On sundials, springs, and atoms

The orderly behavior that occurs when animals are required to deal with time requirements suggests the possibility that they have an internal clock that provides information about the duration of events. After discussing questions inherent in the concept of an internal clock and suggesting criteria that such a device should meet if it exists, data are reviewed involving several different types of experimental procedures. Every procedure produced different conclusions about the nature of timing. Such results, together with observations of behavior outside of the laboratory, suggest that an internal clock has not evolved and, furthermore, is not even necessary for animals to display temporal regularities in their behavior or to respond to temporal demands.