294 Part III. Chapter 3. 
extent of territory. These lakes existed during the Pleistocene and we have 
every reason to believe that the desert flora which now reigns supreme was 
not then widely spread, but its place was taken by a vegetation more typic 
of a humid climate with its forests and other plants dependent on a more 
abundant rainfal. Two elements of the desert flora may be distinguished, 
viz., a Great Basin element and a Mexican element. It is probable 
that the differentiation of the Great Basin element began before the great 
glacial period and was preserved and still further developed, as an arid type 
during the period of more humid climate in the enclosed valleys, such as 
Death Valley and in the Mohave Desert region cut off from the prevailing 
source of rain supply and on the drier hillsides. .With the gradual coming 
on of a dry climate and the disappearance, or contraction of the Pleistocene 
lakes in all probability the endemic Great Basin element of the present flora 
extended its range northward and downward into the areas left bare by the 
disappearance of the fresh water lakes. A present day illustration of this suc- 
cession is afforded by the Salton Sea which was formed in the Colorado Desert 
by the diversion of water from the Colorado River. The freshwater of the 
lake dissolved out much of the alkali of the new shore soil, and as the water 
recedes, since the break in the bank of the river has been closed by engineers, 
the alkali plants occupy the shore line of recession with difficulty. Many 
thousands of the arrow weed, Pluchea sericea, and cat tail, 7ypha angustifolia, 
as well, as other species which occupied the shores of the new lake find it 
difficult to maintain their hold as the shore soil becomes drier and more alka- 
line, so that it is only a question of time when the freshwater vegetation will 
have disappeared and will have been replaced by other vegetation, a8 noticed 
by MacDouGat )). 
At about this time, the Mexican element migrated into the Basin and a5 
the plants constituting it were already from their origin in the ancient deserts 
of Mexico suitably adapted both morphologically and physiologically to dıy 
soil and a dry climate, they naturally spread and usurped much of the tert 
tory formerly controlled by a more humid climate and covered by a morf 
mesophytic vegetation. In all probability, therefore, the spread of the Great 
Basin element began in the Pleistocene and has continued ever since and t : 
the Mexican element was introduced into the region at the close of the Pleisto“ 
cene at a period subsequent to the disappearance .of the Pleistocene lakes. 
C ee 22 n the other hand, in all probability, entered en 
ae # e eistogene for we have evidence of this in the pr a 
en : en eig plants in southern California, not present N nn into 
the ER z en their migration would have been more eis. ib 
San Bernardino a nn f u es ya, telec Sr ey = of 
flora of the Mahre Da ernen Inu. presenich: inch SHE = ith the | 
ve Desert, there is a clear evidence of its affınity Wit 
1) Year Book of the Carnegie Institution of Washington, 1908: 56—57- 
