SOURCES OF THE VEGETATION 71 



ern slopes of the Bitter Roots but usually fail to get over, and 

 again to the north the region is inhospitable on account of its 

 altitude in some places and in others on account of greater aridity. 



Some of the species apparently have come into Montana 

 from the northwest along the Rocky Mountains. Some have a 

 distribution from Alaska to Oregon and in the Rockies to Mon- 

 tana or Wyoming. Others seem to have made their way across 

 from the coast by way of the Okanogan country and the Selkirks. 

 Shaw (57) found in the Selkirk Range most of the species listed 

 above. It is not improbable, as already stated, that formerly 

 conditions were much more favorable for a direct eastward mi- 

 gration across the inland basin which now would be impossible. 



Certain species in this eastward migration have moved on- 

 ward across the main range of the Rockies. The Continental 

 Divide in its general course and altitude has been discussed 

 under topography. It constitutes the main barrier between the 

 eastern and western floras. Many species, however, have crossed 

 it both from the east and the west. In many places, up to 

 7000 feet or more the crest is wooded, sometimes with dense 

 forests, more often with scattered pines and junipers, and here 

 and there a bleak, wind-swept, rocky ridge, tenanted only by 

 low, caespitose perennials of xerophytic habit and structure. 

 While the ridge is not uniformly high, the greater part of its 

 length in Montana is 7000 feet or more in elevation, dropping 

 in some of the passes to less than 6000 feet. These gaps, how- 

 ever, are not wide and climatically are quite different from the 

 Coeur d'Alenes farther west. They share in large measure the 

 rigors of the winters which are long and severe in the neighboring 

 heights. At these elevations the summers are short and the 

 growing season two to three months, with great diurnal range of 

 temperature and with nights frequently marked by heavy frosts. 

 The snow usually disappears during the summer except in patches 

 on sheltered north slopes, or in the glaciers which occupy some 

 of the high depressions in the northern ranges. 



Plants are abundant and over most of the high slopes vig- 

 orous, during the short season allotted to them, but on the whole 

 the region of the high divide is inimical to many species and in 

 general an effective barrier to eastward and westward move- 

 ments. It is interesting, however to note how many of those 



