Before doing this it becomes necessary to bear in mind certain facts and laws without a knowledge of which it is impossible to think intelligently on the subject. It is a matter of common observation that different groups of animals and plants inhabit different regions, even in the same latitude; that some forms are almost world wide in distribution; that others are restricted to very limited areas; that the ranges of very dissimilar species are often geographically coincident; and that, as a rule, animals inhabiting contiguous areas are more nearly related than those inhabiting remote areas. The recognition of these facts early led to the attempt to divide the surface of the earth, according to its animal life, into 'faunal' districts. By the term _fauna_ is meant the sum of the animal life of a region. A comparatively meagre supply of information is sufficient to indicate the principal faunal subdivisions of a country, but for mapping the exact boundaries of such areas a vastly greater and more precise fund of knowledge is necessary. The way in which such maps are prepared is by collecting all available authentic records of localities where the particular species has been found. This is done by compiling published records, by examining labels of specimens in various museums and private collections, and by work in the field. The data thus brought together are arranged on cards under authors and regions, and are tabulated under species. The localities are then indicated by colored spots on an outline map, the space surrounded by the spots being washed in with a paler tint of the same color. A separate map is devoted to each species. Faunal maps are made by combining a large number of species maps. In making such combinations it is found, as a rule, that a considerable percentage of the species maps fall into certain well defined categories whose color patches are essentially coincident. The composite resulting from the coördination of these maps may be held to represent the natural faunal areas of a country. Several such areas may be characterized by the common possession of species not found elsewhere, and may be combined to constitute a faunal province; several provinces, a region; and several regions a realm or primary zoö-geographical division of the earth's surface. Having ascertained the actual extent and limitations of the natural faunal districts, it remains to correlate the facts of distribution with the facts of physiography.