344 TURESSON: SLOPE EXPOSURE AND PSEUDOTSUGA 
presents itself which may explain these strange cases, namely, 
the fact that these islands lie in the lee of the Olympic Mountains 
and therefore have a lesser rainfall. The conditions, therefore, 
more nearly approximate those of the Arid Transition area than 
any other portion of Washington west of the Cascade Mountains.” 
It is not surprising to find Pseudotsuga taxifolia confined to the 
northern slopes of the hills in these islands; in fact, a different 
behavior would be hard to understand, since it is here submitted 
to practically the same climatic conditions as in other arid regions of 
its range, where it, as has been shown, always occupies the northern 
declivity as a result of the effect of slope exposure. 
Rigg (7) has in an interesting paper, dealing with the forest 
distribution in these islands, pointed out the seemingly peculiar 
distribution of Pseudotsuga taxifolia as limited to the north-facing 
slopes of the hills. He is inclined to attribute to the soil the dif- 
ference in the forest distribution. Upon investigation of the soil 
on the barren south-facing slope and on the forested north-facing, 
it was shown that the soil on the barren portion was black and 
powdery, containing a good deal of gravel, while the forested 
portion was everywhere covered with two feet or more of yellow 
clay, containing occasional irregular fragments of rock. 
It is hard to believe that the chemical nature of the different 
soils should cause the difference in the forest distribution, but as 
darker soil is more readily and strongly heated than is that of 
lighter color, this character may in a subordinate way have some 
influence (see Falk, 2; Schimper, 8; Warming, 11). When we on 
the other hand know how profoundly slope exposure affects the 
distribution of Pseudotsuga taxifolia in regions with similar climatic 
conditions to those in the San Juan Islands, the determining factor 
is presumably the same. 
Cowles (1) no doubt is right when stating that a species in 
general can grow in a large number of formations at its center of 
distribution, since there the climatic condition favors it most 
highly. In other regions, especially near its areal limits, it can 
grow only in those formations which resemble most closely in an 
edaphic way the climatic feature at the distribution center. We 
have found how Pseudotsuga taxifolia when growing under condi- 
tions not favorable to its development always establishes itself 
