68 FOREST DISTRIBITIOX 



aud more moderate winds, there is also an historical reason in 

 the events subsequent to the glacial period. The more varied top- 

 ography of the Pacifiv Slope from the mountains to the coast 

 furnished conditions of greater diversity and thus more favorable 

 for varied vegetation. Bodies of water formerly occupied areas 

 now sparsely timbered or prairies and these must, at least during 

 the time of their subsidence, have offered conditions more favor- 

 able for the existence and the movements of arborescent species. 

 The distance to be covered, no less than the character of the 

 intervening country, has doubtless been a factor, for the dis- 

 tance from the Cascades to the Eockies is less than half of that 

 across the plains eastward from the summit, and the time neces- 

 sary for movements would have been correspondingly less even if 

 topographic conditions had been equal. Furthermore the direc- 

 tion of the prevailing winds favor the eastward migrations. It 

 is significant also that of the 63 species enumerated below as 

 having entered from the west 27 have seeds winged or otherwise 

 more or less fitted for wind dispersal. 21 bear succulent 

 fruits with small seeds, such as may be carried readily by birds. 

 The others have small or minute seeds. There are no heavA" seed- 

 ed species, unless the whitebark pine is excepted, and the seeds 

 of this tree are eagerly sought by squirrels, birds and other forest 

 animals. There are, of course, few heavy seeded species on the 

 coast in neighboring latitvides. According to all reports no oaks 

 have yet reached Montana either from the west or the east, al- 

 though they occur in Wyoming. 



Following is a list of the species which have entered the 

 Rocky ilountains from the west or northwest : 

 ^Pinus coniorta ^Befula occidcntalis 



^Pinus monticola * " foiitinaHs 



" albicaiilis " alashana 



Larix occidcntalis " glaiidulosa 



^Larix Lyallii *Ahuis tenuifoUa 



*Psei(dotsuga taxifolia ^Odo.ttcmou Aquifolium 



^Abies graiidis *Iiibcs ccreitm 



^Tsv.ga heterophijUa *P]uladclphus Lewisii 



" Mcrtcnsiann ^Spiraea Douglasii 



*Pic€a Engclmannii ^Holodiscus ariaefolius 



^Tlmja plicata *Rubus parvifJoms 



