The model sketched above is still simple because it assumes a single evolving system. In other words, complex evolution is distributed over different systems. External selection can be considered as adaptation to the requirements of the environment. Internal variation can be viewed as a change which only affects the inner structure or state of the system. External variation finally is a change in which the relations between different systems are changed.
Variety is injected from the outside.
Thus formed higher-order systems may again function as building blocks, to be combined by external variation, generating a system of a still higher order. This process can go on indefinitely, leading recursively to ever more complex systems. Binary interaction Let us work out the case of two interacting systems. The message arriving in B can be viewed as external variation. In this respect the two systems will now behave as one higher-order system.