From the standpoint of ecology, communities may be divided into four general types: first, the primary service community, such as the agricultural town, the fishing, mining, or lumbering community which serves as the first step in the distributive process of the outgoing basic commodity and as the last stage in the distributive process of the product finished for consumption. The size of such communities depends entirely upon the nature and form of utilization of the extractive industry concerned, together with the extent of the surrounding trade area. The community responds in size to any element that affects the productivity of the economic base or the extent of the area from which it draws its sustenance. But, in any event, so long as such a community does not assume any other function in the larger ecological process, it cannot grow in population beyond a few thousand inhabitants. The next type of community is the one that fulfils the secondary function in the distributive process of commodities. It collects the basic materials from the surrounding primary communities and distributes them in the wider markets of the world. On the other hand, it redistributes the products coming from other parts of the world to the primary service communities for final consumption. This is commonly called the commercial community; it may, however, combine other functions as well. The size of this type of community depends upon the extent of its distributive functions. It may vary from a small wholesale town in the center of an agricultural plain to that of a great port city whose hinterland extends halfway across the continent. Growth depends upon the comparative advantages of the site location. The third type of community is the industrial town. It serves as the locus for the manufacturing of commodities. In addition it may combine the functions of the primary service and the commercial types. It may have its local trade area and it may also be the distributing center for the surrounding hinterland. The type is characterized merely by the relative dominance of industry over the other forms of service. There is practically no limit to the size to which an industrial community may develop. Growth is dependent upon the scope and market organization of the particular industries which happen to be located within its boundaries. Industrial communities are of two general types: first, those that have diversified and multiple industries organized on a local sale of products, and, second, those that are dominated by one or two highly developed industries organized on a national or world-sale of products.