272 MONOGBAPHS OF NORTH AMERICAN RODENTIA. 



received from any locality, we find that variety Nultalli finally not only differs 

 markedly in its paler color from variety sylvaticus, but also differs in being 

 smaller; just as, in most cases among the mammals, the pallid forms of the 

 arid plains and deserts prove to he also depauperate varieties of their 

 brighter-colored and more favored or better-fed conspecific allies. 



Geographical variations in the relative size of the feet and ears as com- 

 pared with the general size is not so easily determined from skins as is the 

 difference in general size based on the size of the skull; and the only point 

 in tliis connection that will be specially noticed is the lengthening of the 

 ears to the southward, manifested by nearly all the species, the ears, in many 

 cases, actually increasing in length while the general size diminishes. There 

 is also a marked tendency to an enlargement of the ears in proportion to 

 the aridity of the habitat. Thus, in the L. sylvaticus group, variety Nultalli 

 has the ear generally quite appreciably larger than in variety sylvaticus, the 

 maximum size of the ear being reached in the Great Colorado Desert in 

 variety orizonce, which is chiefly distinguishable from its nearest allies, varie- 

 ties Nultalli and Auduboni, by the enlargement of this member. L. callotis, 

 L. californicus, and L. Trowbridgei also show a very perceptible increase in 

 the size and length of the ear to the southward. In this connection, also, 

 attention may be called to the fact that all of the long-eared species of 

 American Hares are found exclusively over the most arid portions of the 

 continent; just as, in the Old World, all of the longest-eared species are 

 found in the arid portions of Asia and Africa. 



The subject of geographical variation in size being quite fully discussed 

 in connection with the detailed descriptions of the species and varieties given 

 beyond, it is not necessary to devote further space to the subject here. 



II. INDIVIDUAL VARIATION. 



The difference in size between specimens of the same age and sex from 

 the same localities is often quite marked. The amount of this difference is 

 sufficiently indicated by the subjoined measurements of skulls given in 

 Table II. Care has been taken to select only fully adult specimens, and in 

 the table only the maximum and minimum measurements are given. 



