ANCIENT AMERICA. 339 



histories of North and South America have been different, 

 and in some respects strongly contrasted. North America 

 was evidently in very early times so far connected with 

 Europe and Asia as to interchange with those continents 

 the higher types of animal life as they were successively 

 developed in either hemisphere. These more perfectly 

 organised beings rapidly gained the ascendency, and led 

 to the extinction of most of the lower forms which had 

 preceded them. The Nearctic has thus run a course 

 parallel to that of the Palsearctic region, although its 

 fauna is, and perhaps always has been, less diversified 

 and more subject to incursions of lower types from 

 adjacent lands in the southern hemisphere. 



South America, on the other hand, has had a history 

 in many respects parallel to that of Africa. Both have 

 long existed either as continents or groups of large 

 islands in the southern hemisphere, and for the most 

 part completely separated from the northern continents ; 

 and each accordingly developed its peculiar types from 

 those ancestral and lowly- organised forms which first 

 entered it. South America, however, seems to have had 

 a larger area and more favourable conditions, and it 

 remained almost completely isolated till a later period. 

 It was therefore able to develop a more-varied and 

 extensive fauna of its own peculiar types, and its union 

 with the northern continent has been so recent, and is 

 even now maintained by so' narrow an isthmus, that it 

 has never been overrun with the more perfect mammalia 

 to anything like the extent that has occurred in Africa. 

 South America, therefore, almost as completely as Aus- 

 tralia, has preserved for us examples of a number of low 

 and early types of mammalian life, which, had not the 



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