Northward along the continental shelf, with decrease in the strength of the sunlight, the plants and animals lose much of the brilliancy which characterizes many of the denizens of southern waters, and in general assume more subdued colours in harmony with the prevailing gray of their surroundings. The great diversity of animal life to be found in shallow tropical seas decreases as one traces the continental shelf northward, but even in the Arctic Ocean adjacent to the land invertebrate life literally swarms, although the number of species, genera, etc., is comparatively limited. Seaweeds are not absent from the Arctic Ocean, although its shores, owing to the destructive action of ice, usually seem exceedingly barren, and the lower or smaller forms of algæ float in the waters in abundance. Food sufficient for an extensive fauna is thus supplied, and where food is plentiful animals are present also, no matter what the mean annual temperature may be.
The life of the Arctic Ocean has been but inadequately studied, but enough is known concerning it to show that a promising field there awaits the naturalist. On the continental shelf off Point Barrow, the most northern portion of the arctic shore of Alaska (latitude 71° 23'), 180 species of marine invertebrates have been collected. Of these, the molluscs numbered 61 species; the crustaceans, 44 species; the worms, 20 species; and the echinoderms (sea-urchins, starfish, etc.), 17 species.
Nearly all of the Arctic Ocean adjacent to the coast of America is as yet unexplored, and we have therefore no direct testimony as to its flora and fauna. We may reasonably assume, however, that the life is there practically the
same as in the waters of similar depth to the north of Eurasia. Nordenskiöld, in his narrative of the voyage of the Vega, speaks of decapods, worms, mussels, crustacea, and asteroids which crawled in myriads over the beds of clay and sand at the bottom of the Kara Sea. A detailed account is given of one unusually successful haul of the trawl when it brought up large asteroids, sponges, crinoids, holothuria, a gigantic spider, masses of worms, crustacea, etc. This was the most abundant yield of the trawl-net at any one time during the voyage of the Vega on the north coast of Asia, and that, too, from the sea off the northern extremity of the continent. The temperature of the water at the surface was from zero to -1.4° C. (32° to 29.48° F.), and at the bottom from -1.4° to 1.6° C. (29.48° to 34.88° F.). In this connection the same distinguished naturalist remarks: "It is singular that a temperature under the freezing-point of pure water should be advantageous for the development of an animal life so extremely rich as that which is found here, and that this animal life should not suffer any harm from the complete darkness which during the greater portion of the year prevails at the bottom of the ice-covered sea."
To persons who have never visited the far north the statements that travellers in those regions give in reference to the abundance of life in the sea seems scarcely to be credited. The assertion, however, that comparing equal areas in the most populous tropical sea and in portions of the Arctic Ocean, the amount of life, or the tons of living animal matter per square mile in the two regions, would be in favour of the northern station is probably true. In addition to the direct evidence indicated above as to the prolific invertebrate life of the cold waters of the north, we have still more impressive testimony from the vast numbers of birds and large-sized mammals which subsist on this abundance, or feed on fishes, which in turn obtain their subsistence from the invertebrate realm.
Every rocky island and headland at the north is a breeding-place for sea-birds. They are to be numbered by millions, yet their daily food is gathered from the surface of the
sea. The seals live in large numbers about all the arctic shore, and the walruses, each individual weighing about a ton, occur in herds; while whales and narwhals inhabit the same waters. The presence of such numbers of large mammals is proof that the life on which they subsist is abundant.
THE SUBMARINE TOPOGRAPHY OF THE CARIBBEAN REGION
The studies which have given to the world so much information concerning the continental shelf surrounding the main body of North America have been continued, or, perhaps more properly, were initiated, in the West Indian waters. For this important work we are indebted mainly to the United States Coast and Geodetic Survey and the United States Fish Commission. The work of officers of the United States Navy in charge of Coast Survey and Fish Commission vessels, in making accurate measurements of depths, temperatures, strength and direction of currents, character of bottom, etc., has, in some instances, been commemorated on maps of the sea-floor by such names as Brownson Deep, Sigsbee Deep, Bartlett Deep, etc. The routine survey work referred to has been supplemented and extended by the labours of Louis Agassiz, L. F. Pourtales, Alexander Agassiz, and others in studying the life in the sea, the origin and history of the material forming the sea-bottom, and the nature of the shelves, banks, deeps, etc., which give diversity to its topography.
The continental shelf bordering Florida on the east is separated from a similar submarine embankment surrounding the Bahama Islands by a channel 56 miles wide and from 200 to 500 fathoms deep (Fig. 3). This channel when followed northward becomes shallower and broader, and opposite the Carolina coast is no longer discernible in the relief of the broad continental shelf. The Gulf Stream flows northward through this Florida channel, as it is termed, with a current of from 2 to 6 miles per hour. These conditions are such as to suggest that the channel referred to has in part been excavated by the Gulf Stream.