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THE AMERICAN NATURALIST [Vol.LII 



While the statistical certainty of these four types can 

 not be doubted, it must again be insisted that the differ- 

 ences relate to averages rather than to individual ani- 

 mals. The frequency distributions of the various widths, 

 as represented by the histograms (Fig. 5), show this 

 point clearly. There is a certain amount of overlapping, 

 even between the most divergent races. 



Skin Pigmentation.— Certain regions of the skin are 

 colored by dark pigment. The regions showing skin pig- 

 mentation most clearly are the ears, tip of snout, soles of 

 feet, and, in the males, the scrotum. 



Frequent comparisons of considerable numbers of 

 freshly killed specimens have made it plain that, in re- 

 spect to the pigmentation of the ears, our four races can 

 be arranged in the same graded series as was found to 

 hold for coat color. As regards the other three skin char- 

 acters, I have never compared more than two races at a 

 time, but I feel little doubt that all four could be arranged 

 in the same order. No exact measurements are here pos- 

 sible, as in the case of the tail stripe. In a few instances 

 I have, however, graded a given character, according to 

 an arbitrary scale, and have thus been able to express the 

 differences between two races in a roughly quantitative 

 way. The following comparison between 42 sonoriensis 

 and 38 La Jolla gambeli with respect to the pigmentation 

 of the scrotii : ■ • \ 



TABLE II 



A similar tabulation was made in another case, com- 

 paring two lots of specimens of these same races in re- 

 spect to the pigmentation of the foot. 



