106 University of California Publications in Zoology [Vol. 12 



of the Mohave desert forms. There is considerable floral difference in 

 the two regions, and minor climatic differences are well known. Dif- 

 ferent environments thus impinge upon the animals in these widely 

 separated centers, and more or less regular blending of conditions 

 occurs between. Although every factor of environment may be iden- 

 tical immediately on the two sides of the Colorado River, the animals 

 now there have undoubtedly descended from ancestral lines which have 

 invaded the territory from the two opposite directions, bringing with 

 them by inheritance the characters developed under the two different 

 sets of conditions. 



To express the idea otherwise, from each differentiation area there 

 is an outwardly radiating dispersal of descent-lines, involving time as 

 well as space. This dispersive process is going on now as it has 

 through past time. The eastward-flowing tide of Mohave forms would 

 only be arrested by an insuperable barrier, such as the Colorado River. 

 The westward invading descent-lines from the Arizona center would 

 proceed until stopped by the same barrier. Both sets of forms would 

 find themselves along the Colorado Valley under the same associational, 

 faunal and zonal conditions; but each set is continually receiving by 

 the process of inheritance plus invasion the peculiar characters gen- 

 erated on its own side. 



While the Colorado River probably lies in an intermediate position 

 between the Mohave and Arizona faunas, the area of intermediate 

 conditions of environment is probably relatively narrow. This very 

 element of narrowness may be called in to account for the lack of modi- 

 fication displayed by the delimited species of the Colorado River fron- 

 tiers, for example, in the ease of Ammospermophilus. 



Supposing now that the Colorado River does not serve as an 

 insuperable barrier, nor ever has done so ; invasion would have extended 

 from one side to the other as far as associational, faunal or zonal 

 barriers permitted. In animals of wide distribution, intergradation 

 geographically between the remote extremes would in the end be 

 expected to occur. The extremes would not then have differentiated 

 so far, at least in quantity of each character developed, because of 

 inheritance from the opposite type, again involving time and space, 

 concomitantly. Subspecies would have resulted, instead of full species. 

 This condition doubtless obtains in some of the birds, as well as in 

 some of the rodents listed as being the same on the two sides of the 

 river. Take, as an example, Dipodomys merriami merriami of south 

 central Arizona, and Dipodomys merriami simiolus of the Mohave 



