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THE AMEBIC AX NATURALIST [Vol.LII 



III. Differences between the Four Local Races under 

 Consideration 2 

 These differences may be divided, for the sake of con- 

 venience, into pigmental and structural ones. Since the 

 former are the most obvious, they will be discussed first. 



1. Pigmental Differences 

 The pigmental differences relate to (1) the hair, (2) 

 the skin. 



Hair.— Like the other members of the genus Peromys- 

 cus, the mice of the present group are covered with pig- 

 mented hairs upon the dorsal and lateral surfaces, while 

 the ventral surface and to a large extent the feet are cov- 

 ered with white hair. Upon the trunk these white hairs 

 are, to be sure, devoid of pigment only at the distal ends. 

 Parting the pelage at any point, dorsal, ventral or lateral, 

 reveals the presence of a slate-colored basal zone in each 

 hair. 



The most obvious differences between the races under 

 consideration relate to the dorsal coat color (Fig. 4). 

 This is darkest in the animals from the humid redwood 

 district (Eureka), palest in those from the Mojave Desert 

 (Victorville), and of an intermediate hue in the collec- 

 tions from Berkeley and La Jolla. These last two races 

 likewise differ from one another, the former being darker 

 than the latter. Thus we have a series of four grada- 

 tions, which are correlated directly with gradations in 

 the rainfall and atmospheric humidity of their respective 

 habitats. 



It is important to notice, however, that these differences 

 of shade relate rather to averages than to individual cases. 

 All of the Eureka mice are not darker than all of the 

 Berkeley mice. Xor are all of the Berkeley mice darker 

 than all of the La Jolla mice, nor all of the latter darker 

 than all of those from Victorville. In comparing repre- 



