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 and 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 Americia. 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¬ 
ter t,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, R., “ Geographical History of Mammals,” p. 112. 
f Hartert, E., “ Trochilidae,” 
