102 University of California Publications in Geology \Vor-.7 
Possible Inflwences 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*’ 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 Rancho 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- 
oaétus, 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. 
