Fluctuations play a central role in the dynamics of self-maintenance. Any living system can be described in terms of interdependent variables, each of which can vary over a wide range between an upper and a lower limit. All variables oscillate between these limits, so that the system is in a state of continual fluctuation, even when there is no disturbance. Such a state is known as homeostasis. When there is some disturbance, the organism tends to return to its original state, and it does so by adapting in various ways to environmental changes.
Feedback mechanisms come into play and tend to reduce any deviation from the balanced state.
Because of these regulatory mechanisms, also known as negative feedback, the body temperature, blood pressure, and many other important conditions of higher organisms remain relatively constant even when the environment changes considerably. However, negative feedback is only one aspect of self-organisation through fluctuations. The other aspect is positive feedback, which consists in amplifying certain deviations rather than damping them. We shall see that this phenomenon plays a crucial role in the processes of development, learning, and evolution. The ability to adapt to a changing environment is an essential characteristic of living organisms and of social systems.