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Abstract  Homeostasis is an abiding principle of living systems; it is defined as that "relatively stable state of equilibrium or tendency toward such a state between the different but interdependent elements and subsystems of an organism" (Webster); accordingly, metrical trait values (e.g., blood pressure and plasma glucose concentration) are maintained within limits. Whereas Claude Bernard had only observed constancy of the "milieu interieur" (extracellular fluid), homeostasis actually extends to intracellular and subcellular environments and components. Because of biological individuality, each individual will have a particular location within the larger distribution of quantitative values that describe the parameter in the population; the private homeostatic value may then be seen to be displaced because the individual’s system is undermined (by mutation perhaps) or overwhelmed by experience. A displaced value is likely to be disadaptive because evolution of biological diversity by natural selection was the process by which the optimal range of homeostatic values was obtained.

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