54 PRIMAR Y FA CTORS OF ORGANIC E VOL UTION. 



different regions by geographical races or subspecies. 

 In addition to certain differences of color and general 

 size, the ears vary still more strongly. In the form 

 inhabiting the Great Plains, commonly known as the 

 little sage-brush hare (Z. sylvaticus nuttalli}, the ears 

 are considerably longer than in the eastern variety, 

 and increase in size from the north southward, reach- 

 ing their greatest development in Western Arizona and 

 the desert region further westward and southward, 

 where the variety is characterized mainly by the large 

 size of its ears, which are in this race nearly twice the 

 size they attain in the eastern variety. In the long- 

 eared 'jackass' hares of the plains, the ear likewise 

 increases in size to the southward. In Lepus callotis, 

 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 in the north- 

 ern. The little brown hare of the Pacific Coast (Z. 

 trowbridgei} presents a similar increase in the size of 

 the ear southward, as does, to a less extent, the prairie 

 hare (Z. campestris). Not only are all of the long- 

 eared species of American hares confined to the open 

 plains of the arid interior of the continent, but over 

 this same region is the tendency to an enlargement of 

 the ear southward stronger than elsewhere. It is also 

 of interest in this connection that the largest-eared 

 hares of the Old World occur over similar open, half- 

 desert regions, as do also the largest-eared foxes. On 

 our western plains, the deer are represented by a large- 

 eared species. Among the domestic races of cattle, 

 those of the warm temperate and intertropical regions 

 have much larger and longer horns than those of north- 

 ern countries ; as is shown by a comparison of the 

 Texan, Mexican, and South American breeds, with 



