96 University of California Publications in Geology [VOL. > 



phase of Polyborus tharus as represented by a single specimen 

 from Argentina. As no appreciable difference could be noted, 

 the fossil form is referred to the existing species, P. tharus. 



Anomalies in Distribution. According to Ridgway 32 the 

 present distribution of Polyborus tharus is from Amazonia south- 

 ward through South America. The bird thus reaches in the 

 Argentine and the Patagonian climates a set of conditions as 

 rigorous as any that it would be liable to experience in the 

 northern hemisphere in the latitude of Los Angeles. The ex- 

 tremes of climate due to the presence of the ice sheet is thought 

 by Allen to have given rise to the periodical movements of birds 

 which finally merged into the present seasonal migration. 33 

 Would not a plausible explanation be that the polyborine under 

 discussion was driven southward by the cold of the glacial epoch 

 but failed to respond to the later amelioration of climate because 

 of a nature less susceptible to the development of a migratory 

 instinct and therefore remained in the lower latitudes or below 

 the tropics? No record of the true Polyborinae is yet found 

 in the deposits of the southern hemisphere to correspond with 

 the Pliocene form, Palaeoborus umbrosus (Cope), from New 

 Mexico or to extend the occurrence of the group even back to 

 the Pleistocene, as the Bancho La Brea material does so abund- 

 antly for the northern hemisphere. If, on this slender thread 

 of negative evidence, we assume that the group arose in the North 

 Temperate zone, the explanation suggested above seems a 

 plausible one. 



Geranoaetus and Circus present cases similar to that of Poly- 

 borus, while Morphnus differs in that the genus is at present 

 limited to the tropics and probably never reaches a southward 

 distribution which would correspond climatically with the region 

 of Hawver Cave or of Los Angeles. 



These two cases of Polyborus and Morphnus mentioned above 

 are typical of as many classes of change in distribution since 

 the formation of the various Pleistocene deposits. Parallel with 

 Polyborus there appear the following fossil forms whose nearest 



32 Ridgway, R., U. S. Geol. & Geog. Surv. Terr., vol. 1, No. 6, p. 451, 1876. 

 ss Allen, J. A., The geography and distribution of birds, Auk. vol. 10, 

 No. 2, April, 1893. 



