the animals of which differ in a subordinate degree from those of other areas. The study of the modern world, however, would not of itself carry us very far toward the goal of our inquiries, which is an explanation, not merely a statement, of the facts. The present order of things is the outcome of an inimitably long sequence of events and can be understood only in proportion to our knowledge of the past. In other words, it is necessary to treat the problems involved in our inquiry historically ; to trace the evolution of the different mammalian groups from their simpler beginnings to the more complex and highly specialized modern forms ; to determine, so far as that may be done, the place of origin of each group and to follow out their migrations from continent to continent.
While we shall deal chiefly, almost exclusively, with the mammals of the New World, something must be said regarding those of other continents, for, as will be shown in the sequel, both North and South America have, at one time or another, been connected with various land-masses of the eastern hemisphere. By means of those land-connections, there has been an interchange of mammals between the different continents, and each great land-area of the recent world contains a more or less heterogeneous assemblage of forms of very diverse places of origin. Indeed, migration from one region to another has played a most important part in bringing about the present distribution of living things. From what has already been learned as to the past life of the various continents and their shifting connections with one another, it is now feasible to analyze the mammalian faunas of most of them and to separate the indigenous from the immigrant elements. Among the latter may be distinguished those forms which are the much modified descendants of ancient migrants from those which arrived at a much later date and have undergone but little change. To take a few examples from North America, it may be said that the Bears, Moose, Caribou and Bison are late migrants from the Old World ; that the Virginia and Black-
