22 THE COLLECTION OF FOSSIL VERTEBRATES 
The northeast corner of the hall is devoted to a number of 
peculiar groups of SoutH AMERICAN Fossit MAmMaALs, almost all 
North extinct. During the Age of Mammals the two great 
Alcove 9. northern continental areas were joined together from 
SURE time to time, so that there has been an occasional 
Hose interchange of animals and plants among them, the 
Mammals. races developed in one continent spreading to the 
other. The animals of North America therefore, although 
mostly of species distinct from those of Europe and Asia, are 
more or less nearly related to them. But during most of the Age 
of Mammals South America was an island continent, as Aus- 
tralia is still; and its extinct animals are as peculiar and as dif- 
ferent from those of the rest of the world as are the living animals 
of Australia different from those of other continents. It is by 
no means certain where these animals originally came from, but 
there is much evidence to show that both South America and 
Australia were peopled from an Antarctic continent, now sunk 
beneath the ocean or buried in the ice fields of the more frigid 
climate of modern times. 
Of these peculiar South American groups the most extraor- 
dinary are the Edentates, including the Sloths, Armadillos and 
Anteaters which still survive, and the huge MEGATHERIA or 
Ground-Sloths and GLiyptoponts or Tortoise-Armadillos which 
have become extinct. Others were the TOxoDONTs, TYPOTHERES, 
ASTRAPOTHERES and LITOPTERNA, peculiar groups of hoofed ani- 
mals all now extinct. Some of the Litopterna lost their side toes 
and evolved into a one-toed race curiously like the horses of the 
northern hemisphere, although not at all related to them; this is 
one of the most interesting examples of the parallel adaptation of 
two different races of animals to similar conditions of life; the 
horses in the plains and prairies of the north, the litopterna in 
the pampas of the southern continent. 
The best example of the evolution of a race of animals is 
shown in the southeastern corner of the hall. Here is exhibited 
Instances of the Ancestry of the Horse, the specimens from succes- 
Evolution. sive geological strata showing how the modern Horse 
has descended from diminutive ancestors with four toes on each 
forefoot and three on each hind foot, and with teeth and other 
