274. MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
Under the head of each species will be found detailed measurements 
of many specimens collected at identical localities, which further show the 
extent and character of the purely individual variations presented by the 
different species. These tables show that the variation is not confined to 
size, but affects the relative size of the different parts of the body and skull. 
Individual variation in the relative size of the ear, the feet, etc., to the general 
size is also, in many cases, quite noteworthy, as will be fully noticed in con- 
nection with the description of the species and varieties. The variation, 
both in general size and the relative size of different parts, generally ranges 
from 20 to upward of 30 per cent. of the average size, the difference between 
the extremes being generally somewhat in proportion to the extent of the 
series of examples compared. 
III.—GEOGRAPHICAL DISTRIBUTION. 
The family Leporide has representatives throughout the two American 
continents, but is by far the most numerously represented in the temperate 
parts of North America. <A single species only (Lepus timidus) ranges to 
the arctic coast, and is also circumpolar in its distribution, being found also 
in the boreal parts of Asia and Europe. The American representatives of 
this species (forming variety arcticus) differ but slightly from their Old 
World conspecific allies. In America, this species ranges over Greenland, 
the Barren Grounds, Labrador, and portions of Newfoundland, to the south- 
ward its habitat meeting, or slightly overlapping, that of L. americanus. 
LL. americanus extends from the Barren Grounds southward to about the 
isotherm 52° Fahrenheit. Throughout most of this vast region, it forms 
the sole representative of the family. To the southward, its habitat overlaps 
the ranges of L. campestris and L. sylvaticus. Toward the southern border 
of its habitat, it runs into three recognizable varieties, differing, in their 
extreme phases of development, mainly in slight but pretty constant differ- 
ences of color, yet they pass into each other by almost imperceptible grada- 
tions. These varieties are virginianus, Bairdii, and Washingtoni. Variety 
virginianus ranges over Southern New England and the more elevated parts 
of the Middle States, southward to Virginia, and westward to Minnesota ; 
variety Bairdii occupies the dry interior region of the northern portion of 
the Rocky Mountain plateau ; while variety Washingtoni occurs throughout 
a 
