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We believe that this has been the case with evolutionary theory, but that the propensity analysis of fitness which we have described captures the implicit content in biologists’ usage of the term. The propensity interpretation allows us to reconstruct explanations of microevolutionary phenomena in such a way that these explanations appear to be entirely respectable and noncircular. By their form, and by inspection of the premises and conclusion, such explanations appear to satisfy Hempelian adequacy requirements for explanations, and even appear to incorporate recent modifications of the Hempelian model for inductive explanations (Coffa 1974).

Beatty whether it is worth bothering with them. But the fact is that there is a major problem in the foundations of evolutionary theory which remains unsolved, and which continues to give life to the debate. The definition of fitness remains in dispute, and the role of appeals to fitness in biologists’ explanations is a mystery. This is a problem which ought to concern biologists and philosophers of science, quite independent of the vicissitudes of the controversy which it perpetuates. Biologists agree on how to measure fitness, and they routinely appeal to fitness in their explanations, attributing the relative predominance of certain traits to the relative fitness of those traits.

For example, Crow and Kimura (1970) stipulate that ‘‘selection occurs when one genotype leaves a different number of progeny than another’’ (p. 173). Insofar as it is correct to say that the fittest are selected, this definition of ‘‘selection’’ clearly reflects a definition of ‘‘fitness’’ in terms of actual descendant contribution. But surely these definitions (see also Wallace 1963, p. 160; Wilson 1975, p. ’’ For instance, if predatory birds were to kill light- and dark-colored moths indiscriminately, and yet by chance killed more light than dark ones, we would not attribute the differential offspring contribution of light and dark moths to natural selection.