ae | GRINNELL, The California Thrasher. 431 
onto arid chaparral slopes, intermingling with such arid Upper 
Sonoran birds as Scott’s Oriole and the Gray Vireo. It is question- 
able, however, as to what extent faunal restriction really operates 
in this case; for reference to the zone map, again, shows that a 
vast tract of Lower Sonoran, lying to the east of the desert divides, 
extends continuously north to the head of Owens Valley. Really 
the only unbroken bridge of Upper Sonoran towards the east from 
the west-Sierran habitat of Toxostoma redivivum is around the 
southern end of the Sierra Nevada — a very narrow and long route 
of possible emigration, with consequent factors unfavorable to 
invasion, irrespective of either temperature or humidity, such as 
interrupted associational requirements and small aggregate area. 
In this particular bird, therefore, faunal restriction may be of minor 
importance, as compared with zonal and associational controls. 
That faunal conditions have had their influence on the species, 
however, is shown by the fact: of geographic variation within its 
range. The thrasher throughout its habitat-as-a-whole, is sub- 
jected to different degrees of humidity. Amount of rainfall is, 
in a general way, an index of atmospheric humidity, though not 
without conspicuous exceptions. Comparing the map of the ranges 
of the subspecies of 7. redivivum (p. 429) with a climatic map of the 
State, direct concordance is observed between areas of stated rain- 
fall on the latter and the ranges of the respective subspecies. It 
will be seen that the race 7. r. pasadenense occupies an area of 
relatively low humidity, the race T. r. sonome of higher humidity 
and the race 7. r. redivivum of highest humidity, in fact a portion 
of California’s fog-belt. The distinctive color-tones developed are, 
respectively, of gray, slate and brown casts. In the thrasher, 
therefore, we may look to faunal influences as having most to do 
with differentiation within the species. In this case it is the faunal 
variation over the occupied country which is apparently responsible 
for the intra-specific budding, or, in other words, the origination of 
new specific divarications. 
Wherever it occurs, and in whichever of the three subspecies it is 
represented, the California Thrasher evinces strong associational 
predilections. It is a characteristic element in California’s famous 
chaparral belt. Where this belt is broadest and best developed, 
as in the San Diegan district and in the foothill regions bordering 
