138 LAND MAMMALS IN THE WESTERN HEMISPHERE 



chosen as incomparably the more hkely. As a matter of 

 fact, we have definite knowledge that tapirs once ranged all 

 over Europe and North America and doubtless over northern 

 Asia, as well, and, further, that North America was joined to 

 Asia by a land occupying the place of the shallow Bering Sea, 

 at a time when the tapirs were able to take advantage of this 

 means of passing from one continent to the other. Such 

 appears to be the invariable explanation of discontinuous dis- 

 tribution, though we may not always be able to give so clear 

 a proof of it. 



The genera of a family are distributed in much the same 

 fashion as the species of a genus, but, as a rule, much more 

 widely. While no genus of terrestrial mammals is cosmo- 

 politan (i.e. universally distributed), at least as genera are de- 

 fined and limited by most modern systematists, certain families 

 are represented in every continent. If the extremely peculiar 

 and isolated Australian continent be excepted, the number 

 of such cosmopolitan families is considerable and wide separa- 

 tion between the genera is frequent. Of the camel family, 

 for instance, one genus, that of the true Camel (Camelus), 

 is confined to the northern hemisphere and the Old World, 

 the other {Lama), comprising the Llama, Guanaco, etc., is 

 found only in the southern hemisphere and the New World. 

 Less extreme instances of the discontinuous distribution of 

 a family are common enough. 



The principles of distribution are the same when applied 

 to families and orders. Most of the mammalian orders are 

 very widely distributed and many are cosmopolitan, except 

 for Australia, though some are confined to one or two conti- 

 nents. The monotremes are limited to Australia and Tas- 

 mania, the marsupials to Austraha and the Americas, the 

 edentates to the latter, the elephants and hyracoids to Africa 

 and Asia. Carnivores and rodents, on the contrary, are found 

 in every continent, even Australia. 



We have next to inquire what is the nature of the obstacles 



