GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 381 
LIFE REGIONS AND ZONES OF NORTH AMERICA 
In a communication I had the honor to lay before this society two 
years ago (December 4, 1889),* I stated that the Hudsonian and Cana- 
dian zones of the Kast belong to the Boreal region, and extend com- 
pletely across the continent, and that the desert areas of the West 
belong to the Southern or Sonoran region. The pine plateau (Pinus 
ponderosa) of Arizona and other parts of the West was “shown to 
consist of a mixture of Boreal and Sonoran types. - - - In other 
words, it is neutral territory” (North American Fauna, No. 3, Sep- 
tember, 1890, p. 20). [ remarked further that the Carolinian fauna 
‘is suffused with southern forms, and the Alleghanian seems to be 
neutral ground” (Jbid., p. 18), thus implying that the “neutral” or pine- 
plateau zone of Arizona is the western equivalent of the ‘‘Alleghanian 
Fauna” of the East. 
In a subsequent publication (North American Fauna, No. 5, August, 
1891) I went a step further, defining the treeless parts of the ‘“‘ Neutral 
or Transition zone,” and characterizing an ‘* Upper Sonoran zone,” as 
distinguished from the Lower or True Sonoran; but nothing was said 
as to the relations of these zones with those long recognized in the Kast. 
The time has now arrived, however, when it is possible to correlate 
the Sonoran zones of the West with corresponding zones in the East, 
as was done two years ago in the case of the Boreal zones, and as was 
intimated in the case of the Neutral or Transition zone. It can now 
be asserted with some confidence not only that the Transition zone of 
the West is the equivalent of the Alleghanian of the East, but also 
that the Upper Sonoran is the equivalent of the Carolinian, and the 
Lower Sonoran of the Austro-riparian, and that each can be traced 
completely across the continent. Thus, all the major and minor zones 
that have been established in the East are found to be uninterruptedly 
continuous with corresponding zones in the West, though their courses 
are often tortuous, following the lines of equal temperature during the 
season of reproduction, which lines conform in a general way to the 
contours of altitude, rising with increased base-level and falling with 
increased latitude. 
The Boreal region extends obliquely across the entire continent 
from New England and Newfoundland to Alaska and British Columbia, 
and from about latitude 45° north to the Polar Sea, conforming in gen- 
eral direction to the trend of the northern shores of the continent. It 
recedes to about latitude 54° on the plains of the Saskatchewan, and 
gives off three long arms or chains of islands, which reach far south 
along the three great mountain systems of the United States—an 
eastern arm in the Alleghanies, a central arm in the Rocky Mountains, 
*Since published in my report on the ‘‘ Results of a Biological Survey of the San 
Francisco Mountain region in Arizona,” N. dm. Fauna, No.3, September 11, 1890, 
