102 University of California Publications in Geology [VOL. 7 



Possible Influences Conditioning Present Distribution of Cer- 

 tain Groups. In considering the relation of past to present dis- 

 tribution of American birds, at least two principles present them- 

 selves in explanation of the apparent southward retraction of 

 certain forms since Pleistocene time. The first is typified by the 

 case of Polyborus tharus. May this species not have been driven 

 southward across the equator after the time of formation of 

 the asphalt deposits by the advance of a cold period such as sent 

 the mammals of the Ovibos zone as far south as Big Bone Lick 

 and Conard Fissure? 



Extremes of climate due to the presence of the ice sheet are 

 thought by Allen 37 to have given rise to the periodical move- 

 ments of birds which finally merged into the present seasonal 

 migration. The polyborine under discussion may thus have been 

 driven southward, but lacked the incipient migratory instinct and 

 furthermore failed to return northward upon the amelioration 

 of the climate. This failure may have been due to the presence 

 of more virile species blocking the return path, or it may have 

 been due to the limiting tendency of the torrid zone which it 

 would have had to recross in a return to the north. No record 

 of the true Polyborinae has yet been found in the deposits of 

 the southern hemisphere to correspond with the Pliocene Palaeo- 

 borus of New Mexico or to extend the occurrence of the group 

 even back to the Pleistocene, as the Eancho La Brea material 

 does so abundantly 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. The distribution of Circus, Geran- 

 oaetus, Sarcorhamphus, and Euxenura would further uphold this 

 view of the question. These birds are typically of the southern 

 hemisphere in latitudes to the south of the tropics or at high 

 elevations and the Tierra Caliente would act as a more or less 

 effective barrier to their northward dissemination. 



The second hypothesis offered is that the returning annual 

 isotherm has never yet reached the point at which it stood during 

 the deposition of the fossil remains. Sinclair (Op. cit., p. 19) 

 links the Potter Creek Cave deposits pretty closely with the 



37 Allen, J. A., The Auk, vol. X, No. 2, Apr. 1893. 



