The fourth type of community is one which is lacking in a specific economic base. It draws its economic sustenance from other parts of the world, and may serve no function in the production or distribution of commodities. Such communities are exemplified in our recreational resorts, political and educational centers, communities of defense, penal or charitable colonies. From the standpoint of growth or decline such communities are not subject to the same laws that govern the development of towns that play a part in the larger productive and distributive processes.[39] They are much more subject to the vicissitudes of human fancies and decrees than are the basic types of human communities. Of course, any community may and usually does have accretions added to its population as a result of such service. It may, for instance, be the seat of a university, of a state prison, or it may be a recreational resort for at least certain seasons of the year. III. DETERMINING ECOLOGICAL FACTORS IN THE GROWTH OR DECLINE OF COMMUNITY The human community tends to develop in cyclic fashion. Under a given state of natural resources and in a given condition of the arts the community tends to increase in size and structure until it reaches the point of population adjustment to the economic base. In an agricultural community, under present conditions of production and transportation, the point of maximum population seldom exceeds 5,000.[40] The point of maximum development may be termed the point of culmination or climax, to use the term of the plant ecologist.[41] The community tends to remain in this condition of balance between population and resources until some new element enters to disturb the _status quo_, such as the introduction of a new system of communication, a new type of industry, or a different form of utilization of the existing economic base. Whatever the innovation may be that disturbs the equilibrium of the community, there is a tendency toward a new cycle of adjustment. This may act in either a positive or negative manner. It may serve as a _release_ to the community, making for another cycle of growth and differentiation, or it may have a retractive influence, necessitating emigration and readjustment to a more circumscribed base.