of the adaptive plan will be formalized by designating a set of attainable structures. The fact that different in general elicit different performances from a given structure means formally that there will be a different performance measure µEassociated with each E. Each field of study is typified as much by its performance measures as by its structures and operators. For the fields mentioned in connection with examples of structures and operators, we have a corresponding list of performance measures:
Table 2: Typical Performance Measures
Field
Performance Measure
Genetics
Fitness
Economic planning
Utility
Control
Error functions
Physiological psychology
Performance rate (in some contexts, but often unspecified)
Game theory
Payoff
Artificial intelligence
Comparative efficiency (if specified at all)
The successive structural modifications dictated by a plan t amount to a sequence or trajectory through the set . For the plan to be adaptive the trajectory through must depend upon which environment is present. Symbolizing the set of operators by W, this last can be stated another way by saying that the order of application of operators from W must depend upon E.
It is clear that the organization of , the effects of the operators W upon structures in , and the form of the performance measure µEall affect the difficulty of adaptation. Among the specific obstacles confronting an adaptive plan are the following:
1. is large so that there are many alternatives to be tested.
2. The structures in are complicated so that it is difficult to determine which substructures or components (if any) are responsible for good performance.
3. The performance measure µEis a complicated function with many interdependent parameters (e.g., it has many dimensions and is nonlinear, exhibiting local optima, discontinuities, etc.).
of attainable structures. The fact that different 
in general elicit different performances from a given structure 
means formally that there will be a different performance measure µEassociated with each E. Each field of study is typified as much by its performance measures as by its structures and operators. For the fields mentioned in connection with examples of structures and operators, we have a corresponding list of performance measures: