34 



A. Hookerianus and Cottoni being almost the only species of cold 

 climates (A. Ciisickii and jejunus are hot climate modifications of 

 the Hookerianus branch). A. platytropis is of doubtful origin, prob- 

 ably an invasion from warmer regions. 



The present distribution of the Mollissimi would indicate their 

 general distribution in Glacial time on the plains of Mexico and this 

 would account for the differentiation of the group since then by isola- 

 tion In the mountains, being forced up from the Lower Temperate 

 life zone then prevalent on the plains to the same zone now pre- 

 vailing in the mountains. A. Bigelovii and mollissimus are the only 

 species still prevailing on thv plains. 



The occurrence of A. nitidus would indicate its presence then on 

 far southern plains in the Middle Temperate life zone and its dif- 

 ferentiation into the Uliginosi at a very early time. The fact that 

 it does not exist out of the sodded region seems to indicate a lack of 

 access, barriers, in that region, (the Great Basin and Columbia region) 

 but no such barriers exist, though its congener A. agrestis is freely 

 distributed there. The explanation is found in the narrow soil 

 adaptabiity of the species, it will grow in moist me.-^idows. or 

 anywhere but in the peculiar free drainage and small rainfall of the 

 plains, which conditions are not found in the Great Basin. This is 

 shown again by the great dissimilarity in the species of the same 

 zone under apparently similar conditions, and in the absence of 

 many such species in the Sierras, which indicates a much greater 

 climatic diversity than appears on a casual inspection. This is easily 

 understood on inspection of the rainfall curve which shows a great 

 lack in summer rainfall in the region west of the sodded area where 

 though the annual amount is the same, the bulk falls in the summer in 

 the podded area and in the winter (out of the growing season) in 

 the Pacific drainage. 



The total Arctic and alpine species are eight. The meager nv^i- 

 ber of species in all genera and their close relation to those of the 

 forest area do not permit the separation of this treeless region from 

 the other, except as a subdivision of the Upper Temperate life zone 

 as a whole. 



The Upper Temperate life zone, also called the spruce zone which 

 extends from the treeless regions at the north to the deciduous oaks 

 and the upper limit of the sagebrush, and (in Colorado and the 

 Great Basin) the lower limit of the aspen, must have disappeared 

 from the plains of Arizona and northern Mexico giving way to 

 prairies, and fringed the Mogollon slopes and the low^er Sierra foot 

 ard higher plains as (fhe / rntic flora climbed the mountains on the 

 gradual recession of the Ice Age. The forest flora covered the plains 

 region from Kansas eastward. The wet rreadows and similar open 

 spaces and the rocky regions were the only places where Astragali 

 could thrive. The modified ff^rms of A. campestris and montanus 

 evidently grew on the rocks. The new form coming in and adapted 

 to the forests A. tenellns is cosmopolitan and must have come in 

 at this time as shown by its present distribution. It was a marked 

 deviation from the campestris type though not a greajt one and 

 presages the Inflati in A. pauciflorus which dries black in the same 

 way and is not far removed from it genetically. 



At this time the Alpini show differentiation in A. elegans, abori- 

 ginum and Americanus, forms adapted to the forest areas. 



MIDDLE TEMPERATE LIFE ZONE 



With the advent of the Middle Temjjerate climate from the south 

 climbing over the southern slopes of the Great Basin and rejilacing the 

 dense forests with oaks and open parks and i)rairies and supple- 

 mented by the mountain barriers and hot lower regions as barriers 

 there arose many isolated areas with peculiar climates suitable for 

 plant differentiation. The regions of Arizona, southern Nevada and 



