The growth of great towns, one of the last phases of our national development, one which finds its example in the Thames Valley as elsewhere, and one to which we shall allude before closing these notes upon the river, has somewhat obscured the quality of this original accumulation of wealth along the Thames. But when we come to consider the figures of the census at an earlier time, before modern commercialism and the railway had drawn wealth and population into fewer and larger centres, we shall see how considerable was the string of towns which had grown up along the stream. And we shall especially see how fairly divided among them was the population, and, it may be presumed, the wealth and the rateable value, of the valley. The point just mentioned in connection with the larger monastic foundations, and their artificial concentration of economic power, deserves a further elaboration, for the economic importance of a district is one of the aspects of geography which even modern analysis has dealt with very imperfectly. Economists speak of the economic importance of such-and-such a spot because material of use to man-kind is there discovered. Thus, people commonly point to the economic importance of the valleys all round the Pennine Range in England because they contain coal and metals, and to the economic importance of a small district in South Wales for the same reason. A further consideration has admitted that not only places where things useful to mankind are discovered, but places naturally fitted for their exchange have an economic importance peculiarly their own. Indeed, the more history is studied from the point of view of economics, the more does this kind of natural opportunity emerge, and the less does the political importance of purely productive areas appear. The mountain districts of Spain, the Cornish peninsula, were centres of metallic industry of the first importance to the Romans, but they remained poor throughout the period of Roman civilisation. To-day the farmer in the west of America, the miner and the clerk in Johannesburg, are perhaps more numerous, but as a political force no wealthier for the opportunities of their sites: the economic power which they ultimately produce is first concentrated in the centres of exchange where the wealth they produce is handled.