70 FOREST DISTRIBUTION 



all but a few cases means mesophytie conditions, or they are 

 found only along the western boundary of the State. The other 

 species are more or less abundant locally but have no very wide 

 range, or they may be sparingly and uniformly scattered over 

 the area. 



The number of Pacific Northwest species increases with the 

 distance westward from the Divide. As nearly the whole of 

 Montana west of the Kange is drained by the Clark 's Fork of the 

 Columbia and its several main affluents, the Flathead, the Bitter 

 Root, the Blaekfoot. and the Hellgate (the upper continuation 

 of the Clark 's Fork and now known by the same name ) . the re- 

 gion falls naturally into four divisions corresponding to these 

 main drainage basins. Of the two most westerly, the Flathead 

 lying to the north and the Bitter Root to the south, the Flathead 

 presents the most humid conditions and contains the largest 

 number of western species. The Bitter Root, on the other hand, 

 is the least humid and presents fewer mesophytie situations. It 

 lies in the rain shadow of the Bitter Root Mountains. Only in 

 the passes and the deeper gorges of this range are mesophytie 

 conditions found and in these some of the moisture losing species 

 appear, but most of them are excluded by the high barrier to 

 the west. 



In discussing the topography it was pointed out that a high 

 mountain barrier extends along the western border of Montana, 

 except where the Coeur d " Alene Range interrupts the higher ele- 

 vations with a broad plateau-like formation, dissected into nu- 

 merous canyons and relativel.v low summits. Just north cf this 

 lies the channel of the Clark's Fork bearing all the waters of 

 the Continental Divide over a distance of 300 miles of its western 

 slope. North of this the high ranges again arise, extending far 

 to the north. The Coeur d 'Alene Range is occupied by nearly 

 all of the species above ascribed to western origin (30) possibly 

 excepting Lyall's larch and others ordinarily found only on the 

 highest elevations. It is evident that this range is the main 

 highway for the Pacific speeies into western ^Montana. The con- 

 figuration of the land, its low altitude, its proximity to the lake 

 region of northern Idaho, all serve to make this range at once a 

 convenient harbor and highway for the more tender mesophytie 

 species of the Humid Transition. These species as end the west- 



