178 WHEN THE WORLD WAS YOUNG
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 subject have to be satisfied with the skeleton, or with detached 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 without 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 survived 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 complete circle, as in the case of a mammoth skeleton now in