COAST RANGE CORRIDOR IN CALIFORNIA 



171 



Arcto-Tertiary species range around the southern end of the Great 

 Valley at present, but many range around the northern end. 



Trans-valley "leaks" have occurred across the valley barrier 

 but only from west to east at the position of the San Joaquin delta. 

 Apparently the "leaks" became possible during relatively recent 



RED-LEGGED 

 FROG 

 RANA 

 ^ 1 AURORA 



Fig. 7. Distribution of red-legged frog, Rana aurora. Pattern is 

 typical of Arcto-Tertiary forms that have moved southward through 

 Sierra Nevada and coastal corridor. Arrows in Figs. 7-18 indicate proba- 

 ble movements from points of origin. (Figures 7-18 based on Stebbins, 

 with some modifications.) 



fluctuation of humidity in the delta region. The subspecies xanthop- 

 tica of Ensatina eschscholtzii has established a population in the 

 Sierras and is currently showing some interbreeding with the es- 

 tablished Sierran sxihs^ecies platensis (Fig. 15). Similarly, the coastal 

 Aneides lugubris has established a population in about the same area 

 as the subspecies of Ensatina (Fig. 8). In a valuable study of this 

 phenomenon, Rosenthal (1957) points out that no Ijiological or 

 physical factor, other than time itself, limits the Sierran range of the 



