6 ANIMALS OF THE PAST 



brate life began there have been carnivorous 

 animals of some kind to play the role of bone- 

 destroyers. Even were there no carnivores, 

 there were probably then, as now, rats and 

 mice a-plenty, and few suspect the havoc small 

 rodents may play with a bone for the gi-ease it 

 contains, or merely for the sake of exercising 

 their teeth. Now and then we come upon a 

 fossil bone, long since turned into stone, on 

 which are the marks of the little cutting teetli 

 of field mice, put there long, long ago, and yet 

 looking as fresh as if made only last week. 

 These little beasts, however, are indirect rather 

 than direct agents in the destruction of bones 

 by gnawing off the outer layers, and thus per- 

 mitting the more ready entrance of air and 

 water. Plants, as a rule, begin their work after 

 an object has become partly or entirely buried 

 in the soil, when the tiny rootlets find their 

 way into fissures, and, expanding as they grow, 

 act like so many little wedges to force it 

 asunder. 



Thus on dry land there is small opportunity 

 for a bone to become a fossil ; but, if a creat- 

 ure so perishes that its body is swept into the 



