100 University of California Publications in Zoology [Vol. 12 



for local operation of isolation, because more opportunity for radial 

 dispersion to carry the species into distant localities and under extreme 

 conditions, and for the ultimate interposition of more or less efficient 

 barriers. The factor of distance might here replace the operation of 

 associational restriction in segregating descent-line plexuses. 



These are apparently incongruous notions, but the following con- 

 ception tends to harmonize them ; namely, that, granting the three 

 totally different orders of distributionally limiting factors (zonal, 

 faunal and associational). it is probable that different species are 

 restricted unevenly with respect to the three ; thus a certain w y ood-rat 

 {Neotoma intermedia desertorum) is restricted faunally, but ranges 

 widely through zones and associations; a certain wren (Telmatodytes 

 palvstris and subspecies) is tightly restricted associationally, but 

 ranges widely through faunas and zones ; a xerophilous genus of 

 rodents (Perognathus) is closely restricted zonally and associationally, 

 but ranges rather widely as to fauna. So that both the above asser- 

 tions might well be true of a single animal historically and even, in 

 different parts of its range, simultaneously ! Certainly the first serves 

 in explanation of the multiplicity of geographic races or species in 

 several widespread groups of birds and mammals. 



THE COLORADO RIVER AS A HINDRANCE TO THE 

 DISPERSAL OF SPECIES 



Bats and most birds find in the Colorado River no hindrance what- 

 ever to individual travel. Freedom of aerial locomotion gives them 

 superiority over any obstruction on the general level of the country 

 they inhabit. It is possible that in a few of the resident birds of 

 limited flight individuals do not regularly cross the main stream, 

 though they readily could do so if such an exigency as that of fire 

 sweeping the bottom lands should drive them to it. Geococcyx calif or- 

 nianus, Pipilo aberti and Toxostoma crissale are birds which probably 

 do not often cross the river under normal circumstances. 



Among mammals, carnivores are usually of much wider foraging 

 range than rodents. From all the data available it appears that none 

 of the carnivores, not even the cats, are averse to swimming the river 

 if need be. Among rodents, however, our work showed a number 

 of cases in which the Colorado River had effectively checked the 

 distribution of species. The following tables show the situation as 

 regards all the rodents of the region (see also figs. A, B). 



