MUEIDyE— SIGMODONTES— H. LEUCOPUS SONOUIENSIS. 



85 



Table XX. — Measurements of sixty-nine (and list of other) additional specimens of British American and Arctic 

 Hesperomys leucopus, mostly of the "sonoriensis" style — Continued. 



Note — "Wo have brought together our entire Arctic series of H. leucopus under the head of " sonoriensis ", and nearly 

 all our specioion8 will be found labeled " II. leucopus var." It is hardly necessary to add that, however, in this series every 

 grade nf approach to ordinary United States leucopus is found, and that a certain percentage <<! the specimens are identical 

 witii Massachusetts skins, fur example. 



'I'll ruing now to the matter of color, we find that certain differences in 

 this respect have been ascribed to sonoriensis as diagnostic. This we can 

 prove, first, to be so variable with specimens of "sonoriensis" inter se that it 

 cannot distinguish them inter alia; and, secondly to be dependent upon local 

 climatic influences. It fluctuates between wide extremes, and is assuredly 

 unreliable, both on this account and because it is within the ordinary limits 

 ot color-variation of leucopus (see under "gambeli" and "austerus"). The 

 intensity of coloration lias precisely the same relation to hygrometric influ- 

 ences that Mr. Allen pointed out in the case of birds : the depth of color is 

 in direct ratio to the mean annual rain-fall, or, in other words, to average 

 humidity of the atmosphere. In dry, treeless regions, we have the palest 

 forms; while the darker-tinted specimens occur in moist, wooded localities. 

 This is precisely correspondent to the case of " gambeli " and "austerus'', 

 already noted. Some of our palest prairie specimens almost suggest albinism • 

 most of the Nebraska series are remarkably pale, with an indistinct or almost 

 inappreciable darker dorsal wash. Fort Crook specimens are darker than the 

 average (just as the Fort Crook Arvicolas are), approaching austerus in this 

 respect; Utah and New Mexican samples are intermediate. The narrowness 

 of the dark stripe along the upper surface of the tail is simply of a part with 

 the general pallor; so is the. hoariuess of the ears, which appears as a sort of 

 silvery edging in most specimens. (Cf. per contra Baird, op. cit. 474.) 



Recurring now to the Arctic series, we find that instead of being paler 

 than average leucopus, they are almost without exception darker. In this 



