Light. The most recent investigations have shown that light does not penetrate deeper than five or six hundred fathoms, and even there is very dim. Experiments with the photometer have shown that light penetration is greater in tropical than in northern waters. A photographic plate exposed at a depth of 900 fathoms for two hours, was in no way affected. Below the levels reached by sunlight, the sea is in total darkness, and plant forms, with the exception of diatoms, are unknown. There is, however, some phosphorescent light, as the deepsea dredge has brought up many fishes and invertebrates with phosphorescent organs. Such light must, however, be very faint and unevenly distributed.

Notwithstanding the absence of light, a considerable number of deep-sea forms are brilliantly colored, brilliant reds being often found. Most of the deep-sea fishes are, however, black or grayish. Starfishes and crustaceans are frequently red or pink, while holothurians are often violet.

There is a great variation in the size of the eye among deep-sea fishes, which is doubtless dependent to some extent upon the penetration of sunlight or the existence of phosphorescence, their eyes ranging from very minute to abnormally large. A number of the deep-sea species are totally blind.

Temperature. The influence of the warmth to be found in the surface waters does not extend below a few hundred feet, even at the equator where such warmth is greatest. Below 900 fathoms the temperature is always within three or four degrees of the freezing point of fresh water. The low temperature at the bottom is caused by the settling of cold surface water in polar regions. The cold water thus slowly distributed over the floor of the ocean carries with it the oxygen necessary for the maintenance of life in the depths. The bottom of the Black Sea, receiving no such supply of polar water, is devoid of animal life. All deepsea life would doubtless perish were it not for the air thus brought down.

Salinity. Studies of the salinity of the ocean, based on samples of water secured by special forms of apparatus from all depths, have shown various degrees of salinity in different seas. Sea saltness in the Atlantic is higher than in other great open oceans, and highest of all in the Red Sea and in the Mediterranean. It is remarkably low in the Black Sea and in the Baltic, where there are many rivers but no great evaporation.

Pressure. Sea pressure amounts to about a ton to the square inch with each 1,000 fathoms of depth. At 1,000 fathoms it amounts to 180 atmospheres, while at the greatest depth known (over 5,000 fathoms), the pressure would be about six tons to the square inch. It is evident that animal forms of the surface regions could not endure such conditions, but the tissues of deep-sea animals are so permeated by fluids, that a balance is maintained. Most of the deep-sea forms are so soft that when brought to the surface they require various laboratory hardening processes for their preservation as specimens. Both fishes and invertebrates, when brought up from the greater depths, are always dead, and probably die before being dragged far from the bottom. It is interesting to note, however, that the Albatross

has taken alive fishes from 590 fathoms, and large Lithodes crabs from 735 fathoms.

Size of Deep-Sea Forms.- The dredges hitherto used in deep-sea exploration have secured no fishes or invertebrates of large size. The largest fishes taken seldom exceed four or five feet in length. It is not unlikely that by using larger dredges, with wider-meshed nets less liable to become overloaded with mud, larger animals.could be captured.

Bibliography (British and American only). -Agassiz, A., Three Cruises of the Blake' (Boston 1888); 'Bulletins and Memoirs of Museum of Comparative Zoology) (Cambridge, Mass.); Challenger Expedition) (50 vols., ed. by Sir John Murray, Edinburgh); Fowler, 'Science of the Sea' (1912); Hickson, 'Fauna of the Deep Sea (London 1894); Murray, Sir John, and Hjort, Dr. Johan, The Depths of the Ocean (London 1912); Reports and Bulletins (United States Bureau of Fisheries); Tanner, 'Deep Sea Exploration' (Washington 1897); Townsend, 'Records and Bibliography of the Albatross (Washington 1901); Thomson, 'Voyage of the Challenger) (1878).

CHARLES HASKINS TOWNSEND, Director New York Aquarium; formerly in charge of the Deep-Sea Investigations on the U. S. S. "Albatross."

DEEP-SEA LIFE. In this account of midoceanic, or "pelagic," life the writer purposes to restrict himself to the open ocean, that is the spaces of deep water covering the real oceanic basins. The great land-masses are bordered by a submerged rim of varying width, where the water does not exceed an average depth of 100 fathoms, beyond which the bottom falls or slopes to the plain of the original ocean-basin. The sum of these continental borders and similar shallows elsewhere, as in the archipelagic region of the western Pacific, forms only about 15 per cent of the 140,000,000 square miles of salt water, leaving more than four-fifths of it deeper than 100 fathoms, while nearly 10,000,000 square miles exceeds 3,000 fathoms in depth.

We speak of "oceans," but in reality it is all one body of water, which by a constant interchange through currents both horizontal and vertical, maintains a virtual uniformity, yet not a complete one, since distinct variations are observable in temperature, density (salinity), pressure and other physical characteristics that influence animal being. Therefore some parts of the ocean are more populous than others; some are more populous in summer and less so in winter, and there is a definite (although as yet undetermined) geographical distribution of marine life, both horizontal and vertical.

Ocean Floors.-The character of the bottom must be considered. The rivers discharge into the ocean daily an enormous amount of land-material, which is sorted out by weight, readiness to dissolve, etc., and by the action of waves and currents, and is spread out on the bottom to a greater or less distance according to circumstances, but nowhere goes far from the coast. This outer border of deposits from rivers shows a bottom of bluish mud, or, in places, of coral sand or of volcanic dust. It is rich in food-material, and supports in the water above it a far larger assemblage of plants and animals than does the sea outside of