346 University of California Publications in Zoology [Vol. 10 



But we would here express our grave doubts as to there 

 being any intrinsic difference between the results of long-con- 

 tinued hybridization, and "intergradation." 



As illustration of the unevenness in variation from place to 

 place, to be fair, we would instance that different localities pro- 

 duced wood rats of such combinations of characters as: (1) 

 small size, dark coloration and short rostrum; (2) large size, 

 pale coloration and long rostrum; (3) pale coloration, short 

 rostrum and small bullae; (4) pale coloration, long rostrum and 

 small bullae; etc. 



All sorts of degrees of characters in combination are pre- 

 sented, that is, as limited only by the extent of our material, so 

 that the above array of combinations is the result only of judi- 

 cious selection on our part. In no instance were both typical 

 intermedia and desertorum found in the same locality with 

 examples of intermediate nature. Again, it is to be observed 

 that both on the Pacific slope of southern California, and on the 

 desert, intermedia and desertorum -occupy almost identically the 

 same ecologic niche, with resulting similarity in habits. 



The above facts might be interpreted into some form of Men- 

 delian behavior. At any rate they appear to support the theory 

 of hybridization, though the process, of whatever nature, has 

 obviously proceeded so far that the real history is not subject 

 to solution with the data at hand. The possibility of "direct 

 action of environment" in the area of intermediate conditions 

 between the desert and Pacific districts also enters the problem. 

 As previously implied our data furnishes no criterion by which 

 to distinguish here between the results of "geographic inter- 

 gradation" and of hybridization. 



All the facts herein set forth point, by our interpretation, 

 to the probability that intermedia is a Pacific slope race of the 

 species represented also by desertorum of the arid interior, and 

 that through the major portion of the San Jacinto region we 

 find geographic intergradation comparable to the condition in 

 certain species of Perognathus, Peromyscus, Dipodomys, and 

 Onychomys, as elsewhere treated in the present paper. 



The fact that in the white-footed wood rats the opposite 

 extremes are much more diverse than in the cases of the rodents 



