221 



a inouiitaiii barrier dilhciilt of passage, nor a thoroughgoing ' 

 change of cHniatic and soil conditions to bar the way. The 

 percentage of species that are typically Californian of course 

 becomes steadily less as we advance northward; if 50 per cent 

 of the Californian forms reach the Umpqua \alley, not above 

 10 per cent are found in that of the Willamette; but there is no 

 fixed point beyond which they totally disappear. 



As for the second group, any attempt to determine the dis- 

 tribution of species in western Oregon on a purely zonal basis 

 will prove unsatisfactory. In the arrangement of plant zones, 

 that part of Oregon lying west of the Cascades has been assigned 

 to the Humid Transition, while the Upper Sonoran is considered 

 to exist east of the Cascades only. But this arrangement fails 

 to take account of the fact that we have two very different types 

 of climate in western Oregon. During nine months of the year 

 we have heavy rainfall and high humidity; during the other 

 three months the rainfall is negligible and the atmosphere be- 

 comes very dry. During this latter period the conditions of 

 growth in the western valleys are almost identical with those 

 which prevail in the semi-arid region east of the Cascades, and 

 only the most strongly drought-resisting species are able to con- 

 tinue their growth. If therefore a characteristically Upper 

 Sonoran species were introduced into the Willamette Valley at 

 the beginning of the dry season, it would find no difficulty in 

 adjusting itself to its environment; nor would the mild winter 

 offer any serious obstacle to its survival. Certain localities in 

 the region about Salem, where the soil is thin and rocky or 

 gravelly, especially on slopes facing the west, offer conditions 

 before the end of the long summer that are as truly Upper 

 Sonoran as in any part of eastern Oregon. These conditions 

 being uniform in these localities, one would expect the flora to 

 conform; and as a matter of fact, there are localities about Salem 

 where the flora differs very slightly from that of a similar area 

 in the semi-arid districts. A zonal map that was wholly accurate 

 would have its "Humid Transition" region thickly dotted over 

 with larger or smaller patches of the color used for the I'pper 



