22 THE COLLECTION OF FOSSIL VERTEBRATES 



The northeast corner of the hah is devoted to a number of 

 peculiar groups of South American Fossil Mammals, almost all 

 North extinct. During the Age of Mammals the two great 



Alcove 9. northern continental areas were joined together from 

 South time to time, so that there has been an occasional 



American . . , , , , , 



Fossil 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 modem times. 



Of these peculiar South ,\merican 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 Glyptodonts 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 exhiliilcd 

 Instances of the Aiiccstrv of tlic llorsc, 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 h.ind foot, and with teeth and other 



