, the temperature, and 
other conditions of the two subareas are very similar. And the 
conclusions at which we have arrived would be strengthened were 
the investigation to be extended so far beyond our limits as to 
include the depression of Death Valley. Hence it may be 
inferred that the difference in the character of the two floras is 
only in part due to climatic causes, but is largely influenced by 
the topography of the region. In the one case a current of 
migration was able to pass up, encountering no physical barriers, 
from Arizona and Lower California into the Colorado Desert; 
in the other a current from eastern Utah and Nevada would meet 
no considerable obstacle until it reached the San Bernardino 
Range and its continuation. 
The distinctness of the two subareas is further evident from 
a consideration of the zonal distribution of their respective floras. 
_ Much remains to be learned before these zones can be delimited 
accurately and finally; but sufficient data are at hand to permit 
their general disposition to be outlined, eS the details for 
completer knowledge. 
The Larrea belt, which is considered as aedieciens the limits of 
the Lower Sonoran, is present in both deserts. But however 
useful this shrub may be as the biological index of larger 
