Opening Address: Biological Clocks

Abstract

Biologists these days are deeply concerned with phenomena of time measurement in plants and animals. Such phenomena were first recognized many years ago, and experimental studies in this field began earlier than most people realize. A brief survey of these early investigations will provide us with a synopsis of the problems with which we must deal in this Symposium. In this discussion we shall be dealing primarily with the timing mechanism by which organisms mark the passage of the hours of the day. The occurrence of such time measurements in plants and animals has long been known. Originally it was widely believed that such processes operated according to the hourglass principle; that is, it was thought that a stimulus such as sunrise or the ingestion of a meal initiated a biochemical process at the completion of which a signal was sent to certain organs. Analogies were also drawn with processes...