Pavlovian conditioning is found in as wide a range of species as one cares to mention: from neurologically uncomplicated organisms such as planaria (Thompson and McConnell, 1955) and headless insects (Horridge, 1962), to marine mollusks (Carew, Walters, and Kandel, 1981), drosophila (Tompkins, Siegel, Gailey, and Hall, 1983), honeybees (Bitterman, Menzel, Feitz, and Schafer, 1983), nonmammalian vertebrates such as fish (Brandon and Bitterman, 1979; Waxman and McCleave, 1978), amphibia (Goldstein, Spies, and Sepinwall, 1964) and birds (cf. Macphail, 1982: 192–3), as well as in nonhuman mammals (cf. Marler and Terrace, 1984: 419ff) and, of course, humans (cf. Davey, 1987a). Clearly, if such a range of living organisms possesses the ability to learn about the relationship between two environmental stimuli, then it seems reasonable to assume that this ability serves an important biological function. If this is so, then either this ability appeared very early in the evolution of living organisms and serves a generalized biological purpose across the whole gamut of existing species, or it evolved independently in different species or in different biological functions as a means of coping with the different kinds of evolutionary pressures which afflict different species in their own ecological niche. To express some extremes of these possibilities, we might suggest that (1) Pavlovian conditioning evolved because it allows animals to reduce the unpredictability of their environment, or allows them to optimize interaction with a forthcoming biologically important event regardless of what that biologically important event might be (Hollis, 1982); or (2) different species have evolved the capacity of Pavlovian learning to cope with very specific adaptive problems.