ON VARIATION. 53 



biers, the variation often amounting to from ten to 

 fifteen per cent, of the average size of the species. 



"As a general rule, certain parts of the organisms 

 vary more than does general size, there being a marked 

 tendency to enlargement of peripheral parts under 

 high temperature, or toward the tropics, hence south- 

 ward in North America. This is more readily seen in 

 birds than in mammals, in consequence, mainly, of 

 their peculiar type of structure. In mammals it is 

 manifested occasionally in the size of the ears and feet, 

 and in the horns of bovines, but especially and more 

 generally in the pelage. At the northward, in individ- 

 uals of the same species, the hairs are longer and 

 softer, the under fur more abundant, and the ears and 

 the soles of the feet better clothed. This is not only 

 true of individuals of the same species, but of northern 

 species collectively as compared with their nearest 

 southern allies. Southern individuals retain perma- 

 nently, in many cases, the sparsely clothed ears and 

 the naked soles that characterize northern individuals 

 only in summer, as is notably the case among the dif- 

 ferent squirrels and sphermophiles. 



"In mammals which have the external ear largely 

 developed as in 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 apparent. It is 

 more especially marked, however, in species inhabit- 

 ing extensive open plains and semi-desert regions. 

 The little wood hare, or gray rabbit {Lepus sylvaticus], 

 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 



