370 



HISTORICAL GEOLOGY 



cially of the large animals, have become extinct, and the world 

 is now (except for Man) said to be " zoologically impoverished. " 

 The vicissitudes of the climate, i.e. alternations of glacial and 

 interglacial conditions, appear to have " produced a very severe 

 struggle for existence and were fatal to a great many large Mam- 

 mals, causing numerous extinctions over the larger part of the 



Fig. 230 



Great armored Glyptodonts, Doedicurus clavicaudatus and Glyptodon davipes. 



(After W. B. Scott, by permission of The Macmillan Company.) 



world" (W. B. Scott). It is our present purpose to refer to only 

 a few of the most interesting now extinct Pleistocene Mammals. 

 Among the Edentates (Sloths, Armadillos, etc.), which belong to 

 the simplest Placental Mammals, the Megatherium and the 

 Glyptodon are of special interest. The former (see Fig. 229), a sort 

 of giant ground Sloth, was remarkably massive and attained a 

 length of 15 to 18 feet. Its thigh bones were two or three times 

 the thickness of those of the Elephant, and its front feet were about 

 a yard long. The tooth structure shows it to have been a plant 



