1913] Grinnell Swarth: Birds and Mammals of San Jacinto 217 



as the vertebrates arc concerned. Furthermore, the presence of 

 a very small proportion of the elements of a higher zone may be 

 better considered as a dilution of a lower zone rather than as 

 justifying the formal recognition of the high zone. 



We have found ourselves independently led to recognize an 

 upper and a lower division of the Transition zone, this being in 

 accord with Hall's proposal based upon the plants of the region. 

 The modifying terms "upper" and "lower" will thus be fre- 

 quently employed in the discussion of species. Transition, in its 

 entirety, remains the same as in ordinary use. 



In comparing our zone map with that of Hall's (1902, plate 

 2), minor differences will be observed. The chief one is in the 

 vicinity of Hemet Valley, where scattering yellow pines occur 

 over the more nearly level tracts. We have decided this tree. 

 unquestionably Transition in usual zone position, to be best 

 treated as denoting a Transition infiltration into a prevailingly 

 Upper Sonoran area. This is because we found nothing but 

 Upper Sonoran birds and mammals in the debatable area. The 

 Location of certain tracts in Hemet Valley where pines grow 

 most numerously is indicated on our map by blue spots, as the 

 only practicable way of showing this Transition admixture, 

 without undue emphasis. We feel secure in our grounds for 

 showing Thomas Mountain to be capped, merely, by Transition, 

 wholly disconnected from the main Transition area on San 

 Jacinto proper. The slopes of this mountain on all sides are 

 certainly Upper Sonoran and not Transition as indicated on 

 Hall's map. Remarks upon the zonal position of various other 

 localities with reasons for our diagnoses will be found in our 

 "Descriptions of Localities." 



Faunas 

 In the restricted sense in which we believe the term best 

 employed, a fauna is a subdivision of a life zone, based upon 

 conditions of atmospheric humidity. Thus in travelling east- 

 ward across North America from the shore of the Pacific, at 

 San Francisco, keeping within the same zone, say Transition, one 

 passes through a series of belts possessing different assemblages 

 of plants and animals. These divisions are no more sharply 



