THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS 137 



Generic areas differ in character according as the genus 

 is large, that is, comprising many species, or small and having 

 but few species, or, it may be, a single one. The species, as 

 a rule, occupy each its own area, and the areas may be entirely 

 distinct, or they may be contiguous and more or less extensively 

 overlapping, though it seldom happens that two or more 

 species of the same genus inhabit exactly the same area. Often 

 some physical feature, such as a range of high mountains, a 

 great river, the edge of a forest, plain or desert, exactly defines 

 the limits of species of the same genus. The Amazon, for 

 example, acts as such a boundary to many species. It was 

 to this change of related species from one area to another that 

 Darwin referred in the passage quoted above, saying that he 

 had been deeply impressed ''by the manner in which closely 

 allied animals replace one another in proceeding southward 

 over the Continent [i.e. South America]." On the other hand, 

 the overlapping of areas may be very extensive, and one species 

 of great range may cover the whole area of another and much 

 more besides. 



A remarkable example of the widely separated areas of 

 species belonging to the same genus is that of the tapirs. Of 

 this genus there are two or three species in Central and South 

 America and one inhabiting the Malay Peninsula and Borneo, 

 almost as wide a separation as the size of the earth permits. 

 Discontinuous distribution of this character can be explained 

 in terms of the evolutionary theory only in one of two ways. 

 Either (1) the American and Asiatic species developed inde- 

 pendently of one another from different ancestors, or (2) the 

 regions intervening between these widely separated areas once 

 formed a continuous land, occupied by species of the genus 

 which have become extinct. From all that we know concern- 

 ing the operation of the evolutionary process, the first alterna- 

 tive may be set aside as altogether improbable, and, even had 

 we no information concerning the history of the tapirs and 

 their former distribution, the second explanation would be 



