ZOOLOGY OF NORTH AND SOUTH AMERICA. 167 



the migration of species, by affecting the climate, 

 and by forming, with the exception of some valleys 

 and of a fringe of low land on the coast, a broad 

 barrier ; we shall then have the two zoological 

 provinces of North and South America strongly 

 contrasted with each other. Some few species 

 alone have passed the barrier, and may be consid- 

 ered as wanderers from the south, such as the pu- 

 ma, opossum, kinkajou, and peccari. South Amer- 

 ica is characterized by possessing many peculiar 

 gnawers, a family of monkeys, the llama, peccari, 

 tapir, opossums, and, especially, several genera of 

 Edentata, the order which includes the sloths, ant- 

 eaters, and armadillos. North America, on the 

 other hand, is characterized (putting on one side 

 a few wandering species) by numerous peculiar 

 gnawers, and by four genera (the ox, sheep, goat, 

 and antelope) of hollow-horned ruminants, of which 

 great division South America is not kno'wn to pos- 

 sess a single species. Formerly, but within the 

 period when most of the now existing shells were 

 living. North America possessed, besides hollow- 

 homed ruminants, the elephant, mastodon, horse, 

 and three genera of Edentata, namely, the Mega- 

 therium, Megalonyx, and Mylodon. Within nearly 

 this same period (as proved by the shells at Bahia 

 Blanca) South America possessed, as we have just 

 seen, a mastodon, horse, hollow-horned ruminant, 

 and the same three genera (as well as several oth- 

 ers) of the Edentata, Hence it is evident that 

 North and South America, in having within a late 

 geological period these several genera in common, 

 were much more closely related in the character 

 of their terrestrial inhabitants than they now are. 

 The more I reflect on this case, the more interest- 

 ing it appears : I know of no other instance where 

 we can almost mark the period and manner of the 



