338 University of California Publications in Zoology [VOL. 16 



area. This area seems to show homologies to both the upper Sonoran 

 and to the Transition zones as found in other parts of the West. 



Certain species seem to have a different "zonal" position in dif- 

 ferent regions. Those which in one place are restricted to a certain 

 life-zone range elsewhere into areas which must be placed in other life- 

 zones. Many of the species and several of the genera given by Mer- 

 riam (1892 and 1898) as characteristic of the various life-zones are 

 now known to range beyond the limits stated by him. Grinnell and 

 Swarth (1913, p. 217) mention the case of a "Transition infiltration 

 into a prevailing Upper Sonoran area" in the San Jacinto region of 

 southern California. Cases like this indicate very strongly that there 

 is often a lack of homology between the zones of life found in different 

 regions. 



In a restricted region of general climatic similarity the zones of 

 life may usually be easily homologized. In California Grinnell (1902, 

 p. 6) has recognized several zones which are evidently natural divisions 

 of the fauna, and each of which is seemingly homologous throughout 

 its extent in the state. However, the zones of life found in different 

 regions, particularly in regions under different climatic conditions, 

 show much less similarity and in many cases are certainly not directly 

 homologous. 



In some cases the life-zone system seems to be largely dependent 

 upon the distribution of particular associations of plants and animals. 

 The life-zones are based on temperature differences, yet ' ' it is obvious 

 that, throughout considerable portions of the continent, the details of 

 temperature distribution are not known with any approach to pre- 

 cision. Thus, the actual criterion which the field zoologist falls back 

 upon in any given case is the character of the fauna and flora which 

 he finds associated together. The presence of certain species shows 

 him that he chances to be in this or that 'life-zone' " (Sumner, 1915, 

 p. 67). On Alder Creek in northern Nevada, Taylor (1912, p. 331) 

 has placed the vegetation along the stream in the Transition zone, 

 while the treeless slopes away from the narrow strip of vegetation are 

 placed in the Upper Austral zone. There may be a temperature dif- 

 ference between the strip along the stream and the immediately ad- 

 joining timberless slopes sufficiently great to maintain different life- 

 zones in the two places, but there is no proof that such is the case. On 

 the contrary, it seems that the differences are those that would natur- 

 ally be produced by habitat differences. There is no justification for 

 assuming that the differences in this and many other similar cases are 



