What is it that is conserved by urban organization? This is as fundamental a question as any that can be asked in the development of a science. It refers to a characteristic that can be more lasting than the people themselves, or the structures they have put into place, an organizing principle that can be used as a reference both for empirical studies and for theoretical speculation. What is sought is an abstraction that is particularly rich in suggestions for further study, covers a class of phenomena comprehensively, is economical in representation, and persuades the experienced investigators that it isolates invariants that they can trust.
There are several varieties of organizing principles. One of the most powerful, that of cultural evolution, with increasing controls over self and environment, has already been used to simplify and co-ordinate the discussion of predecessor stages to urbanization. It provides an extremely suggestive approach to history which enables archaeologists and prehistorians to establish remarkably precise chronologies and from these chronologies say other things about changes in the organization of human settlements over space and time. Propositions of varying degrees of generality can be formulated and tested, and so on organized body of knowledge gradually accumulates.
Linguists made a huge step forward when they discovered and adopted phonemic analysis. Language seemed to have almost infinite potentials for variation, especially when dialects were taken into account, but when it was shown that phonemes survived while their phonetic representations wondered over a considerable range of possibilities, rules could be devised for predicting the result of contact between two languages, as well as the consequences of cultural isolation. Even in languages not stabilized by the use of script the invariance of phonemic relationships has permitted extrapolation as far back as the archaeologists can find sherds to classify.
These two examples of central concepts have been with us for less than a century. In each instance several hundreds of lifelong careers have been spent in advancing the science since these and other unifying principles were propounded and adopted. The concept of evolution in biology has existed a little more than a century, but it is built upon tens of thousands such careers, a volume of effort that explains in part the general awareness of the concept as well as the exhaustiveness of the recorded knowledge it loosely coordinates. The older principles of this sort came into being more gradually and with greater confusion of language. Perhaps two examples may be drawn from conservation principles that serve as foundations for contemporary thought.
The concept of wealth goes back to the beginnings of history, but the idea of the interchangeability of the property that made up this wealth evolved more gradually. Wealth could be maintained even though the commodities held were continually changing. Thus in early economic thought the conservation principle was a prescription for the survival of the household (or firm): with each market transaction and each management decision there had to be assurance that the value of the capital would not be unintentionally diminished. Thus an accounting system was required which placed a realistic value upon equipment and inventory so that meaningful profit-and-loss statements