is' 4 ] Grinnell: Mammals and Birds of the Colorado Valley 107 



desert. Our Colorado River series is fairly intermediate between the 

 extreme types, though the extremes are not so different as are Ammo- 

 spcrmophilus liarrisi harrisi and Ammospermophilus leucurus leu- 

 curus. As elsewhere explained, the river is believed to be not so 

 much of a barrier to Dipodomys merriami as to the forms of Ammo- 

 spermophilus. 



The degree of hindrance, ranging from the condition, as in the 

 case of Castor and Ondatra, where the river offers no bar to perfect 

 freedom of crossing, to that where the river is an absolute barrier, 

 as in the case of species of Perognathus and Ammospermophilus, 

 accords so closely with degrt < of difference in characters developed on 

 the two sides of the river, that adequate ground is afforded for the 

 belief that intervention of barriers is a prime factor in the differentia- 

 tion of species. And furthermore, it would appear that no two 

 species, in birds and mammals, arise except through geographic segre- 

 gation. 



THE PROBLEM OF BARRIERS WITH REGARD TO BIRDS 

 AND MAMMALS 



The geographical range of any species of animal may be likened to 

 a reservoir of water in a canon. The confining walls are of varying 

 nature. A concrete dam, absolutely impervious, may retain the water 

 at one end. Along either side the basin's walls differ in consistency 

 from place to place. The substratum varies in porosity, at some 

 points impervious like the dam, at others permitting seepage of water 

 to a greater or less distance from the main volume. The water con- 

 tinually presses against its basin walls, as if seeking to enlarge its 

 area. And it may succeed in escaping, by seepage through such por- 

 tions of its barrier as are pervious or soluble, or by free flow through 

 a gap in the walls, if such offers. The area occupied by the water 

 will extend itself most rapidly along the lines of least resistance. 



Every species has a center or centers of abundance in which favor- 

 ing conditions usually give rise to a rate of reproduction more than 

 sufficient to keep the critical area stocked. A tendency to occupy a 

 larger space results because of competition within the species, and 

 individuals and descent-lines multiply and travel radially, extending 

 those segments of the frontier where least resistance is offered. Such 

 radial dispersal takes place slowly in some directions, more rapidly 

 in others, according to the degree of passability of the opposing bar- 



