35 
California replaced the oaks with junipers, pinons, and sagebrush. 
What was the Middle Temperate flora of the Arizona and Mexican 
plains while the Spruce flora filled the Great Basin region never will 
be known, but this much we know, there was little plant differentia- 
tion. A. argophyllus seems to have branched off the Flexuosi. A. 
mollissinus and Humboldtii represented the Mollissimi; A. nitidus 
the Hypoglottides; A. Canadensis the Uliginosi, but none of these 
seem to have differentiated farther, since all the changes seem to 
have come later when this flora had ascended the mountains of 
Mexico and the Great Basin. Probably the Alpini were replaced by 
the Strigulosi, for the Strigulosi do not occur northward. It is prob- 
able that the Debiles branched from the Homalobi at this time, for 
these plants belong rather to the wet meadows of the Middle Tem- 
perate than higher. At the same time the Plains region seems to have 
seen the origin of the Flexuosi and the first of the Hamosi and Mi- 
cranthi, There is good evidence that the Homalobi expanded at this 
time in the junceus group, but the great change in the group did 
uot occur till the Middle Temperate flora occupied the Great Basin 
and the flanks of the Mogollons and Sierras as is shown by present 
distribution. The great fresh water lakes of the Great Basin and 
Columbia region began to dry up at this time, and the oncoming alka- 
linity put an end to the acid soils of the forests and of the forests them- 
selves and all that flora. The spruce area now was confined to the 
lower mountain slopes and higher valleys of the Great Basin and te 
the Idaho region and the middle slopes of Colorado. The drying up of 
the great lakes put an end to the water distribution from the Wasatch 
to the Sierras and from Western Montana to the Cascades, a feature 
so characteristic of the water period, and for the first time the ele- 
ment of barriers began to be felt in east and west distribution, and 
became more prominent as the aridity progressed. That the Middle 
Temperate flora was differentiated somewhat since the Lower Tem- 
perate has crowded it off the floor of the Great Basin and up the ad- 
jacent mountains is evident, but there is little evidence of any change 
at the north. A. campestris has given way to simplicifolius, sericoleu- 
cus, triphyHus and forms of montanus on the Plains, and in the Mogol- 
lon region to A. humillimus and humistratus and the latter species 
has even invaded the southern flanks of the Great Basin. A. simplici- 
folius has spread through western Wyoming to the edge of the Great 
Basin and covered the lower flanks of the Uintas on both slopes, on 
the south slope a new form appears in A. detritalis. A. montanus. 
has become adapted to almost every form of climate prevailing in 
the Great Basin in its various varieties, growing even in the edge of 
the Tropical. A. junceus has split up in the Sierras into Californi- 
cus and inversus; and in the Navajo Basin into Duchesnensis and Col- 
toni and which also run down ipto the Lower Temperate with other 
derivatives such as Episcopus and Woodruffi. A new form A. steno- 
phyllus presaging the Collini has come in on the northwest of the 
Great Basin and throughout the Columbia region. A. montanus seems 
to have had another offshoot in the Columbia region In A. tegetarioi- 
des. The long continued isolation due to barriers, the climatic changes, 
sparseness of vegetation and absence of crowding, and struggle to har- 
monize with increasing alkalinity and temperature has produced many 
new forms. The Navajo Basin the newest geologically had its floor 
covered by this flora, but it has been replaced today by the Lower 
Temperate, and the Middle Temperate flora fills a narrow strip around 
the rim. The same is true of the floor of the Great Rasin as a whole 
except at the extreme north. The localization « the Middle Tem- 
verate flora indicates that most of the species originated since the 
present period of aridity came on, it is therefore not possible to sep- 
