All that, of course, happened long, long ago—many thousands of years before the earliest days mentioned in our history books—at a time about which we know nothing at all save what we can read in that wonderful book of Nature whose pages are the rocks and stones of the earth’s surface. By the study of these rocks and the fossil remains in them we can learn just a few things about the life of those days—the strange kinds of trees which covered the earth from sea to sea, the weird monsters which roamed in the forests and over the hills. Of _man_ we can learn very little. We can get some rough idea of when he first appeared in Britain, and we can tell by the remains preserved in caves, etc., in some small degree what sort of life he lived. But that is all: the picture of England in those days is a very dim one. How and when the prehistoric man of these islands grew to some sort of civilization we cannot say. When first he learned to till the soil and grow his crops, to weave rough clothes for himself, to domesticate certain animals to carry his goods, to make roads along which these animals might travel, to barter his goods with strangers—all these are mysteries which we shall probably never solve. Just this much we can say: prehistoric man probably came to a simple form of civilization a good deal earlier than is commonly supposed. As a rule our history books start with the year of Cæsar’s coming (55 B.C.), and treat everything before that date as belonging to absolute savagery. But there are many evidences which go to show that the Britons of that time were to some considerable extent a civilized people, who traded pretty extensively with Gaul (France, that is), and who knew how to make roads and embankments and, perhaps, even bridges. As early man grew to be civilized, as he learned to drain the flooded lands by the side of the stream and turn them from desolate fens and marshes to smiling productive fields, and as he learned slowly how to get from the hillsides and the plain the full value of his labour, so he realized more and more the possibilities of the great river valley. * * * * * The Thames flows in what may be regarded as an excellent example of a river-basin. A large area, no less than six thousand square miles, is enclosed on practically all sides by ranges of hills, generally chalk hills, which slope down gently into its central plain; and across this area, from Gloucestershire to the North Sea, for more than two hundred miles the River winds slowly seawards, joined here and there by tributaries, which add their share to the stream as they come down from the encompassing heights. [Illustration: THE BIRTH OF THE RIVER.]