INTRODUCTION 23 



and west'ward the prairie and forest alternate with frequency 

 depending in the main upon topographic variation. This di- 

 versification of the vegetation in response to topographic in- 

 fluence in its relation to climate is one of the most interesting 

 aspects of the plant life of the region, and one of the most 

 characteristic. This influence may, indeed, be said to be the 

 dominant one determining the physiognomy of the plant cover- 

 ing over all the western part of the State. 



The flora of the northern Rocky Mountains has been cited 

 as mainly a composite one and such is evidently the case. As 

 the first element may be considered those which are apparently 

 indigenous descendants or derivatives of a former more extensive 

 and probably more uniform vegetation of the Later Tertiary, and 

 those which in subsequent times have evolved from the species 

 from various sources in the migrations subsequent to the close of 

 the glacial period. Most of this region was glaciated and with 

 the recession of the ice was reoceupied from various centers of 

 distribution. "VYe must believe that the forces which then oper- 

 ated in the dispersal of plant life over this area are, at least 

 many of them, still operative and that the invasions from the 

 east, southeast and west, which account for so large a part of 

 the Montana flora, are still in force. However slow the move- 

 ments of a species may be, in competition with others, in regions 

 already well occupied, there is no reason to believe that its 

 present limits of distribution are fixed. 



The vegetation of Montana shows a distinct overlapping 

 of the eastern and western floras. The flora of the Pacific 

 Northwest is well represented on the western slopes of the 

 Rockies in Montana. The number of species, of course, dimin- 

 ishes eastward, yet some of evident Pacific origin have extended 

 their range to the Black Hills and the Great Lakes. Similarly 

 species of the Great Plains appear in the prairies west of the 

 Continental Divide. 



In considering, therefore, the distribution and composition 

 of these forests, special emphasis must be laid on the one hand 

 on the specific individuality of the component species and on 

 the other on the combined influence of topography and climate. 

 As a basis for further discussion, the latter factors will be con- 

 sidered first. 



