A model system in biology refers to an object or process selected for intensive research as an exemplar of a widely observed feature of life (or disease). Model organisms frequently serve as model systems; many, like strains of laboratory mice, have been standardized through breeding and are obtained commercially. An important criterion for selection is accessibility, which may depend on wide availability (the frog), on physical features (size, simplicity, reproduction rate), or simply on an acquired prominence within a field of study (the baboon). Model systems are valued for their particularity, materiality (reality), and complexity. But their key characteristic is surely typicality, as distinguished from universality or generality (in the sense of the general-specific distinction). This suggests that the function of model systems should be related historically to the generic categories of natural history, with a strong emphasis on the function of similarity relations in providing an analytic handle on a genre of similar instances. Finally, model systems exhibit a self-reinforcing quality: the more the model system is studied, and the more perspectives from which it is understood, the more it becomes a model system. Thus historians and philosophers of biology have become interested in the consequences of privileging particular model systems for the shape of knowledge in fields such as genetics (the fly), biochemistry (the Krebs cycle), molecular evolution (the globin genes), and primatology (the baboon). Behind biological principles and experimental approaches often stand specific model systems,