98 University of California Publications in Zoology [Vol. 12 



paratively simple matter. The axiom holds, that, because of the ever- 

 shifting location geographically of associational, faunal and zonal con- 

 ditions, every single element or line of descent, now represented in the 

 biota of any one locality must have come either in its present form 

 or in some antecedent one from somewhere else. This is certainly 

 true of all terrestrial life. Elevation and depression have worked like 

 a seesaw in dislocating faunas. The Colorado valley is a trough, 

 hemmed in associationally on either side, and only capable of influx 

 of riparian elements at either end. Therefore the riparian species of 

 the Colorado fauna can have entered the area under consideration from 

 only two directions : from the north and from the south. 



Only two species are clearly seen to have entered the Colorado val- 

 ley from the north: Castor canadensis frondator, and Ondatra zibethiea 

 pallida. The following species or subspecies are believed to have come 

 in from the south : Pipilo aberti, Guiraca caerulea lazula, Piranga rubra 

 cooperi, Vireo belli arizonae, Vermivora luciae, Toxostoma crissale, 

 Sigmodon hispidus eremicus, Neotoma albigida venusta. 



In the remaining seven cases (Agelaius, Melospiza, Dendroica, 

 Peromyscus, Reithrodontomys, Mephitis and Procyon) no grounds are 

 apparent to the writer for assigning either one over the other direction 

 of invasion, and this in spite of whatever may be the marked Austral 

 or Boreal distributional affinities of each group concerned. The tide 

 of invasion may in fact in these species have tended in one direction 

 at one period, in the opposite at another; or, as in Peromyscus mani- 

 culatus sonoriensis, the Colorado valley may have acted continuously 

 as a narrow bridge where have met and mingled descent-lines from 

 both the north and the south. 



The obvious fact that southern representatives prevail over northern 

 ones is clearly attributable to the present zonal condition obtaining 

 in the region, namely Austral, in its Lower Sonoran division. And 

 evidence elsewhere assembled (Grinnell and Swarth, 1913, p. 383) 

 points towards an increasing temperature throughout the region. This 

 would result in decreasing the favorableness to Boreal forms and 

 increasing the availability of the region for immigration of Austral 

 types. Consideration of the xerophilous vertebrates as well as of the 

 riparian ones leaves little doubt in the writer's mind but that this has 

 been the actual course of events. The northern contingent is on 

 the wane, the southern in the ascendency. 



The query presents itself: is the Colorado fauna full? Are all the 

 ecological niches, which are available in this area and which have 



