MURID^-SIQMODONTES— U. LEUC0PU8 SONOlilBNSlS. 



85 



Tablk XX. — ilcaiuremenUi of »ix/i/->iiiic((iiid lint of other) adiHtioiial upevimem of Brilisli American and Jii/io 

 IlK8rEi!i>M YS LEUCorus, mo»Uy of the " mnorh»iiin " hIijIc — Contiuiicd. 



NOTB — Wohavo Imuislit t«(rptlier our eotini Arctic Rorios of H, IrtwopMiiiiderllio lifad of '-Miuori4'D8lfi", and nrui-ly 

 lilt our HpiH^iiiu'nH vill bo foiintl ln))i>tf;(l " II. IciiRopiin var." It i» liiinliy iicccMary t^i add thnt. Iinw4>ver, in ttiiH Rerit'A evpry 

 ((raclo iif approacli Xn ordinary Utitloil HtatuH letuMpiu ia fuuud, and that a curtain pcn-culii;;)! of Itio 8|K!Ciniena aro idontical 

 Willi Maasacbnauttrt sliius, lor oximiplo. 



Turning now to the matter of color, we find that certain differences in 

 this res|)ect have been ascribed to sonoriensis as diagnostic. This we can 

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

 cannot distinguish them inter alia; and, secondly to be dependerlt 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 

 of color-variation oi leucopm (see imder "gambcli" and "austerUs"). The 

 infonsity of coloration has precisely the same relation to hygrometric influ- 

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

 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 

 Ibrms ; while the darker-tinted s])ecimens occur in moist, wooded localities. 

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

 already noted. Some of our |)alest prairie specimens almosl, suggest albinism ; 

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

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

 average (just as the Fort Crook Aroicolas are), approaching ansterus in this 

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

 of the «lark stripe along the upper surface of the tail is simjjly of a part with 

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

 silvery edging in most specimens. {Cf. per contra B.\1KD, 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 



^^T'lS'i 



