J 912] Taylor: Birds and Reptiles of Northern Nevada. 339 



Transition tree was Populus tremuloides. The occurrence of this 

 species and of Alnus tenuifolia along creeks to 4500 feet altitude, 

 and even in the case of Alnus below that, indicates the downward 

 extension of Transition. Certain species of mammals, ordinarily 

 characteristic of that zone, were found at comparatively low 

 altitudes. Callosperniophilus trepidus and Microtus mordax were 

 both found near Big Creek Ranch, the former on a nearby foot- 

 hill ridge at 5000 feet, and the latter along the creek at the ranch 

 proper. Neotoma cinerea occidentalis was secured at 6500 feet 

 altitude on both Alder and Leonard creeks, and Zapus princeps 

 oregonus at 6000 feet on Alder Creek. 



In an earlier paper (Taylor, 1911, p. 226 and elsewhere in the 

 same paper) the presence of a zone above Transition was rather 

 questioned. A careful analysis of the vertebrate fauna and of 

 the flora leaves no doubt that while there is locally present a 

 Boreal assemblage of forms, still there is no definite tract of 

 pure Boreal. The species which are found elsewhere above 

 Transition and which are present in the Pine Forest Mountains 

 cannot be referred as a whole to either the Canadian or the 

 Hudsonian subdivision of the Boreal, as an inspection of the 

 foregoing lists will show. Such species have been grouped 

 together, and where they occur there is said to be a Boreal 

 infusion. These areas are very limited in extent. In general 

 the pine-covered areas, together with certain shaded cool strips 

 along the streams in the higher parts of the mountains, may be 

 regarded as tracts of such Boreal infusion into a predominating 

 Transition assemblage of species. 



The small extent of the Pine Forest Mountains and their 

 comparative lack of elevation above the level of the plateau (see 

 C. II. Merriam, 1890, p. 27) are obstacles to the plotting of their 

 life zones. These factors decrease their humidity and give this 

 element, as compared with temperature, a disproportionate influ- 

 ence in determining the presence of particular species, and the 

 absence of others (see C. H. Merriam, 1890, p. 26, footnote). 

 This being the case, the life-zone concept here becomes difficult 

 of application, since boundaries of zones cannot be drawn with 

 the precision possible in many other regions. 



