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. 

 mollissimus and Humboldtii represented the Mollissimi; A. nitidus 

 the Hypoglottides; A. Canadensis the Uliginosi, bu't 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 goi d evidence that the Homalobi expanded at this 

 time in the junceus group, but the great change in the group did 

 not 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 to 

 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, triphyllus 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 sou\th 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 Californl- 

 cus and inversus; and in the Navajo Basin into Duchesnensis and Col- 

 toni and which also run down inito 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 Bnsin 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 bnrriers, the climatic changes, 

 sparseness of vegetation and absence of crowding, and struggle to har- 

 UKinize with increasing alkalinity and temperature h.'is produced many 

 new forma, 'i he Navajo B;is:n the newest geologically h;<d its floor 

 covered by this flora, but it has been replaced today by the Ixjwer 

 Temperate, and the Middle Temperate flora fills a narrow strip around 

 the rim. The same is tn)" rf the floor of thp Gre-^t Basin as a whole 

 except at the extreme north. The localization (i' the Middle Tem- 

 perate flora indicates that most of the species originated since the 

 present period of aridity came on, it is therefore not possible to sep- ' 



