[Illustration: FIG. 5.—Diagrammatic section of the eye of _Serolis bromleyana_, a deep-sea species (400–1,975 fathoms), showing the degenerate character of the eye. The corneal facets C, and the crystalline cones V, are the only structures that can be recognised. (After Beddard.)] In every new region they came to, the forms with larger and better eyes would be at an advantage in the fainter light, and would be more likely to survive and transmit their favourable variation in this respect to their offspring than their less fortunate neighbours. Thus down to the depth of the limit of sunlight we should expect to find, as we do find in fishes, large-eyed species. Beyond the limit of direct sunlight the eyes would be of very little use to them, the pigment would disappear and the tissues become degenerate. This is precisely what has occurred in the genus _Serolis_. The disappearance of the sense of sight in the animals of the deep sea is sometimes accompanied by an enormous development of tactile organs. Thus, among fishes we find _Bathypterois_, a form that possesses extremely small eyes, provided with enormously long pectoral fin rays that most probably possess the functions of organs of touch. Among the Crustacea we find the blind form, _Galathodes Antonii_, with an extraordinary development in length of the antennæ, and _Nematocarcinus_, with enormously long antennæ and legs. The subject of the power of emitting phosphorescent light possessed by some deep-sea animals is much more difficult to deal with. The presence of distinct organs in many of the deep-sea fish that can only be reasonably interpreted as phosphorescent organs, the presence of well-developed and evidently functional eyes in many deep-sea animals, and many other considerations render it very highly probable that some, if not many, forms emit a phosphorescent light. The power and constancy of the light emitted, however, must for the present remain a matter of conjecture. We cannot judge at all of the amount of light given out by an animal in deep water by its appearance when thrown out of a dredge upon the deck. Whether the phosphorescent light given out by an Alcyonarian or a Crustacean is more or less at a temperature of 40° Fahr. and a pressure of one ton per square inch than it is at 60° Fahr. and the ordinary barometric pressure of the sea-level, is a question that has not yet been brought to an experimental test. Whatever the answer to this question may be, the fact remains that a greater percentage of animals from the deep sea exhibit some sort of phosphorescent light when brought on deck than animals that live in shallow water. The curious organs possessed by some fishes that are supposed to be organs for the emission of phosphorescent light have recently been subjected to a minute examination by von Lendenfeld.