PHYSICAL CONDITIONS IN (iENESIS OF SPECIES. 38B 



.sylcaticiis)," affords a case in point. This species is represented in 

 some of its varieties across the whole breadth of the continent and 

 from the northern border of the United States southward to Central 

 America, but in different regions by different 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 sagebrush hare {L. .s>/lra- 

 ticus nuttalli), the ears are considerably longer than in the eastern 

 variety and increase in size from the north southward, reaching their 

 greatest development in western Arizona and the desert region 

 farther westward and southward, where the variety of the plains 

 proper passes into still another variety 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 large long-eared " jackass " 

 hares of the plains the ear likeAvise increases in size to the southward. 

 In Lepiis eaUotis,^ for example, which ranges from Wyoming south- 

 ward far into Mexico, the ear is about one-fourth to one-third larger 

 in the southern examples than in the northern. The little brown hare 

 of the Pacific coast (Z. frowhrkk/ei) jjresents a similar increase in 

 the size of the ear southward, as does to a less extent the prairie hare 

 (L. campestris) . Not only are all of the long-eared species of Ameri- 

 can hares confined to the open plains of the arid interior of the conti- 

 nent, 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 rej^resented 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 northern countries, as is shown by a com- 

 j)arison of the Texan, Mexican, and South American breeds with 

 the northern stock, or those of the south of Europe witli the more 

 northern races. In the wild species of the Old World the southern 

 or subtropical are remarkable for the large size of their horns. The 

 horns of the American pronghorn {AntUocapra aTnericana) are also 

 much larger at southern than at northern localities.'" Naturalists 



a The group here referred to as Lepus fiylvaliciis has in rocoiit years been 

 ilividetl into some twenty-five or more forms, mostly witli tlu' rank of sub- 

 species. — Anihor's note, 1906. 



b Lepiis callotis, as now recognized, dues not occur nortli of Mexico; in jjlace 

 of this name may l)e substituted Lepus te.rUiiiux and its sulispecies. — Author's 

 note, 1906. 



c The deer tribe, in which the antlers increase in size toward tlio north, offer 

 an apparent exception to the rule of increase in size of perii)lit'ral parts toward 

 the Tropics. The antlers of the deer, however, are merely seasonal appendages, 



