Althought anticipated by earlier writers, the clearest formulation of the idea of a metropolis as the focus of economic activity is to be found in Professor N. S. B. Gras' The Evolution of the English Corn Market. There he writes:
The metropolitan market may be described as a large district having one center in which is focussed a considerable trade. Trade between outlying parts of course may take place, but it is that between the metropolitan town and the rest of the area that dominates all. This is chiefly the exchange of the raw pro-ducts of the country for the manufactured or imported goods of the town. The prices of all goods sent to the metropolitan center are ‘made’ there, or, in other words, prices diminish as the distance from the center is increased.