SOUTH AMERICAN SLOTHS 367 



in North America. From South America three others are 

 known. The general range of the family Didelphyidae points 

 to South America as the centre of dispersal. Although the 

 genus Didelphys or Peratherium has been met with in the 

 Eocene of North America and France, while it first appears 

 in South America in the Miocene Period, if Dr. Ameghino is 

 correct, the earliest member of the family (Proteodidelphys) 

 occurs in the Lower Cretaceous beds of Patagonia. Even if 

 we look upon these beds, with Professor Osborn, as really of 

 Eocene age, the more primitive characters of Proteodidelphys 

 point to South America as the ancestral home of the family, 

 and on this continent no doubt the genus Didelphys has 

 originated ajid not in south-eastern Asia, as suggested 

 by Mr. Lydekker.* I think the geological history of the 

 opossums, though dating further back than that of the South 

 American monkeys, followed much upon the lines of the 

 groups just considered, at any rate, they seem to have entered 

 Brazil about the same time. 



Among the birds of South America we have precisely 

 similar examples, except that in their case we know unfor- 

 tunately very little of their past history from palaeontological 

 evidence. The wonderful family of humming birds (Trochi- 

 lidae) is comparable in distribution with the opossums, in so 

 far as it ranges all over South and Central America. It 

 has in all likelihood entered North America in later geological 

 times. Not a single species of humming bird is known be- 

 yond the confines of America. It is of importance to note that 

 of the one hundred and eighteen genera admitted by Dr. Har- 

 tert,f the great majority are confined to the west coast. Some 

 of them inhabit Chile, others Peru, Bolivia, Ecuador, 

 Colombia, Central America and Mexico. A few (Oreotro- 

 chilus) live at enormous heights, up to 20,000 feet. Others 

 are limited in their range to the Antilles and Brazil. Only 

 the single genus Avocettula, with one species, is peculiar to 

 Guiana. This seems to suggest that the family originated in 

 western South America, and has only gradually spread east- 

 ward on the mainland. The West Indian area no doubt was 



* Lydekker, E., " Geographical History of Mammals," p. 112. 

 t Hartert, E., " Trochilidae," 



