478 University of California Publications in Zoology [Vol.12 



seem to be a barrier far greater than that between the Sierra 

 Nevada and the Trinity-Cascade mountains, and yet Mustela ari- 

 zonensis is found on both the Rocky Mountains and the Sierra, but 

 is replaced by another species on the Trinity-Cascades ! It may be, 

 of course, that the real efficacy of the Klamath River gap has not 

 been appraised at its true value, and that the effectiveness of the 

 barrier between the Rocky Mountains and the Sierra has been 

 exaggerated. 



Mephitis platyrhina, as previously stated, is found in the Sono- 

 ran valleys around the south end of the Sierra Nevada. This species 

 might be conceived to have arisen through mutation, or Darwinian 

 divergence, or physiological selection, from Mephitis Occident alls 

 holzneri, the range of which partially overlaps the range of 

 platyrhina. The following considerations, however, militate some- 

 what against these possibilities: (1) The speciation, that is, differ- 

 entiation, of the skunk family as a whole seems to have been 

 dependent on migration and geographical isolation. (2) The over- 

 lapping of the ranges of M. platyrhina and M. o. holzneri takes 

 place over a narrow area only. (3) M. platyrhina seems to be most 

 closely related, on the basis of its structure, to M. o. major, which 

 occurs to the northward, and not to M. o. holzneri (Howell, 1901, 

 p. 39). 



One other case: There occur along the Pacific Coast three 

 species of the mammalian genus Aplodontia, very intimately related 

 one to the other, less intimately related to the members of the genus 

 found in the interior mountains (Aplodontia californica and A. 

 chryseola) . The coast species referred to are Aplodontia pacifica, 

 Yaquina Bay, Oregon; Aplodontia nigra, Point Arena, California; 

 and Aplodontia phaea, Marin County, California. 



In the case of these forms the Sierran or mountain stock lias 

 evidently moved westward into the Siskiyou and Trinity mountains, 

 where it is represented by the species chryseola, and even to the 

 coast itself, where it is represented, in the Humboldt Bay region, by 

 an undescribed form nearly identical, cranially, with chryseola. 

 Thus the ranges of the closely related A. pacifica of Oregon and 

 A. nigra of Point Arena, California, are separated by the chryseola 

 stock, which must thus, on the Wagnerian hypothesis, be thought 

 to have carried the western boundary of its range to the sea 

 since the pacifica-nigror-phaea stock attained its broad coastwise dis 

 tribution. 



