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EVOLUTION: A SYSTEM OF THEORIES* RALPH W. LEWISt The many theories in biological evolution are usually presented in a narrative form that fails to give adequate attention to the structure of the theories discussed and to the relations between them. This paper is a preliminary attempt to describe some aspects of theory structure and some kinds of relations between theories found in the study ofevolution. The paper does not attempt to give a complete or well-rounded view of evolution, but I do assume that the patterns discussed and the postulate lists given in the Appendix are an initial step in the presentation of the whole of evolution in a more concise, complete, and logical order. Knowledge in most disciplines is grouped into areas of thought called theories that are built on the pattern of Euclidean geometry. When theories are partially formalized to show some of this pattern explicitly, the intra- and interworkings of theories become more clearly visible, and the total structure of the discipline becomes more evident. When this view is applied to the study of evolution, we find hundreds of theories which have the typical geometric form, albeit the form is often obscured. And we find relations between theories that permit them to be characterized by one or more of these terms: major theory, subtheory , accessory theory, parallel theory, competing theory, and subsumed theory. In this paper only the first three will be discussed. The two major theories in evolution as developed by Darwin are the theory of descent with modification and the theory of natural selection. By the use of the gene theory as an accessory theory, modern biologists have refined and modified natural selection theory to produce what is called the synthetic theory. The postulates of the major theories and several subtheories are given to support the views of the author and to permit the reader to judge *The author thanks Rollin H. Baker, Paul H. Barrett, Alain F. Coreos, Anton E. Lawson, William S. Moore, and a reviewer for their many suggestions and corrections. t Professor, Department of Natural Science, Michigan State University, East Lansing, Michigan 48824.© 1980 by The University of Chicago. 0031-5982/80/2304-0149$01.00 Perspectives in Biology and Medicine · Summer 1980 | 55 1 them. The views of both author and reader are bound to remain tentative until the postulate lists are corroborated or corrected by the best scholars and until the lines of reasoning used by biologists to construct and test the theories have been studied in greater detail. But even in the present tentative state some readers may agree with me that partial formalization of theories can simplify and clarify our knowledge of evolution . The theses in this essay were initiated or supported by the views of many scholars. The words of three of them are a good introduction for some of the arguments that follow. "The progress made in recent decades in the development of unifying concepts has been so great, however, that the presentation of chemistry to students of the present generation can be made in a more simple, straightforward, and logical way than formerly" (Linus Pauling [1, p. viii]). "[Ancient Greece] for the first time created the intellectual miracle of a logical system, . . . , —Euclid's geometry. This marvelous accomplishment of reason gave the human spirit the confidence it needed for its future achievements" (Albert Einstein [2, p. 82]). "Our understanding ofevolution depends on a combination of clearly formulated theories and wide comparative knowledge" (John Maynard Smith [3, p. 241]). The pattern of the Euclidean logical system prevades every theory in every discipline, but only in mathematics and physics is the pattern often made explicit. With some exceptions, the other disciplines expand their theories discursively to such a degree that it is often difficult to identify the basic premises, the postulates, of each theory. And when these are not clearly stated, the reasoning that flows to and from the postulates is cloudy and the subject is needlessly obscure. Only when each discipline makes explicit its Euclidean logical systems, as far as this is practicable, will man be able to teach, learn, use, and enlarge his knowledge with maximum efficiency. Suppes [4] has argued convincingly for...

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