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The article discusses the degree to which species' distributions are at equilibrium with current climate. Distinguishing the relative roles of present and past climates in determining species' distributions is not only of theoretical interest, but is key to understanding responses of species to climate change. Species are said to be at equilibrium with climate if they occur in all climatically suitable areas while being absent from all unsuitable ones. The observation that species are absent from many suitable areas is trivial. Accurate estimates of species' limits of tolerance to climate can be obtained by means of controlled experiments. The ratio between observed and modeled distributions is interpreted as indicating departures of species distributions from equilibrium.