In the world outside the laboratory, life goes on in a chang ing rather than in a constant environment and organisms must continually accommodate to changes in temperature, light, humidity, nutrition, etc. Since studies of the enzymatic process, in vitro, indicate that, in general, biological catalysis can proceed only over limited ranges of temperature, pH, substrate concentration, etc., it seems reasonable to assume that biological systems have an ability to maintain a relatively constant internal milieu in the face of drastic external environmental change. This concept, as applied...