One interesting aspect of this is the draining of population down on to the Road. When a map is drawn up showing the density of population we see upon it separate areas of density, sometimes far apart, and between them areas marked by lesser density or even void. But if one should make an accurate population map of any one moment, plotting down every individual upon it, you would not get this effect of isolated dense districts; you would not get the effect of an archipelago, but of a network; for upon the communications between these districts would be marked a dense chain of units in progress from the one to the other: and one would at once grasp how permanent lesser nuclei arise between the two terminal towns. This aspect of the Road suggests a far more important one. The Road--in the sense of a means of communication--in proportion to its excellence differentiates human society (_a_) Into areas of density and void; (_b_) Into the urban political habit and the agricultural political habit. This is a very important reaction of the Road, which must be allowed for in every historical and contemporary problem. Granted an urban centre, with its special opportunities for inter-communication between human beings, for experiment and for what may be called “the cross-fertilization of knowledge,” the growth of such a centre is, of course, dependent upon many things: its economic basis, either as a market or as the capital of a productive area, or, more commonly, as both; the physical surroundings which may, as in the case of Genoa or Venice for instance, strictly limit that growth, etc. But among the causes affecting it, and chief among them, is the Road: the degree of excellence in communication. The growth of a town is a direct function of this, the most conspicuous example, of course, in the whole of history being the immense growth of London following on the supplementing of the old roads by the railway.