Effects of External Conditions 147 
wise, “there are well-known instances of an increase in the length 
of the tail” (meaning the tail feathers). Much more explicit 
statements are offered regarding mammals, both by Allen and by 
Coues, though it is stated by the former that the responses to 
climatic conditions are less evident in this class than among the 
birds. ‘In mammals which have the external ear largely devel- 
oped, as the wolves, foxes, some of the deer, and especially the 
hares, the larger size of this organ in southern as compared with 
northern individuals of the same species is often strikingly appar- 
ent... .. In Lepus callotus|[‘Lepus texianus and its subspecies’ 
—later note], for example, which ranges from Wyoming southward 
far into Mexico, the ear is about one-fourth to one-third larger in 
the southern examples than inthe northern. . . . . Among 
the domestic races of cattle those of the warm temperate and inter- 
tropical regions have much larger and longer horns than those 
of northern countries. . . . . Naturalists have also recorded 
the existence of larger feet in many of the smaller North American 
mammialia at the southward than at the northward among indi- 
viduals of the same species. . . . .”’ In his monograph on the 
Muridz*? Coues repeatedly makes similar statements. Refer- 
ring to a mouse, “Hesperomys leucopus”? (now Peromyscus leu- 
copus) (p. 66), he says: “The arctic series averages larger than 
the United States specimens, and has shorter feet and ears, as 
well as shorter tail,’ and he alludes later to “the well-known law 
of smallness of peripheral parts in Arctic mammals” (p. 83). 
Comparing the red-backed vole, “Evotomys rutilus gapperi,”’ 
a more southern “variety,” with the species “E. rutilus,’®* he 
finds that the vertebral part of the tail is, on the average, about 
a third of an inch longer in the former, while the foot is 72 hun- 
dredths of an inch in length, as compared with 70 hundredths of an 
inch in the northern form. Relatively, the differences are even 
% Op. cit., 1905, pp. 382-384. 
37 Monographs of North American Rodentia.—Report of the U. S. Geological Survey, vol. xi, 1877, 
pp- I-I091. 
38Tt is quite possible that two or more distinct species are here referred to. I am not sufficiently 
familiar with the classsification of the Muride to know the present status of the various species and 
varieties referred to by Coues. ‘‘E. rutilus gapperi” is now regarded as a true species, Evotomys 
gapperi. 
