34 
A. Hookerianus and Cottoni being almost the only species of cold 
climates (A. Cusickii 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 the 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 ig freely 
distributed there. The explanation is found in the narrow soil 
adaptabiity of the species, it will grow in moist meadows, 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 sodded area and in the winter (out of the growing season) in 
the Pacific drainage. 
The total Arctic and alpine species are eight. The meager num- 
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 lower Sierra foot 
and higher plains as the Arctic flora climbed the mountains on the 
gradual recession of the Ice Age: The forest flora covered the plains 
region from Kansas eastward. The wet meadows and similar open 
spaces and the rocky regions were the only places where Astragali 
could thrive. The modified forms of A. campestris and montanus 
evidently grew on the rocks. The new form coming in and adapted 
to the forests A. tenellus 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 great 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 Temperate climate from the south 
climbing over the southern slopes of the Great Basin and replacing the 
