FOKKST MSTRir.rTION 



all but a iVw cases means mesophytic conditions, or they arc 

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

 species arc 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 Range is drained by the (Mark's Fork of the 

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

 Knot, the Blackfoot. and the Hellgate (the upper continuation 

 of the (lark'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 

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



:its 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 mesophytic situations. It 

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

 the passes and the deeper gorges of this range are mesophytic 

 conditions found and in these some of the moisture loving species 

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

 the west. 



In dismissing the topography it was pointed out that a hiuli 

 mountain harrier extends along the western border of Montana, 

 except when- the Coeiir d'Alciie Raii'je interrupts the higher ele- 

 vations with a br.iad plateau-like formation, dissected into nu- 

 merous canyons and relatively low summits. Just north < f this 

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

 the Continental Divide over a distance of :>()( miles of its western 

 North of this the hiuh ranges auain arise, extending far 

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

 all of the species abov.- ascri'xd to western origin CiO) possibly 

 'x-ept }\\'_ r Ly.-ill's lar<-h and others ordinarily found only on the 

 hiirhest elevations. It is evident that this ran.izv is the main 

 highway for the 1'acjfic species into western Montana. The con- 

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

 region n f northern Idaho, all serve to make Ibis ran^e at once a 

 onvcnicnt harbor and highway for the more tender mesophytic 

 spe.-ics of the Humid Transition. These species as.-cnd the west- 



