“On the coast of Chili,” says the same author, “a few leagues north of Concepcion, the _Beagle_ one day passed through great bands of muddy water; and again a degree south of Valparaiso, the same appearance was still more extensive. Mr. Sullivan, having drawn up some water in a glass, distinguished by the aid of a lens moving points. The water was slightly stained, as if by red dust, and after leaving it for some time quiet a cloud collected at the bottom. With a slightly magnifying lens, small hyaline points could be seen darting about with great rapidity and frequently exploding. Examined with a much higher power, their shape was found to be oval, and contracted by a ring round the middle, from which line curved little setæ proceeded on all sides, and these were the organs of motion. Their minuteness was such that they were individually quite invisible to the naked eye, each covering a space equal only to the one-thousandth of an inch, and their number was infinite, for the smallest drop of water contained very many. In one day we passed through two spaces of water thus stained, one of which alone must have extended over several square miles. The color of the water was like that of a river which has flowed through a red clay district, and a strictly defined line separated the red stream from the blue water.” In the neighborhood of Callao, the Pacific has an olive-green color, owing to a greenish matter which is also found at the bottom of the sea in a depth of 800 feet. In its natural state it has no smell, but when cast on the fire it emits the odor of burned animal substances. Near Cape Palmas, on the coast of Guinea, Captain Tuckey’s ship seemed to sail through milk, a phenomenon which was owing to an immense number of little white animals swimming on the surface and concealing the natural tint of the water. The peculiar coloring of the Red Sea, from which it has derived its name, is owing to the presence of a microscopic alga, _sui generis_, floating at the surface of the sea and even less remarkable for its beautiful red color than for its prodigious fecundity. I could add many more examples, where, either from minute _algæ_, or from small animals, the deep blue sea suddenly appeared in stripes of white, yellow, blue, brown, orange, or red. For fear, however, of tiring the reader’s patience, I shall merely mention the _olive green_ water which covers a considerable part of the Greenland seas. It is found between 74° and 80° N. lat., but its position varies with the currents, often forming isolated stripes, and sometimes spreading over two or three degrees of latitude. Small yellowish Medusæ, of from one-thirtieth to one-twentieth of an inch in diameter, are the principal agents that change the pure ultramarine of the Arctic Ocean into a muddy green.