332 Univ< rsity of California Publications in Zoology [Vol. 10 



evidently the riparian association within the Transition zone. 

 From this focus it invades both above and below along favorable 

 routes. 



At Garnet Queen Mine this mouse was found up the shaded 

 canon sides beneath golden oaks, as well as down along the stream. 

 In Strawberry Valley many were caught along the alder-lined 

 creeks, often beneath undercut banks overhung with gooseberries 

 and tangles of roots. At still higher altitudes those caught evi- 

 dently had their headquarters in the clumps of willows outlining 

 the veratrum meadows. 



The three specimens from the desert edge, at Dos Palmos 

 Spring, were trapped along the water course marked for a short 

 distance by a scanty seepage. A remarkable circumstance was 

 the occurrence at this point, in fact within the course of one line 

 of traps, of no fewer than four species of the one genus Peromys- 

 cus, namely, stephensi, eremicus, martirensis, and rowleyi. This 

 situation evidently resulted from the commingling of representa- 

 tives from the two faunas which abut at this point, the first two 

 species belonging to the arid contingent. 



Fully as interesting as those cases where there is a clear 

 transition through tile San Jacinto region from one subspecies 

 to another, as in certain species of Perognathus and Veotoma, 

 are those cases where no trace of such modification is evinced. 

 Peromyscus l>. rowleyi offers an instance of the latter sort, as 

 it ranges from one extreme of geographical conditions almost to 

 the opposite, and as far as we are able to detect there is not 

 the slightest variation displayed other than due to age, sex. and 

 individual peculiarity. 



As a possible explanation of this indifference we may offer, 

 that the associat ional predilections are in this mammal of more 

 weight than zonal, and the association followed is nearly uniform. 

 Further. /'. b. rowleyi is an animal whose focus of dispersal 

 is in a high zone, so that upon a mountain like San Jacinto its 

 range fits over the topography like a mantle. It appears to be 

 altogether those animals of Upper and Lower Sonoran, and of 

 wide rani;e in adjacent areas where they have separately differ- 

 entiated, which exhibit geographical modification within the 

 relatively short distance embraced in the north-and-south diam- 



