More important than either in its effect on life is temperature, the factor determining to a great extent both the amount and the distribu tion of marine life. In the middle of the At lantic, near the equator (Sargasso Sea), the water at the surface in summer, will be about 22.30° C., and at 100 fathoms of depth about 16.70° C., below which it diminishes more slowly to about 2.25° C. at 2,000 fathoms, and to below the zero of centigrade (freezing-point of Fahrenheit) in the deeps. The temperature, density, etc., below a few hundred fathoms are probably fairly constant for each measure of descent, so that the water may be regarded as consisting of a series of layers bounded by thermometric limits, which are substantially permanent, and are biological strata as well. At the surface, however, temperature varies with latitude, with seasons, and with circumstances,. especially under. the influence of winds and rents— those rivers that flow 'con the face of the here warm, there cold, each one fos tering and carrying far the kinds of life agree able to it.
A fourth factor conditioning deep-sea life is that of light, which penetrates to a distance only 'recently understood; • but the rays at the blue end of the spectrum go much further than do the red rays—a fact of considerable biologi cal significance. The result of late trials with the best photographic plates show that the blue rays at least penetrate to a depth of about 800 fathoms. Below that is absolute darkness, il lumined only by the phosphorescent glow of the lanterns carried by animals living in those Stygian depths.
Such in broad outline are the con-, ditions of plant and animal life in midocean; and there, is good reason to believe that they have changed little since the beginning of earthly time.
Oceanic Plants the Basis of Subsistence, —Now how do organic beings maintain life in the world of sea water? What do they get •to eat? Well, here, as elsewhere, the larger and more advanced feed on the weaker; and at the base of the list, as on land, are plants. Leaving aside the vexed question of origin, investigation has shown that not only near shore, but all over the ocean, plants exist in myriads. Most of them are lowly algae invisible to the unaided eye, yet of vast variety in form, degree of or., ganization and nutritive value. They find their nourishment dissolved in the sea-water and dis tributed uniformly around them, hence they abound everywhere within the “photic zone,A or surface-layer penetrated by sunlight; and at certain times and places they may be so plenti-, ful as to give the water a tinge of color.
The majority are single-celled, silicious-, framed diatoms. As they are heavier than the water all would sink were it not that some have means of propulsion, 'others parachute-like sus pension-organs. They may have bladder-like attachments, or be flattened into twisted ribbons, or drawn out like a hair, or be furnished with long projections (flagellm). Still more remark able suspension-organs and varieties of form belong to the Pendinee, which are more com plicated, mobile alga often highly phosphoresi., cent. A third series of pelagic algae are the brown flagellates, which possess calcareous frames forming globules called cocoliths and other shapes. Microscopic Thrown algae" occupy the most important place in the economy of the sea, and their shields of lime may be met with in geological deposits as far back as the Cam. brian, practically •identical with those of the present day.
Masses of seaweed are found floating in various parts of the ocean. These have been uprooted from their fastenings on shore, and collect in quiet, eddy-like places. The greatest and most permanent collection is that the Sargasso Sea, in the Atlantic just north of the equator, where the plant Sargassum predomi nates. These continue to live and grow, but do not propagate. Such floating rafts of seaweed are the home and hiding place of a great variety of marine animals.
These algae and seaweeds form the basis of food-supply for the whole of the marine animal-life, since those creatures that feed upon them directly are themselves the prey of all the rest, from shore to shore, from the surface to the depths and from animalcules to whales.
Character of Plankton.,—Associated with the almost invisible plants swarming in the sur face-layers of the ocean, is an assemblage of animals, varying from single-celled globules of protoplasm (Protozoa) to all the higher classes of invertebrates. These together constitute the (plankton,' the swimming or floating sea-life, and its zone is regarded as about 100 fathoms thick. It is far more populous near most shores than in mid-sea, and in warm than in colder waters, and varies in density of life according to season, temperature, and other regional con ditions, yet is everywhere present. Efforts have been made to estimate the amount of plankton, as that might help to determine the amount of fish-food present, but with little success.