1915 DISTRIBUTIONAL AREAS 11 
Arctic (or Alpine-Arctic) 
BorEAL Hudsonian 
Canadian 
Transition 
Upper Sonoran 
AUSTRAL 
Lower Sonoran 
TROPICAL 
While all of the zones from Lower Sonoran to Alpine-Arctic are represented 
in California, the accompanying map takes account only of the Lower Sonoran 
(orange), Upper Sonoran (yellow), and Transition (blue), separately, the three 
uppermost subdivisions being lumped into one under the major designation 
Boreal (green). This lumping is advisable for two reasons: (1) the detail on a 
map of small scale would be too fine for practical portrayal; (2) the three divi- 
sions of Boreal are not, in California, as sharply demarked as in a north-and- 
south section of the continent interiorly, zonation in California in this respect 
being confused locally through the effects of small area, and factors other than 
temperature. 
Transition is, as the name implies, a zone of overlapping of Boreal and 
Austral,—where certain types from these opposite categories occur over a defi- 
nite interval on common ground; and there are also perfectly characteristic 
breeding species, chiefly if not exclusively among migrant forms, which render 
this zone easily recognizable. 
Turning again to the faunal divisions of California, we find that the factor 
here involved is undoubtedly humidity of the atmosphere, directly or indirectly. 
Roughly, the western portion of the United States can be separated into an arid 
interior province (Great Basin plus southwestern desert tracts), and a humid 
coastal strip, the latter of increasing width from south to north. In California 
this line of demarcation appears to lie, approximately, along the crest of the 
great Sierran divide the whole length of the state, from the Oregon line in the 
vicinity of Mount Shasta, to the Mexican boundary below, and in line with, the 
Cuyamaca Mountains. Distance from the ocean, prevailing direction of air eur- 
rents, and height of intervening elevations of land crossed by these, seem to be 
the chief conditions modifying the atmospheric humidity of a locality. 
The faunal and subfaunal areas here recognized may be classified as follows. 
The relationship of zones to faunas is also suggested. 
