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



These amphibia are well suited for "waif" dispersal and the main- 

 land opposite the islands has numerous stream channels that fre- 

 quently sweep enormous amounts of sediment and floating debris 

 into the ocean. In the case of Batrachoseps, the older species {paci- 

 ficus) on the mainland has populated the largest number (5) of the 

 offshore islands. The younger species {attenuakis) has reached but 

 one island (Fig. 16). 



The reptilian insular fauna appears erratic in distribution, and 

 significantly there are no chelonians. Certainly the distribution of 

 insular herpetofauna does not support the theory of "tied" islands, 

 and the relationship of mainland herpetofauna to the Coast Range 

 corridor suggests a generally high degree of sensitivity to land 

 routes. 



CONCLUSIONS 



The fossil record at present contributes little to a detailed study 

 of origins and dispersals of the modern herpetofauna of western 

 North America. However, a synthesis of the data from geology, 

 paleontology, and herpetology indicates that the far west, particu- 

 larly California, was the locus of physical events during the late 

 Cenozoic era that shaped the destiny of the modern herpetofauna. 

 The synthesis reveals an especially intriguing example of the physi- 

 cal factors inherent in biological evolution — the coincidence of time, 

 place, and climate contributing to the randomness and opportunism 

 of evolution. The synthesis suggests that such fortuitous interac- 

 tions of time, place, and climate might be considered a major factor 

 in biologic evolution, along with genetic variability, selective pres- 

 sure, and geographic isolation. Perhaps this might be considered a 

 fourth major factor in biologic evolution along with genetic varia- 

 bility, selective pressure, and geographic isolation. Perhaps this 

 factor could be included in the term "geodynamics." 



A major result of the synthesis is the concept of a Pacific Coast 

 Range corridor which took shape in Pliocene and early Pleistocene 

 time as a zoogeographic peninsula connected to the mainland on the 

 north, and separated by a marine barrier strait from mainland to 

 the south. After mid-Pleistocene time the peninsula became a con- 

 tinuous corridor allowing dispersal in both directions, but causing 

 a high incidence of sympatry in the herpetofauna of southern Cali- 

 fornia. The study of origins and dispersals relative to the evolution 



