ANCIENT AMERICA. 337 



families of rodents and Edentata are here abundant, 

 many of the genera being the same but several new ones 

 also appearing. There are also horses, peccaries, a mas- 

 todon, llamas, and deer ; but besides these there are a 

 number of altogether peculiar forms, such as the 

 Macrauchenia, allied to the Tapir 'and Palseotherium ; 

 the Homalodontotherium, allied to the miocene Hyraco- 

 don of North America ; and the Toxodontidae, a group 

 of very large animals having affinities to Ungulates, 

 rodents, Edentata, and Sirenia, and therefore probably 

 the representative of a very ancient type. 



Here then we meet with a mixture of highly developed 

 and recent, with low and ancient types, but the latter 

 largely predominate ; and the most probable explana- 

 tion seems to be that the same concurrence of favourable 

 conditions which allowed the megatherium and mega- 

 lonyx to enter North America also led to an immigration 

 of horses, deer, mastodons, and many of the Felidae into 

 South America. These inter-migrations appear to have 

 taken place at several remote intervals, the northern and 

 southern continents being for the most part quite sepa- 

 rated, and each developing its own peculiar forms of life. 

 This view is supported by the curious fact of a large 

 number of the marine fishes of the two sides of Central 

 America being absolutely identical implying a recent 

 union of the two oceans and separation of the continents 

 while the mollusca of the Pacific coast of America 

 bear so close a relation to those of the Caribbean Sea and 

 the Atlantic coasts, as to indicate a somewhat more 

 remote but longer continued sea-passage. The straits 

 connecting the two oceans were probably situated in 

 Nicaragua and to the south of Panama, leaving the 



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