It has been asserted that circulation or interaction is a result of areal differentiation. To a degree this is true, but mere differentiation does not produce interchange. Numerous different areas in the world have no connection with each other. In order for two areas to interact, there must be a demand in one and a supply in the other. Thus an automobile industry in one area would use the tires produced in another but not the buggy whips produced in still another.
Specific complementarity is required before interchange takes place.
So important is complementarity that relatively low-value bulk products move all over the world, usually utilizing, it is true, relatively cheap water transport for most of the haul. Some cheap products in the distant interior of continents, however, also move long distances. Thus, when the steel mills were built in Chicago, they reached out as far as Went Virginia to get suitable supplies of coking coal, in spite of the fact that the distance was more than five hundred miles by land transport and that the coal was of relatively low value. One large plant may be so much more economical than several smaller ones that it can afford to import raw materials and ship finished products great distances, such as specialized logging equipment from Washington to forest areas of the South. In this case, the similarity of the two regions in other respects provides the market and encourages the interaction.