The Unity of the Senses

This chapter discusses sensory processes, which include the evolution of a visual system out of an undifferentiated sensitivity took one of its most important early steps with the development of special light-sensitive pigments.

Abstract

This chapter discusses sensory processes. In these many forms, the unity of the senses reflects fundamental facts of phylogenetic and ontogenetic development. It is commonly believed that all of the senses trace their evolutionary history back to a single primitive sense, a simple undifferentiated responsiveness to external stimulation. It is not difficult to imagine some early form of life, a relatively simple agglutination of cells that wriggled or withdrew when bombarded with virtually any sharp stimulus whether mechanical, radiant, or chemical. The evolution of a visual system out of an undifferentiated sensitivity took one of its most important early steps with the development of special light-sensitive pigments. It has been hypothesized that the senses of hearing and vision, both evolved from an earlier touch sense. That there is currently a close kinship between hearing and the modern touch sense is clear. Both modalities are excited by mechanical energy, that is, by changes in patterns of pressure at the receptors, and both show phenomenological as well as psychophysical similarities.