Fresh-water algæ have a like economic value. The green surface on stagnant pools is a vegetable growth whose function is to assimilate the matter which makes the pool offensive. A submerged district soon becomes covered with scum, or minute plants (/Sphæoplea annulina/), which grow with great rapidity, using up the materials of the decaying vegetation, and in great measure counteracting the ill effects, in the atmosphere, of such decay. When the waters subside, the plants shrivel up and appear like thin paper covering the ground. This ephemeral substance soon [pg038] disappears, without giving evidence of its nature in dust or gases, its body seeming to be a machine which transmutes, but does not hold, the substances on which it grows. Algæ, as has been said above, grow in definite zones, and each zone has also a definite animal life which finds there its food. Darwin says: "In all parts of the world a rocky and partially protected shore perhaps supports in a given space a greater number of individual animals than any other station." And speaking of the /Laminariaceæ/, he adds: "I can only compare these great aquatic forests of the southern hemisphere with the terrestrial ones in the intertropical regions. Yet if in any country a forest was destroyed I do not believe nearly so many species of animals would perish as would here from the destruction of the kelp." The same may be said of the Sargasso Sea, where millions of living creatures make their home. In every kind of marine fauna there are species which derive, if not the whole, at least a part of their nourishment from the seaweeds. The vegetation in the narrow boundary of the three zones is palpably inadequate to supply the needs of the animal life which exists in deeper waters. But over the broad area of the ocean there exists a vast number of pelagic, free-floating algæ, which, although microscopical in size, are almost infinite in numbers. In illustration of this it has been estimated that, although they are not especially numerous in the Sargasso Sea, yet if all the seaweed there were gathered into one mass and the free-floating algæ into another, the bulk of the latter would exceed that of the former. The pelagic flora consists of /Diatomaceæ/, /Protococcaceæ/, /Peridinieæ/, and others. Undoubtedly it is on these pastures that fishes feed, as well as other organisms which in turn are food for fishes.