The power of regenerating organic structures diminishes with increasing complexity of the organism. Even though they are capable of maintaining and repairing themselves, no complex organisms can function indefinitely. They gradually deteriorate in the process of ageing and, eventually, succumb to exhaustion even when relatively undamaged. Instead of replacing the damaged or worn-out parts they replace the whole organism. This, or course, is the phenomenon of reproduction, which is characteristic of all life.
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.