244 l^li^ DOCTRINE OF DESCENT. 



the magnolia in IMexico ; 40° — 50° of latitude separate 

 the extremes which meet in the Himalayas, and the vast 

 plains and huge river systems seem almost to solicit 

 immigration. The accordance of the whole faunas of 

 Mexico 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 shall 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 

 Palaeochoeridse. 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 

 buffaloes ; 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-opera- 

 tion, 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 



