forms. The first of these is that it is a secondary sexual character; the second, it is protective in purpose; third, it is a lure for prey. Other less general or less obvious uses may be shown, but in the great majority of cases one or the other of these explanations of the use of the light-producing function will be found adequate, and sometimes more than one will apply to a given species.
The simplest luminous forms are the bacteria. It is certainly difficult to see that photogenicity can be of any especial service to these unicellular organisms. Most, if not all, of the photogenic bacteria are cf marine origin, and it is possible that some of them may be pathogenic to certain marine creatures when ingested, and the light thus serves to warn the bacterivorous plankton, etc., that these bacteria are dangerous food. Some species have been found to be pathogenic to Talitrus, a crustacean, which may serve as a case in point.
Luminous fungi are quite common, and here again we must leave the question with at most only a poor attempt at explanation. Protective or warning it may be; but the author has never heard of luminosity in the most poisonous of all fungi, the Amanitas, and certainly the luminous fungi he has seen did not appear to form any exception to the usual fate of fungi in general of being abundantly attacked by various species of insects.
In both of these cases, the photogenicity may be like the fluorescence of extracts of the common firefly, Photinus pyralis, and of cultures of Bacillus fluorescens liquefaciens—merely a property of some chemical substance elaborated by the life processes of the organism, and having no bearing upon its economy.
In simple marine forms like Noctiluca miliaris, the Pyrocystæ, Pyrosoma, etc., it is possible, though unlikely, that the luminosity has a warning significance and it is obviously not a sexual character. These creatures exist in swarms, or in the form of communities—compound, or rather composite animals—and it would appear very probable that in them the possession of the power to produce light finds its usefulness in the fact that by its means they are enabled to communicate in such ways as their low state of organization may require.
It is perhaps a digression, but a few words may not be amiss as to why the emission of light would be more useful to marine creatures than some other modes of communication in use among land forms. The sea-water is full of currents, ever changing and wavering; it varies in density slightly in different portions, owing to slight variation in concentration and in temperature. Therefore, the emission of a substance to produce olfactory or gustatory sensations would be of little use as a means of communication, especially to a creature capable of motion, as many luminous forms are; they would have far more control over their own movements than over that of any emission. The pro-