What is equally interesting, when a high civilisation after its little effort begins to decline into one of those long periods of repose into which all such periods of energy do at last decline, the river reassumes its importance. There is a very interesting example of this in the history of France. Before Roman civilisation reached the north of Gaul the Seine and its tributary streams were evidently the chief economic factor in the life of the people: this may be seen in the sites of their strongholds and in the relation of the tribes to one another, as for instance, the dependence of the Parisians upon Sens. The five centuries of active Roman civilisation saw the river replaced by the system of Roman roads; the great artificial track from north to south, for instance, takes on a peculiar importance; but when the end of that period has come, and the energies of the Roman state are beginning to drag, when the money cannot be collected to repair the great highways, and these fall into decay--then the Seine and its tributaries reassume their old importance. Paris, the junction of the various waterways, becomes the capital of a new state, and the influence of its kings leads out upon every side along the river valleys which fall into the main valley of the Seine. There are but two considerable modifications to the use for habitation of slow and constant rivers: their value is lessened or interrupted by precipitous banks or they are rendered unapproachable by marshes. The first of these causes, for instance, has singularly cut off one from the other the groups of population residing upon the upper and the lower Meuse, as it has also, to quote another example, cut off even in language the upper from the lower Elbe. From this first species of interruption the Thames is, of course, singularly free. There is no river in England, with the exception of the Trent, whose banks interfere so little with the settlement of men in any place on account of their steepness. As to the second, the Thames presents a somewhat rare character.