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The concept of an evolutionary scale has been an inherent part of the rationale of comparative psychologists in their attempts to understand the relation between behavioral and evolutionary processes. Although the concept has been criticized as inappropriate to the study of phyletic origins of behavior, it is the thesis of this article that such a concept is quite appropriate to the study of biological improvement. This article deals with the concept of biological improvement, or anagenesis, its definition, its application to the construction of grades (units of anagenesis), and its application to the comparative study of behavior.The suggestion that animals could be ranked on a single graded scale (scala naturae) was introduced by Aristotle (1910, 1912a, 1912b). He attempted to classify animals by various criteria, including "possession of blood," breeding habits, and number of legs. The scale consisted of eleven grades, with man at the top and zoophytes at the bottom. Hodos and Campbell (1969) noted that it is incorrect to suggest that a scale like the scala naturae depicts the phylogenetic origins of animals on the scale. A scale that results in the order bird-monkey-man, regardless of the criteria used in its construction, cannot be used as a model of the origin of man for the simple reason that birds were not ancestral to man. Hodos and Campbell suggested that there are two approaches to the study of behavioral evolution. One approach stresses the search for the origins of a particular behavior and is characteristic of the study of behavioral homologies. The second approach studies the interaction between animals and their environment in order to understand principles of adaptation and survival. As we shall see, for neither of these approaches is the construction of evolutionary

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Pigeons were exposed to a multiple schedule which provided equally frequent reinforcement in the presence of two stimuli but which produced markedly different rates of key-pecking. Generalization gradients were displaced away from the stimulus associated with the lower rate of key-pecking. Another group of pigeons had similar training, except that a low rate of key-pecking was established in a stimulus with a much higher frequency of food reinforcement. In this case, the generalization gradients were not affected by the training on the schedule producing a low response rate.