142 IP s t e I s i a 



species extend southward into the forests of the 

 Sierra Nevada as alpine and subalpine trees. 

 One? noticeable fact is that nearly all these trees 

 are able to withstand considerably more severe 

 winter conditions than they undergo in Van- 

 couver Island. vSeveral of them occur far north 

 along the Alaska coast, while others occur in 

 the interior mountain districts where the winters 

 are long and severe. 



In this connection it should be noted that the 

 geographical distrilnition of species appears quite 

 incom])atible with tlie theory that the present 

 plants of the region are post-glacial immigrants 

 either from the south or from the north, though 

 it is quite ])r()bable that the plants of the north- 

 ern ])art of the region have emigrated there from 

 the more southern parts. As it appears im- 

 possible that there should have been an extensive 

 post-glacial migration across the Cascade Moun- 

 tains, the only logical alternative is that the 

 present Pacific coast forest, at least its southern 

 half, represents the survival of the pre-glacial 

 forests of the region. In western Vancouver, 

 while many of the valleys have been glaciated, 



