180 



F. E. PEABODY AND J. M. SAVAGE 



currence on six offshore islands, while the other species has managed 

 only one insular invasion. Fossil trackways of Mio-Pliocene age, 

 near Sonora in the Sierra Nevadas (Peabody, 1940) suggest that 

 B. pacijicus was derived from the north in pre- Pleistocene time. 

 B. attenuatus represents the appearance of a younger species in 

 southern California, more advanced in a trend toward attenuation, 

 and one that shows the forked distribution pattern of a boreal species 

 with respect to the Great Valley barrier. 



DISJUNCT 



i 



MUSCOSA 



Fig. 17. Distribution of yellow-legged frogs, Rana boylii and R. 

 muscosa. The latter is found at high elevations in Sierra Nevada and in 

 montane situations in southern California. Sympatry with R. boylii 

 occurs at one locality in San Gabriel Mountains. 



3. The distribution pattern of the two species of the yellow- 

 legged frogs, Rana boylii and Rayia muscosa (Fig. 17), displays a 

 situation resembling that of Ensatina (Zweifel, 1955). The younger 

 of the two species, boylii, ranges around the Great Valley barrier in 

 the familiar forked pattern. The range is more or less continuous 

 except for a small, disjunct population at the southern end, in the 

 San Gabriel Mountains. Here there is a limited sympatry with the 

 older species, muscosa. The older species ranges at high elevations in 

 the Sierra Nevada and has undergone post-Pleistocene fragmenta- 



