ANCESTORS OF MAMMALS 23 



ance very much like that of the ordinary sloth of to-day. 

 What the weight of such a monster could have been is 

 a question, but that it weighed many tons can readily be 

 believed. Another of these giant sloths, the Mylodon, 

 not less interesting, was eleven feet long. 



These animals roamed the forests of South America 

 in the Pleistocene period, when North America was cov- 

 ered in part by an ice sheet — the so-called glacial period, 

 during which the mammoth lived. In studying these 

 giants, and the times in which they lived, a fascinating 

 field is opened up, and the question, why such huge ani- 

 mals should have been entirely wiped out of existence, 



Fig. 16. — The Megatherium. 



arises. It is believed that the huge Megatherium (Fig. 16) 

 was the victim of droughts, which from time to time sweep 

 over South America, killing the cattle. These sometimes 

 last for three years, and then the great rivers are mere 

 streams running between vast walls of soft mud. To 

 reach the water, animals of all kinds make their way into 



