ON VARIATION. 55 



the northern stock, or those of the South of Europe 

 with the more northern races. In the wild species of 

 the Old World, the southern or sub-tropical are re- 

 markable for the large size of their horns. The horns 

 of the American prong-horn (Antilocapra americana] are 

 also much larger at southern than at northern locali- 

 ties. 1 Naturalists have also recorded the existence of 

 larger feet in many of the smaller North American 

 Mammalia at the southward than at the northward, 

 among individuals of the same species, especially 

 among the wild mice, in some of the squirrels, the 

 opossum and raccoon, as well as in other species. 



"In birds, the enlargement of peripheral parts, es- 

 pecially of the bill, claws, and tail, is far more obvious 

 and more general than in mammals. The bill is par- 

 ticularly susceptible to variation in this regard, in 

 many instances being very much larger, among indi- 

 viduals of unquestionably the same species, at the 

 southward than at the northward. This accords with 

 the general fact that all the ornithic types in which the 

 bill is remarkably enlarged occur in the intertropical 

 regions. The southward enlargement of the bill within 

 specific groups may be illustrated by reference to al- 

 most any group of North American birds, or to those 

 of any portion of the continent. As in other features 

 of geographical variation, the greatest differences in 

 the size of the bill are met with among species having 

 the widest distribution in latitude. Among the species 

 inhabiting eastern North America we find several strik- 



1 The deer tribe, in which the antlers increase in size toward the north, 

 offer an apparent exception to the rule of increase in size of peripheral parts 

 toward the tropics. The antlers of the deer, however, are merely seasonal 

 appendages, being annually cast and renewed, and are thus entirely different 

 physiologically from the horns of bovines, which retain a high degree of 

 vitality throughout the life of the animal. 



