1912] Miller: Pacific Coast Avian Palaeontology 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 

 cases of Circus, Polyborus, Sarcorhamphus, Geranoaetus 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 arctic 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 antarctic, 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 con- 

 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. 



