Geographical Distribution 163 



Survey, although the studies of other workers, especially 

 those of Allen on mammals and Jordan on fish, have contrib- 

 uted largely to our knowledge of this subject. 



The Biological Survey divides North America into zones of 

 plant and animal life based primarily on temperature, which 

 zones may be subdivided to a large extent on the basis of mois- 

 ture. These zones follow in a very broad way the parallels 

 of latitude in the lower country and the levels of altitude in 

 the mountains. Let us take for an example San Fran- 

 cisco Mountain in northern Arizona, whose life zones have 

 been studied by Doctor Merriam, the former chief of the Sur- 

 vey. The zonal distribution of life shows somewhat more 



Arctic Alpine HaJioniau C-Jinaetton transition Upper Stonoran Lowir Sonoron 



PROFILE OF SAN FRANCISCO AND O'LEARY PEAKS IN ARIZONA 

 To illustrate their life zones. The left side of the diagram is S. W., 

 the right is N. E. The horizontal lines indicate contour intervals of 

 1,000 feet. Modified after Merriam 's "Biological Survey of the San 

 Francisco Mountain Region . . . Arizona," North American Fauna, 

 No. 3. 



clearly in the case of an isolated group of mountains such as 

 those of which San Francisco Mountain forms the principal 

 peak, than it does in an extended range such as the Rockies or 

 the Sierra Nevada, where the zones are more or less broken up 

 by the irregular contour of the mountains, with their jum- 

 bled masses of peaks and valleys. These mountains further 

 include more zones from base to summit than do those of like 

 altitude further north, where the temperature range from 

 base to summit is less. 



San Francisco Mountain is located in north central Ari- 

 zona on the elevated plateau through which the Colorado 

 River has cut its titanic chasm. The town of Flagstaff, site of 

 the Lowell Observatory, from which the late Professor Lowell 



