142 Postelsia 
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. Several 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 distribution of species appears quite 
incompatible with the theory that the present 
plants of the region are post-glacial immigrants 
either from the south or from the north, though 
it is quite probable that the plants of the north- 
ern part 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, 
