4 LAND MAMMALS IN THE WESTERN HEMISPHERE 



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 regard- 

 ing 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 hemi- 

 sphere. 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 

 m.odified 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- 



