390 University of California Publications in Zoology [Vol. 10 



mesquite belt, may be cited, as from the desert fauna; and the 

 Hutton vireo, of the live oak association, and Bell sparrow, of the 

 Adenostoma association, as belonging to the San Diegan fauna. 

 These species must be of extreme dependence upon one or more 

 factors of their surroundings. 



The third category shows an intermediate degree of adapta- 

 bility or indifference as compared with the first and second. In 

 four cases species of prevalently desert habitat range over on the 

 Pacific side of the San Jacintos, either continuously, as with 

 Lepus c. deserticola and Peromyscus in. sonoriensis, or are repre- 

 sented within "islands" of the Lower Sonoran zone, as with 

 Ammospermophilus 1. leuctirus and Uryobatcs s. cactophilus in 

 the San Jacinto Valley. On the other hand, and in a greater 

 number of cases, San Diegan species range to a considerable dis- 

 tance down over the arid side of the mountains, becoming thus 

 exposed, in some cases, to an extreme of desert conditions. All 

 those listed are of practically continuous range, except the San 

 Diego song sparrow, the desert side colonies of this riparian 

 species being of necessity cut off from their parent stock (sec 

 p- 279). 



The fourth category includes some of the species classified 

 in the third. These and the additional ones are peculiar in that 

 there are complementary forms in the two contiguous faunas. 

 Some of these blend together gradually over the neutral strip 

 and thus in each case constitute subspecies of a common species, 

 in accordance with the understanding of the term subspecies 

 among American vertebrate systematists at the present time ; for 

 example, the Lower Sonoran Perognathus panamintinus, with its 

 subspecies bangsi and brevinasus intergrading through San Gor- 

 gonio Pass. Others of the forms are connected by a broken 

 series of intermediates (see under Neotoma intermedia and 

 desertorum, pp. 336-347). In a few cases there are unquestioned 

 hybrids of more or less frequent occurrence, as between the 

 quails, Lophortyx gambeli and L. c. vallicola. And finally there 

 are perfectly distinct though obviously closely related forms in 

 the two faunas, as, for example, Polioptila plumbea and P. 

 calif ornica, and Dryobates s. cactophilus and D. nut I alii. 



