The Biological Clock describes the rhythmic processes in a great variety of plants and animals. This book is an outgrowth of the 1969 James Arthur Lecture Series on "Time and its Mysteries" held at New York University. This three-chapter work begins with the basic principles of biological rhythms and clocks, along with various diagrams to illustrate some aspects of circadian rhythms in animals. The second chapter discusses the hypothesis of environmental timing of the clock. This chapter explores numerous research studies on phenomenon of biological rhythms, the nature of the rhythmic mechanism, and hormonal regulation. The third chapter examines the cellular-biochemical clock hypothesis and its contribution in the progress of understanding the complexity of biological rhythm. This book is intended primarily for biologists, behaviorists, and researchers.