Adaptive system — whether on the biological, psychological, or sociocultural level — must manifest (1) some degree of "plasticity" and "irritability" vis-a-vis its environment such that it carries on a constant interchange with acting on and reacting to it; (2) some source or mechanism for variety, to act as a potential pool of adaptive variability to meet the problem of mapping new or more detailed variety and constraints in a changeable environment; (3) a set of selective criteria or mechanisms against which the "variety pool" may be sifted into those variations in the organization or system that more closely map the environment and those that do not; and (4) an arrangement for preserving and/or propagating these "successful" mappings.

p. 491. - Sociology and modern systems theory (1967)