226 Forestry Quarterly. 



modified by their environment and isolation. The specific status 

 of both the spruce and the birch is in doubt, some authorities 

 make distinct species of both. 



The species from the east have, in most cases, been modified 

 both botanically and silvically. In some cases this modification 

 took place on the long journey across the plains, in others since 

 the arrival in the Hills. The narrow-leaved cottonwood (Popu- 

 lus acuminata) , a varient of the common cottonwood of the 

 plains, is an example of the first. The various willows are ex- 

 amples of the second. These after the cool moist canyons of 

 the eastern Hills after their long prairie journey under such 

 vastly different conditions, have after the well known habit of 

 willows varied greatly. It is almost impossible to separate them 

 into recognized species. Bur oak and American elm, being old 

 and stable species, have not varied botanically, but were dwarfed 

 by the long plains journey, and have only partly succeeded in 

 regaining their size. Perhaps the relatively dry climate is a fac- 

 tor in this. 



The cosmopolitan Yellow pine has not varied botanically in 

 the Hills as it has in its further eastward journey in the sand 

 hills of Nebraska, where it has developed into a sub-species 

 (Pinus ponderosa var. scopulorum) as a result of its changed en- 

 vironment. Nevertheless Black Hills Yellow pine is silvically 

 quite distinct from that in the Rockies. 



If the supposition that a narrow strip of forest once connected 

 the Black Hills' with the great northern forest of the continent 

 is correct, it is probable that at one time other species of the 

 northern forest occurred there. Eastern larch, Balsam fir and 

 Jack pine or forms very closely resembling them, would likely 

 have been represented. A climatic change or the competition of 

 the Yellow pine could account for their disappearance. 



The path from the Rocky Mountains was closed by the ad- 

 vancing aridity of the region, perhaps aided by forest fires. 



The migration from the east is still going on. Many hard- 

 wood species, such as the backberry and the ash, have scarcely 

 yet penetrated beyond the eastern foothills. Few of those which 

 have, have as yet been able to adapt themselves to grow upon the 

 uplands, Bur oak and aspen being those which have come closest 

 to it. If natural conditions were not interfered with in time it 

 might be that some of the eastern hardwoods would regain the 



