96 University of California Publications in Geology (VoL. 7 
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*? the 
present distributicn 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 lable 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.** 
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 Plocene form, Palaeoborus umbrosus (Cope), from New 
Mexico or to extend the occurrence of the group even back to 
the Pleistocene, as the Rancho 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. 
Geranoaétus 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. 
83 Allen, J. A., The geography and distribution of birds, Auk. vol. 10, 
No. 2, April, 1893. 
