604 



PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 87 



ences to suggest reason for this strange discrepancy between north- 

 ward and southward migration. That pressure from overpopulation, 

 which may have been greater in the south, could adequately account 

 for this difference seems to me improbable. Note that the dry 

 Sonoran region apparently was a deterrent to migration of moisture- 

 loving Anura. It was effective in stopping the northward movement 

 of leptodactylids, but the hylids swarmed by. But that does not 

 solve the question of the bar to southward migration, for the frogs 



Figure 155.— Geographic distribution of the Ranidao. Eana occurs in all the stippled areas and in the 

 blackened areas, except Madagascar and the Seychelles, but in South America its only representative is 

 R. palmipes. Other genera than Rana occur in the areas indicated in black. The occurrence of Cerato- 

 batrachus in the Solomon Islands is indicated by a black cross. 



and toads, with their Opalinas, got by the desert going southward in 

 numbers and were then stopped at the Isthmus. 



Ranids other than Rana possibly came from eastern Asia-Malaysia 

 to tropical America by way of the circum-Pacific land-strip, in the 

 Cretaceous period, the route used by Bvjo, though a trans-Atlantic 

 route is more probable. They did not bring Opalina, which is not 

 now in South America.^ For the same reason Rana was not in this 

 migration. Rana and Opalina apparently were in Asia-Malaysia too 

 late to avail themselves of the Cretaceous Pacific land-strip. When, 

 in the Tertiary period, they reached North America by the then 

 opened Siberia-Alaska route, access to South America was shut off 

 by interruption of the land-strip north of South America and, as the 

 Isthmus of Panama was not yet formed, the way to further south- 

 ward passage was barred. 



« Since this was first written, knowledge of certain facts makes it questionable, e. g., Opalinas are said to 

 be in South America (Carini, 1937). 



