408 University of California Publications in Zoology [Vol.12 



1899, pp. 69-86) must to the writer's mind be reconsidered. Zone 

 must be thought of in conjunction with conditions of varying humid- 

 ity, associational features, and relative sizes of all the segregated 

 areas concerned. Species may be delimited in their ranges by other 

 factors than that of temperature. This remark is not, however, 

 meant as implying a denial that temperature is the one factor most 

 frequently limiting the ranges of species. A narrow Transition-zone 

 gap between two Boreal areas, as that at Sisson, between mounts 

 Eddy and Shasta, cannot serve as a particularly effective barrier 

 between the Boreal species encountering it, unless it be coupled with, 

 on opposite sides, marked differences in humidity or associational 

 conditions, which, however, do not appear to exist in the present 

 instance. 



The problem here arises, how to treat the Trinity region in faunal 

 nomenclature. "What is the criterion for nominal separation of 

 faunas ? What percentage of its species have to be peculiar, or what 

 minimum ratio of differences obtain in comparison with the animal 

 life of adjacent areas, to render the Trinity assemblage of forms 

 worthy of separate recognition by name on our fanual maps? Mount 

 Shasta has but two mammals and two birds (even these doubtful), 

 which do not occur on the central Sierras; it is chiefly characterized 

 by absence of forms (due to its small area). We are therefore led 

 to include Mount Shasta with the Sierra Nevada in the "Sierra 

 Nevadan Faunal Area". But we do not hesitate to recognize as 

 distinct the Cascade fauna and the Sierra Xevadan fauna. And the 

 Humid Coast fauna is conspicuously peculiar. 



The difficulties here encountered have been alluded to in a pre- 

 ceding paragraph. The Trinity region in its vertebrate animal life 

 resembles much more closely the Sierras than the Cascades; it is in 

 a way intermediate, but has in addition some humid coast elements. 

 The differences between the Trinity and Sierran faunas are con- 

 cerned with only eleven percent of its species all told. Is this a 

 sufficient amount of difference to warrant their recognition as dis- 

 tinct faunal areas, as indicated on a current distributional map 

 (Grinned, 1913, pi. 16) ? Possibly not. though the writer believes 

 that for purposes of faunal analysis such fine distinctions may be 

 useful. It will depend on circumstances, for, with wider problems 

 in view, it might for the moment be better to lump the Trinity area 

 in with the Sierra Nevada under a common designation. Expediency 

 will fix the criterion for recognizing faunal division, and this may 



