22 Merriam Geographic Distribution of Life. 



and Canadian zones of the East belong to the Boreal region and 

 extend completely across the continent, and that the desert 

 areas of the West belong to the Southern or Sonoran region. The 

 pine plateau (Pinus ponderoso) 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, September, 1890, p. 20). I remarked 

 further that the Carolinian fauna " is suffused with southern forms, 

 and the Alleghanian seems to be neutral ground" (Ibid. 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, denning 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 East. 



The time has now arrived, however, when it is possible to cor- 

 relate 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 Transi- 

 tion 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 

 Austroriparian, 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 tempera- 

 ture 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 conti- 

 nent from New England and Newfoundland to Alaska and Brit- 

 ish Columbia, and from about latitude 45 north to the Polar 

 Sea, conforming in general 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 



