[Definition and use of the terms "ultimate factor", "proximate factor" and "zeitgeber"].

The use of the terms, which have not always been employed uniformly throughout the literature, is discussed and an exact definition is tried. Application of the terms "ultimate" and "proximate factors" should not be restricted to annual and circadian periodicities. Instead they should be used for all temporally or spatially restricted processes for which a difference between selecting and regulating mechanisms is to be found. "Ultimate factors" are environmental factors which in the course of evolution have led, through natural selection, to the relevant restriction. Proximate factors may be defined as those external stimuli which initiate or maintain biological processes under most favourable ecological conditions. The term "Zeitgeber" finally, should only be used where endogenous circadian or circennial periodicities are synchronized with environmental changes through external factors. In this sense "Zeitgeber" is merely a special case of "proximate factor".