Hierarchical structure and combination into systems of ever higher order is characteristic of reality as a whole and of fundamental importance especially in biology, psychology, and sociology. It is an essential feature of stable complex systems, whether they are inanimate systems, living organisms, social organizations, or patterns of behaviour, and such systems tend to evolve more quickly when hierarchically organized. As a tree structure, hierarchies may serve to represent evolution as a process, and its projection in taxonomic systems; it may equally represent the step-wise differentiation in embryonic development; it may serve as a structural diagram of the parts-within-parts architecture of organisms or galaxies; or as a functional schema for the analysis of instinctive behaviour by ethologists; or of the phrase-generating machinery by the psycholinguist.Hierarchy is also the most conspicuous part of the formal structure of any social organization. (As such, it has frequently been falsely identified with the totality of formal structure, and the adjustment of hierarchic relationships has been falsely identified with the totality of the administrative process). The essence of hierarchy is then the distinction between the role of superior expected to exercise authority over one or more subordinates, who in turn function as superiors with respect to a lower level of subordinates.