But, lovely as the Earth is, we should not perhaps have thought so much of it if there had been nothing to discover. We see that it has been prepared for us an immensely long time ago; and when we know a little, we want to search further and find out what the whole plan of Creation is, so far as we can. You will be surprised when you know how many signs of past life there are around you—many more than you can see with the eye. The Earth is one great burying-place of creatures which have passed away. You are walking over their dead and fossil bodies at almost every step. They are built into the walls of our houses, and there are millions of them in some of the commonest stones of the pavement. Those round, smooth pebbles, called flint stones, which we pick out of the gravel walks, were once partly such soft tender things as sponges; but time has hardened them, and they have been rolled together in seas and rivers by the always moving water until they have become quite different to look at from the rough blue flints they were when they were washed out of the chalk beds. When you are walking along the sands of some seacoasts, you are treading on little specks of these small flints which have been ground down fine in that great mill, the ocean. The sponges, then, did some part in the building up of the Earth. The very chalk you draw with is composed of the shells of sea-animals. Your slates and slate pencils were once a fine mud at the bottom of the sea, since become so hard that it is used for covering the roofs of our houses, and in this mud lived myriads of small shell-fish which have sometimes left their frail houses in the slate beds to tell us how they were made. That slate is the hardened mud of an old sea bottom, there is no doubt at all. There are many other things in common use which show us the life that was. Perhaps you did not know that coals are _compressed plants_, and that we are now burning the vegetation of the past time! But these will be described in their right places by-and-by, and you will see how certain it is that some of the commonest things we use were living creatures and graceful plants. Here is “the framework” of the puzzle, and I think you will agree with me that we shall have pleasure in putting it together with all the queerly-shaped pieces we shall find in the following chapters. We have fossil plants to show us what grew upon the Earth, fossil bones to tell us what animals lived here, and thousands of different kinds of fossil shells and fishes to show us that the seas in the long past time were crowded with life; and besides, though there are no written histories of the men whom we shall read about, they, too, have left many things which they used in the caves where they lived and in their graves, to make us feel certain that they were some of the oldest people that ever lived. With all these things to help us, it will be strange if we cannot make out a great deal of the history of life upon our Earth.