1912] Miller: Pacific Coast Avian Palacontology 103 
Upper San Pedro series of marine deposits and the San Pablo 
Bay oyster beds at Rodeo. These shell deposits are considered by 
western palaeontologists to represent a time of higher average 
annual temperatures than prevail in the region at present. The 
cases of Morphnus, Micropallas, Geococcyx (*) and Pavo make 
a strong aggregate in favor of this theory. To harmonize the 
eases of Circus, Polyborus, Sarcorhamphus, Geranoaétus and 
Ciconia with those of the more tropical species, it would be neces- 
sary to assume nothing further than that these forms, since the 
partial amelioration of the climate, had developed powers of 
resistance to cold and had extended their ranges to the southward 
instead of remaining intertropical species. The extension of 
range took place from the tropics southward instead of to the 
northward again because of overcrowded conditions in the north. 
The advance of arctie cold toward the equator would drive north- 
ern animals into narrower and narrower quarters, while the 
forms of the southern hemisphere, under like encroachment of 
the antaretic, would experience the opposite effect. The conver- 
gence of all the Boreal species into the Austral on the continent 
of North America would be in effect like crowding the basal eon- 
tents of a cone into its apex. The result would be an enormous 
intensification of the natural attrition of species upon species 
with a resultant stimulus to the surviving form. In the southern 
hemisphere conditions would be reversed and the advance of 
polar cold, whether synchronous with or alternating with the 
northern fluctuations, would have much less serious effect. As- 
suming the various faunal zones to be fully populated, the driv- 
ing of the Patagonian fauna into the wide expanse of Argentina 
and southern Brazil would serve to dilute greatly the Boreal 
fauna without materially disturbing the Austral. A form that 
had been obliged to flee the rigorous conditions resulting from 
an advance of the cold in North America might find, upon the 
return of milder conditions, that the path of least resistance to 
expanding range from the tropics led toward the south. 
Bird Remains as Indicators of Climatic Conditions.—Certain 
appearances in the deposits at Rancho La Brea might be inter- 
preted as evidence that the climate during deposition of the beds 
was warmer and more moist than it is at present in the region. 
