   For every such definition instantiated by a population, there is the synthetic truth that the members of the population, or all of its subpopulations, instantiate the definition, and the further broader truth that many populations of otherwise diverse species instantiate a small sub-set of the models, and the yet broader truth that all populations instantiate one of this set of really quite similar models. Presumably, all these statements of varying degrees of generality need explanation. Consequently, those who resist the “semantic interpretation” of theories argue that we might as well treat each as a substitution instance of the principle of natural selection generated by a different probabilistic propensity measure of fitness as a distinct empirical generalization to begin with instead of a model-theoretic definition. But, one will want to ask, does this set of generalizations with similar antecedents and identical consequents have something in common, which in turn explains and unifies them, or are they the fundamental principles of the theory of natural selection? The question is obviously rhetorical. For each of the members of the set of functions that turn the definition-schema for fitness into a complete property is a measurement of the fitness of some actual or potential individuals or populations. Each is a measurement appropriate to its particular environmental circumstances, a measurement inappropriate for the same or different individuals at different locations or times in the same environment.