During the course of development of an invasion into a new area, either of use or type, there takes place a process of displacement and selection determined by the character of the invader and of the area invaded. The early stages are usually marked by keenness of competition which frequently manifests itself in outward clashes. Business failures are common in such areas and the rules of competition are violated. As the process continues, competition forces associational groupings. Utilities making similar or complementary demands of the area tend to group in close proximity to one another, giving rise to subformations with definite service functions. Such associations as amusement areas, retail districts, market sections, financial sections, and automobile rows are examples of this tendency. The climax stage is reached in the invasion process, once the dominant type of ecological organization emerges which is able to withstand the intrusions of other forms of invasion. For example, in the development of a residential district, when it is not controlled in advance by building restrictions, the early stages of growth are usually marked by wide variations in the type and value of buildings constructed. But, in the process of development, a uniform cost type of structure tends to dominate, gradually eliminating all other types that vary widely from the norm, so that it is customary to find a considerable degree of economic homogeneity in all established residential districts. The same process operates in areas devoted to business uses, competition segregates utilities of similar economic strength into areas of corresponding land values, and at the same time forces into close proximity those particular forms of service which profit from mutual association such as financial establishments or automobile display-rooms. Once a dominant use becomes established within an area, competition becomes less ruthless among the associational units, rules of control emerge, and invasion of a different use is for a time obstructed. The general effect of the continuous processes of invasions and accommodations is to give to the developed community well-defined areas, each having its own peculiar selective and cultural characteristics. Such units of communal life may be termed “natural areas,”[53] or formations, to use the term of the plant ecologist. In any case, these areas of selection and function may comprise many subformations or associations which become part of the organic structure of the district or of the community as a whole. It has been suggested that these natural areas or formations may be defined in terms of land values,[54] the point of highest land value representing the center or head of the formation (not necessarily the geographic center but the economic or cultural center), while the points of lowest land value represent the periphery of the formation or boundary line between two adjacent formations.