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Acclimatization refers to any of the numerous gradual, long-term responses of an organism to changes in its environment. Such responses are more or less habitual and reversible should environmental conditions to an earlier state. The numerous sudden changes that rapid and short-term responses via the nervous and hormonal systems are not examples of acclimatization. An individual organism can regulate its internal processes rapidly to sustain itself within the usual range of environmental changes that it hourly or daily. But this rapid regulation, or homeostasis, is limited in its operation to a small range of environmental variations. Homeostatic regulation usually cannot meet effectively large environmental changes such as those that would allow a plant or animal living in the warmth of summer to in the cold of winter. As summer wanes, organisms change their substance and their habits in seeming anticipation of the coming winter. This gradual adjustment to conditions is acclimatization.