62 Charles Darwin, 



here. He had named this American Equus curvi- 

 dens. Certainly it is a marvellous fact in the history 

 of the mammalia, that in South America a native 

 horse should have lived and disappeared, to be suc- 

 ceeded in after ages by the countless herds descended 

 from the few introduced with the Spanish colonists ! " 



In later years. Professor Marsh has shown that a 

 long line of horses once lived in America, this being 

 one of the most interesting and perfect links of 

 evidence in the geological chain. 



These remains suggested to the mind of Darwin 

 many questions pregnant with interest, not the least 

 of which was the geographical distribution of ani- 

 mals. He divided North and South America at the 

 lofty table-land of Mexico, which would naturally 

 affect migration, and saw in the separation two im- 

 portant and strongly contrasted zoological fields. 

 " Some few species," he writes, " alone have passed 

 the barrier, and may be considered as wanderers 

 from the south, such as the puma, opossum, kinka- 

 jou, and peccary. South America is characterised by 

 possessing many peculiar gnawers, a family of mon- 

 keys, the llama, peccary, tapir, opossums, and, espe- 

 cially, several genera of Edentata, the order which 

 includes the sloths, ant-eaters, and armadillos. North 

 America, on the other hand, is characterised (putting 

 on one side a few wandering species) by numerous 

 peculiar gnawers, and by four genera of hollow- 

 horned ruminants, of which great division South 

 America is not known to possess a single species. 

 Formerly, but within the period when most of the 

 now existing shells were living, North America 



