6 ANIMALS OF THE PAST 
brate life began there have been carnivorous 
animals of some kind to play the réle 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 grease 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 teeth 
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 
