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This book is a translation of the second Gennan edition (1963). In a few places modifications have been made to take into account important works that have since appeared. The aim has not been an all-embracing survey of experimental facts on the subject, but rather to emphasize the physiological kinship of phenomena having a wide variety of names, and to show thereby the great extent to which the physio logical clock influences quite different phenomena in the "behaviour" of uni cellular organisms, of higher plants, of animals, and of human beings. Furthennore, an attempt has been made to make as thorough an analysis of the mechanism of the clock as is possible at the present time. The first draft of the translation was made by Dr. LARS LORCHER (now of the of Radiation and Organisms, Smithsonian Institution, Washington, D. C.). Division The whole script was then thoroughly revised and corrected by Prof. Dr. FRANK SALISBURY (Department of Botany, Colorado State University, Fort Collins, Colorado). Finally, I made amendments and additions to the presentation, as they seemed to me to be necessary, in order to keep abreast of recent developments. Mr. DEREK MCCULLOCH (at present Lecturer in English Language in the Uni versity of Tiibingen) then checked the manuscript. I should like to thank all those involved in the translation for their efforts.