The conditions which initiate invasions are legion. The following are some of the more important: (1) changes in forms and routes of transportation;[[51]] (2) obsolescence resulting from physical deterioration or from changes in use or fashion; (3) the erection of important public or private structures, buildings, bridges, institutions, which have either attractive or repellent significance; (4) the introduction of new types of industry, or even a change in the organization of existing industries; (5) changes in the economic base which make for redistribution of income, thus necessitating change of residence; (6) real estate promotion creating sudden demands for special location sites, etc.
Invasions may be classified according to stage of development into (a) initial stage, (b) secondary or developmental stage, (c) climax. The initial stage of an invasion has to do with the point of entry, the resistance or inducement offered the invader by the prior inhabitants of the area, the effect upon land values and rentals. The invasion, of course, may be into an unoccupied territory or into territory with various degrees of occupancy. The resistance to invasion depends upon the type of the invader together with the degree of solidarity of the present occupants. The undesirable invader, whether in population type or in use form, usually makes entry (that is, within an area already completely occupied) at the point of greatest mobility. It is a common observation that foreign races and other undesirable invaders, with few exceptions, take up residence near the business center of the community or at other points of high mobility and low resistance. Once established they gradually push their way out along business or transportation thoroughfares to the periphery of the community.
The commencement of an invasion tends to be reflected in changes in land value. If the invasion is one of change in use the value of the land generally advances and the value of the building declines. This condition furnishes the basis for disorganization. The normal improvements and repairs are, as a rule, omitted, and the owner is placed under the economic urge of renting his property to parasitic and transitory services which may be economically strong but socially disreputable and therefore able and obliged to pay higher rentals than the legitimate utilities can afford. It is a well-known fact that the vices under the surveillance of the police usually segregate in such transitional areas.[[52]]
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.
Each formation or ecological organization within a community serves as a selective or magnetic force attracting to itself appropriate population elements and repelling incongruous units, thus making for biological and cultural subdivisions of a city’s population. Everyone knows how racial and linguistic colonies develop in all of our large cities, but the age and sex segregations which take place are not quite so obvious to common perception. In the city of Seattle, which has in general a sex composition of 113 males to 100 females, the downtown district, comprising an area inscribed by a radius of half a mile or so, has from 300 to 500 males to every 100 females. But in the outlying districts of the city, except in one or two industrial sections, these ratios are reversed. Females predominate in numbers over males in all the residential neighborhoods and in the suburbs of the city. This same condition is true with regard to the age distribution of population. The school census shows an absolute decline in the number of children of school age in the central districts of the city although the total population for this area has shown an increase for each decade. It is obvious, then, that the settler type of population, the married couples with children, withdraw from the center of the city while the more mobile and less responsible adults herd together in the hotel and apartment regions near the heart of the community.
This process of population-sifting produces not only increasing mobility with approach from the periphery to the center of the formation, but also different cultural areas representing different mores, attitudes, and degrees of civic interest. The neighborhoods in which the settler type of population resides, with their preponderance of women and children, serve as the custodians of the stabilizing and repressive mores. It is in the Seattle neighborhoods, especially those on the hill-tops, that the conservative, law-abiding, civic-minded population elements dwell. The downtown section and the valleys, which are usually industrial sites, are populated by a class of people who are not only more mobile but whose mores and attitudes, as tested by voting habits, are more vagrant and radical.
R. D. McKenzie