THE AMEBIC AN NATURALIST [Vol. XLIX 



Now do we find any instances of correlation between 

 these differences of structure or color and differences in 

 environmental conditions? The most conspicuous of the 

 structural differences relate to the greater length of the 

 tail and foot of the Eureka race {rubidus) as compared 

 with any of the other three races here considered. It is 

 of considerable interest to note that there is here an in- 

 crease in the length of these appendages as we pass to the 

 northward, a circumstance which is still further empha- 

 sized by the condition of certain Alaskan subspecies. In 

 fact, so far as these coastal subspecies of Peromyscus 

 are concerned, there seems to be, within certain limits, a 

 reversal of Allen's principle of the shortening of "periph- 

 eral parts" as we pass from south to north. The facts 

 here revealed are likewise out of harmony with my own 

 experimental results from white mice, which showed con- 

 clusively that low temperature and high humidity led to 

 a decrease, rather than an increase in the length of the 

 tail and foot. 12 A little later I shall point out the pro- 

 nounced effect of certain other artificial influences upon 

 the length of these appendages in Peromyscus, though I 

 must admit that these later experimental results furnish 

 no more satisfactory clue to the origin of these differ- 

 ences in nature. 



On the whole, then, these preliminary researches do 

 not offer much ground for believing that the differences 

 found in the tails and feet of these wild races of Pero- 

 myscus result directly from any differences in environ- 

 mental stimuli, or for expecting that they will respond 

 appreciably to artificial climatic changes. 



Passing to color differences, we seem to have here a 

 good illustration of that correlation between atmospheric 

 humidity and depth of pigmentation which has long been 

 recognized to hold for mammals, birds and some other 

 animals. If we arrange our four environments in ascend- 

 ing order with respect to their atmospheric humidity 



12 Cf. Journal of Experimental Zoology, April, 1915, and earlier papers 



