Within the limits of each of these categories we may note that there are many degrees of specialization or adaptation to particular modes of life. Thus, for example, among the marine mammals, the Whales and their allies, Porpoises, etc., are so completely adapted to a life in the seas that they cannot come upon the land, and even stranding is fatal to them, while the Seals frequently land and move about upon the shore. It should further be observed that mammals of the most diverse groups are adapted to similar modes of existence. Thus in one natural group or _order_ of related forms, occur terrestrial, burrowing, arboreal and aquatic members, and the converse statement is of course equally true, that animals of similar life-habits are not necessarily related to one another, and very frequently, in fact, are not so related. Among the typically marine mammals, for example, there are at least three and probably four distinct series, which have independently become adapted to life in the sea. * * * * * Before attempting to set forth an outline of what has been learned regarding the history of mammalian life in the western hemisphere, it is essential to give the reader some conception of the manner in which that knowledge has been obtained. Without such an understanding of the methods employed in the investigation the reader can only blindly accept or as blindly reject what purports to be the logical inference from well-established evidence. How is that evidence to be discovered? and how may trustworthy conclusions be derived from it? The first and most obvious step is to gather all possible information concerning the mammals of the present day, their structure (comparative anatomy), functions (physiology), and their geographical arrangement. This latter domain, of the geographical distribution of mammals, is one of peculiar significance. Not only do the animals of North America differ radically from those of Central and South America, but within the limits of each continent are more or less well-defined areas, 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 illimitably 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.