Stanley Salthe:
EVOLVING HIERARCHICAL SYSTEMS (Columbia University Press, 1985)

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A novel take (from a philosophical perspective) at traditional issues of evolutionary theory. By combining the metaphysics of Justus Buchler and Michael Conrad's "statistical state model" of the evolutionary process, Salthe develops an ontology of the world, a formal theory of hierarchies and a model of the evolution of the world.
The world is viewed as a determinate machine of unlimited complexity. Within complexity discontinuities arise. The basic structure of this world must allow for complexity that is spontaneously stable and that can be broken down in things separated by boundaries. A possible solution is a hierarchical structure, which is also implied by Buchler's principle of ordinality: nature (i.e., our representation of the world) is a hierarchy of entities existing at different levels of organization. Hierarchical structure is a consequence of complexity. Entities are defined by four criteria: boundaries, scale, integration, continuity: an entity has size, is limited by boundaries, and consists of an integrated system which varies continously in time.
Salthe develops a formal theory of hierarchical structures. Entities at different levels interact through mutual constraints, each constraint carrying information for the level it operates upon. A process can be described by a triad of contigous levels: the one it occurs at, its context (Bunge's environment) and its causes (Bunge's structure). In general, a lower level provides initiating conditions for a process and an upper level provides boundary conditions. Representing a dynamical system hierarchically requires a triadic structure.
Aggregation occurs consequent upon differentiation. Differentiation interpolates levels between the original two and the new entities aggregate in such a way that affects the structure of the upper levels: every time a new level emerges, the entire hierarchy must reorganize itself.
Salthe also recalls Pattee's view of complexity as the result of interactions between physical and symbolic systems, where a physical system is dependent on the rates at which processes occur and a symbolic system is not. Symbolic systems frequently serve as constraints within which physical systems operate, and frequently appear as products of the activity of physical systems (e.g., the genome in a cell). A physical system is complex when a part of it functions as a symbolic system (as a representation, and therefore as an observer) for another part of it.
These abstract principles are then applied to organic evolution.
Over time nature generates entities of gradually more limited scope and more precise form and behavior. This process populates the hierarchy of intermediate levels of organization as the hierarchy spontaneously reorganizes itself. This model applies to all open systems, whether organisms or ecosystems or planets.

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