244 THE DOCTRINE OF DESCENT. 
the magnolia in Mexico; 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 
Paleochceride. 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 
