244 "^^^ DOCTRINE OF DESCENT. 



in Mexico; 40° — 50° of latitude separate the extremes 

 which meet in tlie Himalayas, and the vast plains and 

 huge river systems seem almost to solicit immigra- 

 tion. The accordance of the whole faunas of Mex- 

 ico and Guiana, moreover, shows how little the isthmus 

 of Panama checks the advance to South America, 

 where again one mighty fluvial system trenches upon 

 the other without any lofty partitions; nor is there any 

 arid desert in the whole extent from the Canadian seas 

 to Patagonia. 



" We «hall probably not be wrong in ascribing the 

 remarkable extension of fossil and present mammals of 

 America in a great measure to this circumstance. As 

 we have seen, the Miocene fauna of Nebraska is the 

 offspring of the Eocene fauna of the Old World. The 

 Pliocene animals of Niobrara, which are buried in the 

 same district as Nebraska, but on more recent arenaceous 

 strata, still further corroborate this statement: elephants, 

 tapirs, and many species of horses, scarcely differ from 

 those of the Old World; the pigs, judging by their 

 dentition, are descendants of European miocene 

 Palseochoeridse. The ruminants are represented by the 

 same genera, and partially by the same species, as in 

 the analogous strata of Europe, as deer, sheep, and buf- 

 faloes; neither do the carnivora or the minute animal 

 life offer an exception. Many genera of an entirely 

 Old- World cast have in the lapse of time penetrated far 

 into South America, and there died out shortly before 

 the arrival of man, or perhaps by his co-operation, as 

 was the case with the two species of mammoth of the 

 Cordilleras and the South American horse, whose present 

 successors reached this insular continent by a far shorter 



