A fourth factor conditioning deep-sea life is that of light. The sunlight penetrates to a much greater distance than was formerly believed; and experiments with photographic plates show that the blue rays may sink as far as 800 fathoms, but the red rays go much less down. Below that glimmer is absolute darkness, illuminated only by the phosphorescent glow of the lanterns carried by the animals moving about in that Stygian and icy abode--which would seem to us the most dreadful fate to which any creature on the globe is born. It has been said that the ocean depths seem to be divided into horizontal zones, certain groups of animals being confined, when adults, within limits of depth determined by conditions suitable to them, one zone above the other. Practically, however, these intermediate life-zones can hardly be defined, and vary in different seas, and under changing conditions, as of season, and so forth. Animals taken only by deep hauls of the nets within the tropics, for instance, may be captured in cooler latitudes near the surface; furthermore, the vertical distribution of fishes, as a class, may differ from that of crustaceans as a class. Nevertheless it is true in general that many sorts of pelagic animals dwell at intermediate depths, from which, when they have become mature, they cannot either rise or descend any great distance. Among them are representatives of all the classes of marine life. Let us now consider the creatures of the lowest level--those abysmal depths where eternal cold, stillness, darkness, and equability unite to make an environment so forbidding that human imagination would refuse to people it with living beings; yet where life and strife do actually exist, although by no means uniformly distributed. We know most about it as it exists in the bed of the north Atlantic. The real bottom animals are mainly fixed--sponges, hydroids, sea anemones, bryozoans, brittle-stars, crinoids, brachiopods, holothurians, worms and mollusks. They are nowhere numerous remote from a shore, and below 2,500 fathoms are very scarce, to judge by the results of dredging. Their food comes wholly from the surface, apparently, some catching it as it falls and others sucking it out of the ooze. Moving about among these, and feeding on them, is a scanty population of snails, squids, crabs, and fishes, making their living upon or close to the bottom; and a larger and more varied company of relatives swim in the water above them up to, say, the 2,000-fathoms line. All these are of forms different in many respects from kindred species at or near the surface; and some brought up by the deep-sea dredge can hardly be distinguished from fossils entombed in the oldest fossiliferous rocks--so unchangeable is the environment in which their race has been propagated for perhaps fifty millions of years.