Replication and extension of Skinner's "superstition" experiment showed the development of two kinds of behavior at asymptote: interim activities (related to adjunctive behavior) occurred just after food delivery; the terminal response (a discriminated operant) occurred toward the end of the interval and continued until food delivery. These data suggest a view of operant conditioning (the terminal response) in terms of two sets of principles: principles of behavioral variation that describe the origins of behavior "appropriate" to a situation, in advance of reinforcement; and principles of reinforcement that describe the selective elimination of behavior so produced. This approach was supported by (a) an account of the parallels between the Law of Effect and evolution by means of natural selection, (fc) its ability to shed light on persistent problems in learning (e.g., continuity vs. noncontinuity, variability associated with extinction, the relationship between classical and instrumental conditioning, the controversy between behaviorist and cognitive approaches to learning), and (c) its ability to deal with a number of recent anomalies in the learning literature ("instinctive drift," auto-shaping, and auto-maintenance). The interim activities were interpreted in terms of interactions among motivational systems, and this view was supported by a review of the literature on adjunctive behavior and by comparison with similar phenomena in ethology (displacement, redirection, and "vacuum" activities). The proposed theoretical scheme represents a shift away from hypothetical "laws of learning" toward an interpretation of behavioral change in terms of interaction and competition among tendencies to action according to principles evolved in phylogeny.The field of learning has undergone in-At times, one senses a widespread feeling of discreasing fractionation in recent years. In-couragement about the prospects of ever getting , !.•«••,. i »> j * clear on the fundamentals of conditioning. Attempts terest in miniature systems and exact to arrive at firm dedsions abou f alternat f ve theories of local effects has grown _ to the formulations rarely produce incisive results. Every detriment of any attempt at overall integra-finding seems capable of many explanations. Issues tion. Consequently, as one perceptive ob-become old, shopworn, and disappear without a server has noted:proper burial [Jenkins ' 197 °-™ 107 -108 1

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