The problem of communication under early conditions is serious. Even in a high civilisation the maintenance of roads is of greater moment, and imposes a greater burden, than most of the citizens who support it know; but before the means or the knowledge exist to survey and to harden roads, with their causeways over marshes and their bridges over rivers, the supply of food in time of scarcity or of succour in time of danger is never secure: a little narrow path kept up by nothing but the continual passage of men and animals is all the channel a community of men have for communicating with their neighbours by land. And it must be remembered that upon such communication depend not only the present existence, but the future development of the society, which cannot proceed except by that fertilisation, as it were, which comes from the mixture of varied experiences and of varied traditions: every great change in history has necessarily been accompanied by some new activity of travel. Under the primitive conditions of which we speak a river of moderate depth, not too rapid in its current and perennial in its supply, is much the best means by which men may communicate. It will easily carry, by the exertions of a couple of men, some hundred times the weight the same men could have carried as porters by land. It furnishes, if it is broad, a certain security from attack during the journey; it will permit the rapid passage of a large number abreast where the wood tracks and paths of the land compel a long procession; and it furnishes the first of the necessities of life continually as the journey proceeds. Upon all these accounts a river, during the natural centuries which precede and follow the epochs of high civilisation, is as much more important than the road or the path as, let us say, a railway to-day is more important than a turnpike.