Perception as substitute trial and error.

The Darwinian concept of random variation and natural selection through differential survival provides a general model for the deterministic description of adaptive phenomena. Recently this model has seen increased application to the adaptive phenomena described by psychologists as problem solving or learning. It is the purpose of this paper to suggest the application of the model to a third level, that is to say, to the adaptive behavior demonstrated in carrying out or executing a well-learned habit. Such an application seems to necessitate the conceptualization of sensory and perceptual processes implied in the title. The early comparative biologists and evolutionists assembled impressive evidence of the adaptive fit between organismic structure and environmental possibilities. To explain such fit, three principal alternatives were available. The first involved the detailed a priori planning of a prescient deity. The second involved appropriate or corrective structural modifications based on experience with the environment in question. But this model also involves prescience, however modest and distributed, in that the organism somehow foresees which modifications will fit better. Where the Lamarckian notion of inheritable habituation could be applied, some plausibility might be gained. But most instances of adaptive fit could not thus be explained. The third model was the Darwinian theory of natural selection. For this model, unlike the second, the modifications or variations are blind, are random, are individually nonappropriate, are not of the order of corrections. But by chance there do occur those which provide better fit, and these survive and are duplicated. While Darwinian theory of evolution has undergone considerable elaboration and modification, and while there has been disagreement as to the mechanism and the magnitude of the variations involved, his basic model of natural selection is uniformly accepted today, and stands as one of the great conceptual achievements of the 19th century. In its abstract or formal aspects, it is a model which may be applied to other adaptive processes, or other apparently teleological series of events in which modifications seem guided by outcome. 1 Recently this model has been applied to embryological morphogenesis and differentiation. While this application is yet incomplete, it is worthy of psychology's attention, inasmuch as the "organizing fields" of the experimental embryologists have provided an influential model for field theory in psychology. The problem has been to explain how genetic control is exercised to account for growth and differentiation into adult form. A tempting early model involved the notion that the genes of the germ plasm, containing the total blueprint for the adult organism, became distributed in the process of mitosis, so that each somatic cell contained only those genes appropriate to its body part. This model required that the genes collectively contain an almost incredible detail, with every phase of growth predestined at a cell-by-cell level. The model has been invalidated by the experimental facts of wound healing, regeneration, and, most dramatically, by the transplantation experiments done on amphibian embryos. These show that, up to a certain stage, transplanted cells will develop in conformity with the locality in which they are placed, rather than in terms of the locality of origin. From such experiments have come concepts of organizing fields with excitatory and inhibitory gradients. Such concepts provide useful summaries of the facts, and may be stated in mathematical form, but are un-