As the community grows there is not merely a multiplication of houses and roads but a process of differentiation and segregation takes place as well. Residences and institutions spread out in centrifugal fashion from the central point of the community, while business concentrates more and more around the spot of highest land values. Each cyclic increase of population is accompanied by greater differentiation in both service and location. There is a struggle among utilities for the vantage-points of position. This makes for increasing value of land and increasing height of buildings at the geographic center of the community. As competition for advantageous sites becomes keener with the growth of population, the first and economically weaker types of utilities are forced out to less accessible and lower-priced areas. By the time the community has reached a population of about ten or twelve thousand, a fairly well-differentiated structure is attained. The central part is a clearly defined business area with the bank, the drugstore, the department store, and the hotel holding the sites of highest land value. Industries and factories usually comprise independent formations within the city, grouping around railroad tracks and routes of water traffic. Residence sections become established, segregated into two or more types, depending upon the economic and racial composition of the population. The structural growth of community takes place in successional sequence not unlike the successional stages in the development of the plant formation. Certain specialized forms of utilities and uses do not appear in the human community until a certain stage of development has been attained, just as the beech or pine forest is preceded by successional dominance of other plant species. And just as in plant communities successions are the products of invasion, so also in the human community the formations, segregations, and associations that appear constitute the outcome of a series of invasions.[50] There are many kinds of intra-community invasions, but in general they may be grouped into two main classes: those resulting in change in use of land, and those which introduce merely change in type of occupant. By the former is meant change from one general use to another, such as of a residential area into a business area or of a business into an industrial district. The latter embraces all changes of type within a particular use area, such as the changes which constantly take place in the racial and economic complexion of residence neighborhoods, or of the type of service utility within a business section. Invasions produce successional stages of different qualitative significance, that is, the economic character of the district may rise or fall as the result of certain types of invasion. This qualitative aspect is reflected in the fluctuations of land or rental values.