A body which emits light because of its (high) temperature is said to be incandescent and we speak of temperature radiation. We know, however, of many cases where substances give off light at temperatures much below 525° C. They do not follow the Stefan-Boltzmann law. The light emission is stimulated by some other means than heat. Such bodies we speak of as _luminescent_, and in this category belong all luminous animals. The distinction between light and luminescence was first pointed out by Wiedemann (1888). It is usual to classify luminescences, according to the means of exciting the light, into the following groups: Thermoluminescence Phosphorescence and Fluorescence Photoluminescence Cathodoluminescence Anodoluminescence Radioluminescence Triboluminescence and Piezoluminescence Crystalloluminescence Chemiluminescence The luminescence which appears in a vacuum tube when an electric current is passed through it is sometimes spoken of as _electroluminescence_. As electroluminescence and also thermoluminescence are really special cases of phosphorescence or fluorescence and tribo-and crystalloluminescence are closely allied, the classification has only the merit of emphasizing the means of producing light. Let us examine each kind in turn in order that we may place the light of animals, _organoluminescence_ or _bioluminescence_ (or _biophotogenesis_), in one of these classes. All are examples of "cold light," light produced at temperature far below those observed in incandescent solids. In this category should be placed also the light from salts in the bunsen flame, for flame spectra and line spectra in general, while only obtained at relatively high temperatures, are not to be confused with the purely temperature radiation from the incandescent particles of carbon in a gas or candle light. The sodium or lithium flame, etc., is not a simple function of temperature and has been spoken of as a luminescence, _pyroluminescence_. As the luminescence of organisms could in no manner be regarded as a pyroluminescence, occurring at temperatures far above those compatible with life, a consideration of this form of luminescence will be omitted. Some other low temperature flames are known, such as that of CS_{2} in air, rich in ultra-violet rays, despite its relatively low temperature. While these are of interest to the physicist and chemist, they can have no direct bearing on the luminescence of animals and their consideration will also be omitted. (See Bancroft and Weiser, 1914-1915.)