mankind sets up everywhere his nocturnal flames and electric lamps, the previously unimportant useless survival of an overpowering impulse to rush to nocturnal lights, reveals itself as a serious and death-dealing "disharmony." We must suppose, on this theory, that the other insects, such as moths and certain flies (by no means all insects), which also madly fly into nocturnal lights to their own destruction, have had luminous ancestors and a similar early history. This is a legitimate supposition, since there are several very distinct kinds of insects known at the present day which are luminous at night, although no existing moths or butterflies are known to be so. A fact bearing on the explanation of the insects' perilous rush to flame is that birds when migrating are attracted by the great brilliant lamps of lighthouses, and, flying towards them, strike against their glass coverings, and are killed in considerable numbers. In that case, it may be that the flying towards the sun has become instinctive, and that the bright light of the lighthouse acts upon a certain number of birds (perhaps the less well-adjusted individuals) so as to call forth the same response in the direction of flight as that exercised by the sun's globe. The truth or error of this suggestion should be tested by an examination of the species of birds which kill themselves against lighthouse lanterns, and a knowledge of the season and direction of their migration. As to luminous or phosphorescent (often called "luminescent") insects and other animals, there are a great many curious and interesting facts known. There are luminescent bacteria (common on old meat bones and dead fish and in the sea generally), animalcules of various species, jelly-fish, star-fish, worms, shell-fish, and crustaceans and true fishes. Inhabitants of the great depths of the ocean of all kinds are usually luminescent. The light is caused by the oxidation of a peculiar fatty substance. Without free oxygen there is no luminescence, and yet no heat is produced but merely light, as when a stick of damp phosphorus glows. The luminescence of living things (often, but undesirably, called phosphorescence) is a process differing greatly from that called "phosphorescence" in minerals and crystals, such as the emission of light by a lump of white loaf-sugar when crushed. You may see that kind of phosphorescence by standing in front of a looking-glass in a dark room and crushing a lump of loaf-sugar with the teeth, keeping the lips raised. It seems that in many organisms luminescence occurs without any consequent use or service to the organism. But in higher forms the power of emitting light has been seized upon by natural selection having become of value in attracting the individuals of a species to one another, or in attracting prey, or again in scaring enemies. The luminescent matter is concentrated in certain definite organs, and the access to it of oxygen and even its