626 Bulletin American Museum of Natural History [Vol. XLVI 



entiated at a much earlier stage than the former, and I regard their 

 present isolation as "relict distribution." They were probably elements 

 of the older Lower Californian fauna co-existing with the earlier and 

 more widely differentiated species of the peninsula, such as Uta thalas- 

 sina, while the less distinct forms have become differentiated since the 

 relatively recent faulting or submergence which has separated the islands 

 from the peninsula. 



SPECULATIONS ON THE ORIGIN OF THE FAUNA 



The factors in the geologic history of Lower California which seem 

 to bear most directly on the origin of its present fauna are: (1) the an- 

 cient (pre-Cretaceous) granitoid character of the southern tip of the 

 peninsula; (2) the extensive Miocene lava-flows of the middle of the 

 peninsula; and (3) the recent (post-Pliocene) submergence of the middle 

 of the peninsula, where marine fossils and beaches are found to a height 

 of over 3000 feet. 



Although recent beaches are not recorded from the immediate 

 vicinity of La Paz, the submergence to the north indicates that the hypo- 

 thesis of Eisen (1895, p. 755) and Nelson (1921, p. 53), that the Cape area 

 south of La Paz has been separated from the peninsula at a relatively 

 recent date, is probably correct. I believe, however, that Nelson is right 

 in regarding this separation as of comparatively little importance in the 

 history of the fauna. The islands which parallel the gulf coast of the 

 peninsula were' quite certainly part of the peninsula at a fairly recent 

 date. The topographical similarity of many of the islands with isolated 

 capes and mountains of the adjacent coast is very strikingly illustrated 

 in Nelson's map (loc. cit., PI. xxxv). 



As the bulk of the middle part of the peninsula consists of Miocene 

 deposits with extensive late Miocene lava-flows (Darton, 1921, p. 720, 

 ff.) it is evident that, if there are remnants of a pre-Miocene fauna in 

 Lower California, they must be looked for in the southern tip of the 

 peninsula. 



It is possible that the apparently oldest element in the fauna (see 

 below) antedates the Miocene igneus activity. The larger part of the 

 fauna, and that with a more characteristically desert facies, however, 

 must have entered the peninsula from the north subsequent to the late 

 Tertiary volcanic activity. It is possible that another element of the 

 fauna might be dated from the most recent submergence (late Pleisto- 

 cene) and this probably consists of the Colorado Desert species (such 

 as Crotaphytus wislizenii, Callisaurus ventralis gabbii, Coluber flagellum 



