178 WHEN THE WORLD WAS YOUXG 



They did not all live at the same time either. One race 

 would hold sway for more ages than we can guess, and 

 then would die out, perhaps affected by some change of 

 climate, and by-and-by another would take its place, also 

 to disappear when its turn came. 



Now, how can we know anything at all about animals 

 which died thousands of years ago ? In two ways. From 

 their bones (long since become like a stone in substance), 

 or the impressions of them which have been preserved in 

 the rocks, and from the bodies which have sometimes 

 been found quite complete, with skin, hair, and even eyes, 

 in the frozen marshes of Northern Asia. 



But the discovery of creatures in this condition is 

 very rare. In general, scientific men who study the sub- 

 ject have to be satisfied with the skeleton, or with de- 

 tached parts of the frame, and with this help they have 

 worked wonders. One of the most important things in 

 building up the history of fossil animals is the teeth, 

 and with the aid of these it is possible to find out whether 

 the dead monster fed upon flesh, or upon herbs and leaves, 

 or even if it preferred the wood of the branches. A 

 lightness in the upper part of the body, combined with a 

 small head and short forelegs, tells us that the quadruped 

 could rear itself up on its hind legs, like a kangaroo, while 

 in creatures of the elephant kind, which own a long nose 

 or proboscis, we shall find that the neck is so short that it 

 could not reach its food in the trees or on the ground with- 

 out help of this sort. 



Most of these animals lived long long ago, thousands 

 of years before we have any idea of ; but one or two sur- 

 vived till a race of men inhabited the earth, or at any rate 

 some parts of it. The best known of these great creatures 

 is the mammoth, which was very like an elephant 

 in shape, and like him had huge ivory tusks, curving 

 inwards and upwards, instead of being comparatively 

 straight. Sometimes the curve nearly made a com- 

 plete circle, as in the case of a mammoth skeleton now in 



