1913] Grinnell: Californian Species of Sorex 193 



San Pablo Bay) ; and (3) the marshes lying east of the Straits 

 of Carquinez and extending thence to the confluence of the Sacra- 

 mento and San Joaquin rivers, hence including the Suisun Bay 

 district. 



The first-named area of differentiation is wholly cut off from 

 the second by the open water-way leading from the Straits of 

 Carquinez out through Golden Gate. The third area of dif- 

 ferentiation centering in the Suisun marshes, is cut off from both 

 of the other areas by the ranges of hills which rise abruptly from 

 the shore on either side of the Straits of Carquinez, there being 

 thus an extensive littoral interval lacking any trace of palustrine 

 conditions. 



Attention to the eleven species of birds and mammals above 

 enumerated shows allotment by areas, as follows: (1) South arm 

 of San Francisco Bay, six species: Geothlypis t. sinuosa, Passer- 

 culus s. bryanti, Melospiza m. pusillula* Ballus obsoh his,' /,'< ith- 

 rodontomys raviventris* and Sorer halicoetes* The four 

 species starred are found only in this area. (2) North side of 

 San Francisco Bay, four species: Geothlypis I. sinuosa, Passer- 

 culus s. bryanti, Melospiza m. samuelis* Reithrodontomys hali- 

 coetes. Of these only the single species starred is restricted to 

 this area. (3) Suisun district, rive species: Passercuhis s. 

 bryanti, Melospiza m. maxillaris* Reithrodontomys halicoi l< s, 

 Mic ml us edax* Sorex sinuosus* The three starred species are 

 confined exclusively to this area. 



It will be observed that the mammals show more restriction 

 than the birds; which is to be expected from the relative abilities 

 in the two classes to transgress geographic barriers. It is 

 further to be seen that the first and third differentiation areas 

 have produced the greatest number of, and must divergent, types, 

 while the second area is of smaller size and is approximately 

 intermediate in location; both of these contingencies may account 

 for its poverty in species and ineffectiveness, as a center of dif- 

 ferentiation. 



The most interest attends those cases where the salt marsh 

 has produced races distinct from the supposedly parent stock 

 of the immediately adjacent uplands, no obvious barrier being 

 interposed. Most pertinent in this regard are the following seven 



