A city, then, is a blending of biological communities and the technological structures and devices placed there by man, and it has at least two functions, It must provide for the maintenance of the biological communities housed there, for the exchange of language-based information, and for the creation of wealth. If the demands of the technological aspects of the city ignore or overshadow the biological components of the city, the biological community suffers and will deteriorate. Conversely, if the biological components deny full expression of the technological development, the ultimate potential of the city is not reached.The city as a biological community must have the attributes of all biological communities, and one of these attributes is minimum maintenance. There is no waste in a well-run ecosystem. So long as there is an uninterrupted flow of energy into a biological community it is a self-maintaining, self-regulating system. An ecosystem is stabilized as it acquires an increasing number of elements and a diversity of structural and functional forms. The greater the diversity, the greater the ability to use efficiently the inflow of energy. As an ecosystem or a biological community de-stabililizes, it becomes simplified, is less able to use the energy entering it, and therefore is less efficient. An unstable ecosystem undergoes relatively rapid change due to inefficient use of the energy flow. If one considers all the elements that support a city as maintenance functions, it follows that cities that are good biological communities will require the least maintenance and unstable communities will require more. There are many indicators to the stability or instability of the city as a biological community.

Some of the more meaningful indicators can be found in the rate of infant mortality, the rate of school dropout, the rate of drug abuse, the rate of communicable disease, the number of crimes against persons and property, the number of people on welfare, the number of people unemployed and underemployed, and the degree of malnutrition. The fact that a city needs urban renewal may indicate that it is a failing community. Urban renewal does not necessarily replace a poor biological community with a good one, but may only create an environment in which all the buildings are new in a neighborhood that is hostile to human life. Not long ago a skid row section of a major Midwest city was replaced by urban renewal. Prior to urban renewal the streets were relatively safe because there were people on them most of the day and night.

Furthermore, it was one of the few places in the city where an individual could be self-supporting on the minimum social security payment. The area is now a jungle of high-rise apartment and office buildings that must be guarded. The apartment buildings are locked and a guard must open the door for residents after 10:00 p.m. In short, a viable biological community was replaced with a technological conglomeration of buildings, some of which have won architectural prizes, that stand in an ecological no-man's land after dark. Cities must be viewed as biological communities if man is to be happy in them. The cities will not be abandoned because they produce great wealth, but they need not be places of great technical achievement at the expense of the humanness of man. First and foremost, the biological needs of man must be provided. Commerce and industry must be placed in a matrix with the human community in a way that will provide a desirable and stable habitat. Properly run, cities should not be difficult to operate and maintain, for as well-balanced ecological communities they should maintain themselves. The economic benefits from cities can be significant by the most rigorous cost-benefit analysis, but if they are exploited for short-term gains they will be burdensome and costly to maintain and will bring into question their value and the desirability of perpetuating them. The city is a crossroad, a conglomeration of human beings, a haven, or a jungle, but most of all it is a place where man the thinker shares thoughts with other thinkers.

Successful cities have retained their generalized biological forms and functions; those that have not have perished. Over-specialization has always been an evolutionary Achilles' heel, and overspecialization has caused the demise of many cities and has prevented many villages and towns from ever becoming cities.

In a simple environment with few complications man discovered that cities were a means of creating and regulating wealth. This concept was exploited so successfully that cities inevitably became the center of man's technological development. The wealth of the city was not derived from the commodities that were brought there, such wealth was in the mines, fields, and rivers of the surrounding hinterland, but the city provided for the system of creating wealth by giving value to things. Things become valuable because of what is known about them, or because of their location, or because a buyer for them is known, or a supplier of them is known. In short, the value and function of the city is to provide the means for men to communicate with each other for their mutual benefit. It was this communication that made it possible for commodities to change hands and for wealth to be accrued, spent, and lost thus increasing the probability of success and conferring value upon the commodity by incorporating it into the system. As soon as cities became financially more prosperous undertakings than hunting, they were built, destroyed, and rebuilt; established, abandoned, razed, and plowed with salt: burned, added to, and subtracted from. In short, the history of man on earth, once the city was invented, consists of experiments with the form, size, structure, function, and purpose of cities. From their inception cities have been located where people wanted to be, where they went for excitement, to gain their fortune, or to have a good time. Cities have functioned in essentially the same manner from the first built to the latest being built today. (Housing developments and suburbia should not be confused with cities. They are places to sequester families while making wealth in the city.)

The congregation of people with diverse interests interacting with each other and the ferment generated by the business of the city, namely, the creation of wealth, made the cities melting pots of human intellect and they have boiled out a stream of technology for which no end is in sight. This technology has steadily increased man's control over his environment and his insights into the operation of the universe, relentlessly created wealth, destroyed and exhausted resources, polluted air and water, and, for better or worse, has continued to increase logarithmically despite its long history.

The successful biological community, whether stationary or mobile, is capable of occupying a site or territory, of maintaining itself on the site or territory, and of reproducing itself there. It is a self-regenerating and self-renewing system capable of sustaining itself and extending its borders. It usually occupies the best sites in an area. The successful biological community is characterized by variety; variety of biotypes, variety of ages, and variety of functions, and it is controlled by dominant elements that provide its main aspect and its ability to interact with the environment, It includes a variety of other elements which survive and thrive in the environment created by the dominant elements. The elements of the successful biological community reach their maximum development and stability through the replacement of unsuccessful or inefficient individuals, generally the aged or those no longer useful, slowly and over a long period of time. The elimination or destruction of dominant elements opens the community to newcomers and the space is usually filled by younger members with the same developmental potential. When changes occur in a successful biological community without seeming to alter its aspect, maturity and subsequent stability have been achieved. As long as the conditions favorable to the mix of elements of a mature community continue to prevail, the community will inexorably reproduce and renew itself.

Cities have come and gone. On some sites there have been as many as seven to nine cities built one atop another. Some cities are brand new and some have existed from ancient days, yet each city with its living survivors is really a thing of the present. The important element is the diversity and stability of the city as it appears to its current inhabitants, for each new generation sees the city anew. How long does it take to assemble the elements of a thriving biological city? There probably is no single answer but many simple-minded indications. Kinship groups within the context of stable family configurations may take three generations to develop, and trees planted along the streets and rights-of-way take from 60 to 100 years to mature. The collective wisdom of the community bent on determining the most stable configuration of neighborhood through the mechanism of the creative activity of the inhabitants of the neighborhood may take 20 or even 50 years, and if the wealth-producing, interconnecting communications network does not form, the city may never become viable.

How long does it take for a newborn member of the community to learn enough about the life of the community and the city to participate in its activities? If all these factors are weighed with the time scale of human interaction on any level (getting to know one's neighbors, the grocer, the filling station attendant, the banker, wholesaler, broker, manufacturer, mayor, and local police officer) against the known rate of migration into and out of neighborhoods, the problem becomes quite complex. The simplest estimates tend to point to a long time, It may take 100 years for a city to mature, growing and developing along with its street plantings. If properly cared for, the city should last millennia—not necessarily in exactly the same form and with each ancient element preserved, but in conjunction with new elements whose functional equivalents determine the biological viability because it is built into the system. A city should extend itself, but it cannot do so if it is infested with even-aged, even-sized structures having a single function and only partial diurnal-nocturnal occupancy and if great distances separate families from children and children from the city. The city has a life of its own, it has the biological energy and vitality of any biological system relentlessly moving toward its most stable configuration.