duction of a sound would be better so far as transmission is concerned, for water is an excellent transmitter of sound; but to produce a sound, especially under water, a not inconsiderable amount of power is required—as may easily be demonstrated by trying to clap one's bands under water—and this amount of power is far and away beyond any possessed by the luminous marine creatures. True, some fish emit sounds—e. g., the drum-fish—but those marine creatures which do so are of considerable power and a quite high degree of organization. All living creatures, probably without exception, produce a certain amount of heat through their life-processes; but heat is obviously of no value so far as the purposes of communication are concerned, especially in an immense body of liquid of high specific heat. Variations in pressure more especially vibrations of longer interval than those of sound, may, of course, be produced and transmitted very effectively, but here again much power is required. As a matter of fact, some species of fishes have been found to possess along their lateral line, organs susceptible to vibration-frequencies approaching six per second in the water. Electricity is a possibility, but in all cases of electrical tissues so far studied, considerable masses of muscular tissue have been found as the site of the electrogenic phenomena, again a matter out of the question for a simple organism. With light the problem is different: All that is necessary is the elaboration by the cell through its vital processes of a substance which, when in contact with the oxygen dissolved in the seawater, will produce light. Since certain bacteria can produce such a material from a compound as comparatively simple as asparagin (aminosuccinamidic acid, , this is a matter of comparative ease and requiring nothing more than the metabolic processes which might be ordinarily expected. After synthesis, the substance when brought into contact with the sea-water would be oxidized with the evolution of light. Light knows little of water currents, and but little more of differences in concentration; it would spread in all directions from the point of emission, and to the delicate structures of the ocean fauna and flora, would, however weak it might appear to human eyes, be sensible for considerable distances. Hence light is an ideal method of communication for marine forms of low organization and indeed for many of those of higher organization.
Returning again to a discussion of the various forms: Perhaps the first case, in the upward scale, where we may apply with any degree of certainty one of the uses mentioned before, is that of certain marine worms, the Annelids. Professor W. T. Galloway has recently shown the use of photogenicity in a species of Odontosyllid as a mating adaptation, with evidence which leaves little ground for doubt. In this case propagation of the species appears to be entirely dependent upon a periodic photogenicity limited to certain more or less definite por-
