68 FOKKST DISTHIBUTION 



and more moderate winds, then- is also an historical reason in 

 tli.- events subsequent to tlie glacial period. The more varied top- 

 ography of Hie Pacific 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 arid these must, at least during 

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

 able for the existence and the movements of arboreseent 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 Rockies 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. 

 Tlie others have small or minute seeds. There are no heavy 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 latitudes. 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 .Mountains from the west or northwest : 

 *Pinus contort a ~l<lul<i <><'<'i<l< nlu/i* 



s monticola * " fonlimih's 



" albicaulis " nl<isl,;in<i 



>'!> n talis " glandulosa 



-/.<//;, /,//,///;; *Alnus >< n HI folia 



*PseudotSUgQ l<irif<>'i<; ; 1)<losi< niun A'/iii folium 



"A 1 "Us *Kibcs 



<!<t luh i-nfthiilln 



Kertensiano 



*ri<-(U Enf/flmfiHiiii -I/ulotliscus a rtic 



-Thuja ftlirata Rubus 



