144 LAND MAMMALS IN THE WESTERN HEMISPHERE 



subsistence, each new generation ranging farther and farther 

 from the original home of the species and continuing so to 

 extend until some insuperable obstacle is encountered. When 

 a sea-barrier is removed by upheaval and the newly formed 

 land rendered habitable for mammals through the invasion 

 of plants, the interrupted process is resumed and an inter- 

 change of species between the areas thus connected is brought 

 about. The interchange is, however, always an incomplete 

 one, certain forms not being able so to extend their range^ 

 because of climatic differences, pre-occupation or some such 

 barrier. 



It is customary to give a graphic expression to the facts 

 of animal distribution by dividing the land surface of the earth 

 into districts which are characterized by their faunas. It is 

 not possible to construct a geographical scheme which will 

 be equally satisfactory for all classes of animals, because the 

 geological date of most rapid development and diffusion was 

 so different in the various classes. The geographical and 

 climatic conditions which favoured a particular geographical 

 arrangement of one class had been so completely altered that 

 the class coming in later could not attain a similar distribution. 

 For this reason, land mammals are chosen as affording the best 

 criteria ; their adaptability is such that they are found all 

 over the earth, their dispersal is primarily dependent upon the 

 arrangement and connections of the continental land-masses, 

 modified by the topographical and climatic conditions, and 

 they, with the birds, are the latest of the vertebrate classes to 

 assume a dominating importance. Their history is the most 

 fully known and falls within the best understood portion of the 

 earth's history, making it possible to follow their migrations 

 with a precision which is seldom feasible for the other classes 

 of animals, and thus to correlate the successive physical and 

 organic changes. A particularly great advantage which mam- 

 mals possess for this purpose is that the mutual relationships 

 of the various kinds are better understood than in the case of 



