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The purpose of this chapter is to discuss the effects of a parameter which is present in every learning experiment, but about which we know relatively little--the species being tested. The effects of the species variable have been studied relatively infrequently by animal learning psychologists for at least three reasons. (1) Psychologists have tended to assume that a few general processes will account entirely for most, if not all, of the important phenomena of animal learning in a broad range of species. This assumption has been criticized frequently in recent years (e.g., Seligman, 1970; Seligman & Hager, 1972), but still dominates the field of animal learning. (2) There are serious conceptual problems in attempting to interpret species differences in performance in a learning experiment (Bitterman, 1960, 1965). The basic problem is that if one species performs better than another on a learning task, this does not necessarily represent a species difference in learning ability. It may represent an inadequacy in the design of the experiment for one (or both) species. (3) Psychologists have been without a systematic, biologically sensible method of selecting species for comparative study. As Hodos and Campbell (1969) have pointed out, the psychological view of species has been dominated by the"scala naturae" concept.