430 GRINNELL, The California Thrasher. anes 
provides good illustrations of this. A tongue or belt of Lower 
Sonoran extends from the Mohave Desert over the low axial moun- 
tain ridge at the head of Kelso Creek and thence down along the 
valley of the South Fork of the Kern River nearly to Isabella. 
Leconte’s Thrasher is a conspicuous element in this Lower 
Sonoran invasion, but no California Thrashers were met with in 
this region below the belt of good Upper Sonoran on the flanking 
mountain sides, as marked by the presence of digger pine, blue oak, 
sumach, silk-tassel bush, and other good zone-plants. Similar 
zonal relationships are on record from San Gorgonio Pass, River- 
side County, as well as elsewhere. 
Reference now to the general range of the bird under considera- 
tion (see p. 429), as compared with a life-zone map of California 
(Pacific Coast Avifauna No. 11, Pls. I, I), will show to a remark- 
able degree how closely the former coincides with the Upper Sono- 
ran zone. The thrasher is, to be sure, one of the elements upon the 
presence of which this zone was marked on the map; but it was 
only one of many, both plant and animal; and it is concordance 
with the aggregate that is significant. Diagnosis of zonation simi- 
larly is possible in scores of places where change in altitude (which 
as a rule means change in temperature) is the obvious factor, as up 
the west flank of the Sierra Nevada, or the north wall of the South 
Fork valley, already referred to, in Kern County, or on the north 
wall of the San Jacinto Mountains. The California Thrasher is 
unquestionably delimited in its range in ultimate analysis by tem- 
perature conditions. The isothermic area it occupies is in zonal 
parlance, Upper Sonoran. 
The second order of restriction is faunal, using this term in its 
narrowed sense, indicating dependence upon atmospheric humidity. 
The California Thrasher does not range interiorly into excessively 
arid country, although the Upper Sonoran zone may, as around the 
southern end of the Sierra Nevada, continue uninterruptedly 
towards the interior in a generally latitudinal direction. This 
is true where extensive areas are considered, but locally, as with 
zones, individuals or descent-lines may have invaded short dis- 
tances beyond the normally preferred conditions. An example 
of this situation is to be found on the north and west slopes of the 
San Jacinto Mountains, where California Thrashers range around 
