[Illustration: +Fig.+ 296.—General relations of ocean basins to the lithosphere. Lat. 20° S. Depth of the water (black) and height of land exaggerated ten times. (Data from Murray, Scot. Geogr. Mag., Vol. XV, 1899.)] The reason for this profound difference is readily found. On the land, the dominant processes which shape the details of the surface are degradational, and though the final result of degradation is flatness (base-level), the immediate result is relief, and, most commonly, relief of the hill-and-valley type. In the sea, the dominant processes are aggradational, and tend to monotonous planeness. =Distribution of marine life.=—Marine life has been of such importance in the history of the earth that the elementary facts concerning its distribution and the principles which control it are here recalled. The distribution of marine life is influenced by many factors, chief among which are _temperature_ and _depth of water_. Not only is life more abundant in the warmer parts of the ocean than in the colder, but the species inhabiting cold waters are different from those in warm, and few species range through great variations. Many forms of life are restricted to shallow water. Many more, especially those which do not live on the bottom, swim about freely without reference to the depth of the water beneath them, while relatively few are restricted to great depths. Many species are also influenced by the _salinity of the water_, which varies notably along coasts where the fresh waters from the land are discharged; by the _character of the sediment_ at the bottom, some species preferring mud, others sand, and others gravel; by the _movement of the waters_, some species preferring still waters and others rough; and some species by the _abundance and nature of the food-supply_, and by _rival and hostile species_. Subject to the exceptions determined by temperature, etc., plant life abounds in shallow water out to depths of 100 fathoms or so, and is found in abundance at the surface where the depth is much greater. Animal life abounds in shallow water, both at the bottom and above it, out to depths of 200 or 300 fathoms, and occurs in great profusion in the surface-waters of temperate and tropical regions without regard to the depth. The great body of the ocean water lying below a depth of some few hundred fathoms is nearly tenantless, though life reappears sparingly at the bottom, even where the depth is great. For further discussion of this topic, see Chapter XI. PROCESSES IN OPERATION IN THE SEA. Within the area of the sea, as on the area of the land, three sets of processes are at work—_diastrophism_, _vulcanism_, and _gradation_.