The above consideration indicates that at present many of the experi mental facts on PS in animals can be quantitatively explained within the limits of the "universal" photoreceptor membrane concept. Of course, existence of preferential orientation of the absorbing dipoles in the tubuli of the rhabdomeres can not be totally rejected. We hope that the concept of the "universal" photoreceptor membrane may serve as the useful instrument when dealing with newly discovered properties of visual cells so that true mechanisms of electrical and optical coupling will be searched for instead of assumptions being made on additional properties of the photoreceptor membrane in every new animal under study. 5. Absorption Spectrum of the Universal Photoreceptor Membrane and Spectral Sensitivity of the Photoreceptor 5. 1 Preliminary Notes It seems nearly self-evident that the absorption spectrum of the pho toreceptor membrane coincides exactly with that of the visual pigment it contains. Hence, the membrane must exhibit three bands of absorp tion - the principal band with its peak within the limits of visible spectrum (or a-peak); the secondary band between 340 and 380 nm (S peak); and the third, protein band, in the ultraviolet (UV) at 280 nm (COLLINS et al., 1952). The main peak of absorption is located within the range 433-575 nm for retinol-based pigments and between 438 and 620 nm for 3-dehydroretinol-based pigments, the position of Amax de pending on many ecological factors."

In the comparative physiology of photoreception by the Protista and the invertebrates two aspects are emphasized: (1) the diversity of visual processes in these groups and (2) their bearing upon general mechanisms of photoreception. Invertebrates have evolved a far greater variety of adaptations than vertebrates modifications aiding survival in the remarkably different biotopes they occupy. The number of species in itself suggests this multiformity; each of them has peculiarities of its own, in morphology as well as in physiology and behavior. But these special adaptations are variations on a few great themes. Although the catalogue of invertebrate species is immense, the literature concerning them nearly rivals it in extent-even if one considers only that fraction dealing with visual physiology. Taxonomy proceeds by grouping the species, categorizing them in genera, families, orders, and progressively larger units. Similarly, comparative physiology aims at an analogous, more or less compre hensive, classification. This Part A of Volume VII/6, like Part B that follows it, emphasizes the broad questions that concern groups larger than the individual species; in some cases these questions have general applicability. The middle course between approaches that are too specialized and those that are too general is often elusive, but here we attempt to follow it. The vast number of special adaptations-probably, as we have said, as large as the number of species-is beyond the range even of a handbook."

P. Marler* and H. S. Terrace** *The Rockefeller University Field Research Center Millbrook, NY 12545 **Dept. of Psychology, Columbia University New York, NY 10027, USA For the first half of this century, theories of animal conditioning were regarded as the most promising approach to the study of learning - both animal and human. For a variety of reasons, disillusionment with this point of view has become widespread during recent years. One prominent source of disenchantment with conditioning theory is a large body of ethological observations of both learned and unlearned natural behavior. These challenge the generality of principles of animal learning as derived from the intensive study of a few species in specialized laboratory situations. From another direction, the complexities of human language acquisition, surely the most impressive of learned achievements, have prompted developmental psychologists to doubt the relevance of principles of animal learning. Even within the realm of traditional studies of animal learning, it has become apparent that no single set of currently available principles can cope with the myriad of new empirical findings. These are emerging at an accelerating rate from studies of such phenomena as selective attention and learning, conditioned food aversion, complex problem solving behavior, and the nature of reinforcement. Not very surprisingly, as a reaction against the long-held but essentially unrealized promise of general theories of learning, many psychologists have asked an obvious question: does learning theory have a future? 2 r. Marler and B. S."