Flying squirrels are circumpolar in distribution. In the Old World they occupy forested areas in eastern Europe, and nearly all of Asia. In the New World they are peculiar to North America, where they frequent nearly all the wooded parts from the Arctic Circle to the Mexican border, and in forests in Mexico along the eastern border of the highlands as well as through Chiapas and Guatemala. In Asia, the center of development of these interesting rodents, many extraordinary forms occur. Some are giants of their kind, measuring nearly four feet in total length. In America there are two groups of species, the smaller and better known of which, the subject of this sketch, occupies the eastern United States and southward. The northern and western animals are larger, some of them more than twice the weight of the eastern species. In many parts of the United States flying squirrels are common and even abundant, but their habits are so strictly nocturnal that they are infrequently seen. They make their homes in woodpecker holes, knot-holes, and hollows in limbs, and trunks of trees and stubs. In addition they take possession of many odd places for residence, among which may be mentioned bird-boxes, dove-cotes, attics, cupboards, boxes, and other nooks in occupied or unoccupied houses that are located within or at the borders of woods. They also make nests of leaves, lining them with fine fibrous bark, grass, moss, fur, or other soft material placed securely in the branches or in forks in trees. They often remodel old bird or squirrel nests into snug homes for themselves. The size and construction of these outside nests vary according to the locality and the material available. As a rule, the nests are small and accommodate only a single pair with their young, and sometimes hold only a single individual, but numerous exceptions to this have been observed. In southern Illinois fifty flying squirrels were discovered in one nest in a tree; in Indiana fifteen were found in a hollow stump; and near Philadelphia thirty were evicted from a martin box they had usurped. In the southern part of their range flying squirrels are active throughout the year, but in the North they become more or less sluggish if they do not actually reach the stage of real hibernation during the severest weather.