particular, as Jennings ( 1906/ 1976, 1908) realized, even the simplest of animals must be capable of modulating its behavior (e.g., its path) to adapt to variable and changing environmental circumstances. Thus, the evolutionarily primitive case of animal activity is a simple metazoan moving through its environment and regulating the changes in its activity pattern on the basis of environmental variation, not as a function of internal states. Note that this is not an argument for some highly adapted or efficient state: the ability to regulate one's action patterns in accordance with a complexly variable environment is a minimal condition for safety and life itself.
From an ecological point of view the CNS is not a commander of the body nor a storehouse of ideas; it is something much more amazing: a system that serves to maintain an animal's functional contact with its surroundings. The units of this system-- neuronal firings--are not themselves signals, messengers, or symbols of either ideas or actions, ass previous psychological theories assume. Instead, these units embody a population of variable activity that allows for an animals perceptions and actions to be selected via environmental constraints, and thus come to be adapted to the environment even within the constraints of ontogenetic and behavioral time.
Selectionist accounts of neural processing have been proposed before (and the present account relies heavily on Edelman "Neural Darwinism") but none of these neural selectionist theories has provided an account of the environmental structures involved in the selection process. I have argued here that the relatively persisting affordances of the environment and similarly persisting information for those affordances constitute the environmental basis of this neuronal selection process. Because affordances and the information for them tend to persist across many multiple populations of varying neural firings, it is these external factors that provide the selective shaping of neural functional systems. Furthermore, because the physical dynamics of using affordances are so different from the dynamics of accessing information, perhaps the most fundamental difference among functional systems is the ubiquitous separation of exploratory perceptual systems from performatory action systems.