But it is none the less certain that the presence of this light cannot always be designed to serve this purpose, for many of the animals so endowed are blind. The phosphorescent centipedes are without eyes, like all the other members of the genus (Geophilus) to which they belong, and probably the majority of phosphorescent marine forms are likewise destitute of organs of sight.
Another suggestion is that the light derived from these marine forms, and especially from deep-sea Alcyonarians, is what enables the members of the deep-sea fauna that are possessed of eyes (which are always enormously enlarged) to see. Such is the suggestion of Dr. Carpenter, Sir Wyville Thomson, and Mr. Gwyn Jeffries; and it is possible that this actually is one of the effects of the phosphorescent property. But if so, it remains to inquire how the forms endowed with it came to be possessed of a power useful in that way to other forms, but not to themselves. According to the Darwinian doctrine of development, the powers that are developed in different organisms by the process of natural selection are such as are useful to themselves and not to others, unless incidentally.
This consideration has led to another suggestion, namely, that the property of phosphorescence serves as a protection to the forms possessing it, driving away enemies in one way or another: it may be by warning them of the fact that they are unpalatable food, as is believed to be the case with the colors of certain brilliantly-colored caterpillars; it may be in other ways. In Kirby and Spence one case is recorded in which the phosphorescence of the common phosphorescent centipede (Geophilus electricus) was actually seen apparently to serve as a means of defence against an enemy. "Mr. Shepherd," says that authority, "once noticed a scarabeus running round the last-mentioned insect when shining, as if wishing, but afraid to attack it." In the case of the jelly-fishes, it has been pointed out that their well-known urticating or stinging powers would make them at least unpleasant, if not dangerous, food for fishes; and that consequently the luminosity by which so many of them are characterized at night may serve at once as a warning to predatory fishes and as a protection to themselves. The experience of the unpleasant properties of many phosphorescent animals may likewise have taught fishes to avoid all forms possessing this attribute, even though many of them might be quite harmless.
Lastly, it has been suggested that the phosphorescence in the female glow-worm may be designed to attract the male; and that it will actually have this effect may readily be taken for granted. Observation shows that the male glow-worm is very apt to be attracted by a light. Gilbert White of Selborne mentions that they, attracted by the light of the candles, came into his parlor. Another observer states that by the same light he captured as many as forty male glow-worms in one night.
