170 F. E. PEABODY AND J. M. SAVAGE 



the present herpetofauna exist In the stable Santa Lucia positive 

 area of the peninsula. Geological and climatic events of the late 

 Cenozoic preclude such endemism. Finally, the profound and com- 

 plex influences brought to bear on the biota of the California region 

 are shown to be the result of an entirely fortuitous combination of 

 interacting geological and climatic changes having peak effect dur- 

 ing Pliocene and Pleistocene time: evolution of the Coast Ranges; 

 southward shift of marine and continental isotherms on a worldwide 

 basis; rise of the Sierra Nevada crestline with concomitant drying 

 of the interior; continental uplift. 



CORRIDOR EFFECTS 



Arcto-Tertiary Species 



Major migrations of western geofloras described by Axelrod (1957) 

 are important to an understanding of distribution changes in the 

 contemporaneous herpetofaunas. The writers believe that the late 

 Cenozoic herpetofauna of the west may be correlated broadly with 

 the Arcto-Tertiary and Madro-Tertiary geofloras. Salamanders are 

 fundamentally boreal and Arcto-Tertiary, the lizards and snakes 

 are fundamentally Sonoran and Madro-Tertiary in historical rela- 

 tionship. The frogs and toads are transitional in that some forms 

 appear to be Arcto-Tertiary elements while others are of Madro- 

 Tertiary relationships. Biijo boreas and Rana aurora are examples of 

 the former; Bufo microscaphus, Rana boylii, and Rana miiscosa of 

 the latter. As a result of the combination of geological and climatic 

 events described above, the Arcto-Tertiary salamanders and frogs 

 tended to move southward and split around east and west sides of 

 the Great Valley depression. However, the western route down the 

 corridor was not complete until the mid-Pleistocene. Thus until the 

 last half of the Pleistocene a "dam" was in force which would allow 

 accumulation of genetic differences between east (Sierran) and west 

 (Coast Range) arms of Arcto-Tertiary dispersals. Unless the "dam" 

 was in force until relatively late in the Pleistocene, the flow of 

 genetic material down the corridor should have merged compatibly 

 with the flow down the mainland to eastward. Apparently this was 

 not the case. Once the corridor was in operation it was possible for 

 an Arcto-Tertiary species to disperse southward, subject to fluctua- 

 tions, in a pattern like that of Rana aurora (Fig. 7). Development of 

 clines along the route would be expected, and do occur. Few of the 



