As the _Rattlesnake_ sailed through the tropical seas Huxley came in contact with the very peculiar and interesting inhabitants of the surface of the sea, known now to naturalists as pelagic life or "plankton." Although a poet has spoken of the "unvintageable sea," all parts of the ocean surface teem with life. Sometimes, as in high latitudes, the cold is so great that only the simplest microscopic forms are able to maintain existence. In the tropics, animals and plants are abundant, and sometimes by their numbers colour great areas of water; or, as in the drift of the Gulf Stream, make a tangle of animal and plant life through which a boat travels only with difficulty. The basis of the food supply of this vast and hungry floating life is, as on land, vegetable life; for plants are the only creatures capable of building up food from the gases of the air and the simple chemical salts found dissolved in water. Occasionally, in shallow or warm seas, marine floating plants, large and visible like the sea-weeds of the coast, form the floating masses known as Sargasso seas; more often the plants are minute, microscopic specks visible only when a drop of water is placed under the microscope, but occurring in incredible numbers, and, like the green vegetation of the earth, forming the ultimate food-supply of all the living things around them. Innumerable animals, great and small, live on the plants or upon their fellows, and, however far he may be from land, the naturalist has always abundant material got by his daily use of the tow-net. This drifting population floats at the mercy of the waves. Most of the animals are delicate, transparent creatures, their transparency helping to protect them from the attacks of hungry fellows. Nerves, muscles, skin, and the organs generally are clear, pale, and hardly visible. Such structures as the liver, the reproductive organs, and the stomach, which cannot easily become transparent, are grouped together into small knots, coloured brown like little masses of sea-weed. Other floating creatures are vividly coloured, but the hues are bright blues and greens closely similar to the sparkling tints of sea-water in sunlight. The different members of this marine flotsam frequently rise and fall periodically: some of them sinking by day to escape the light, others rising only by day; others, again, appearing on the surface in spring, keeping deep down in winter. Perhaps the majority of them are phosphorescent, sometimes shining by their own light, sometimes borrowing a glory from innumerable phosphorescent bacteria with which they are infested. Nearly every class of the animal kingdom contributes members to this strange population. The young forms of many fish, as for instance of conger, flying gurnards, and some flatfish, are pelagic and have colourless blood, and pale, transparent, gelatinous or cartilaginous skeletons. The tadpole-like stages of the sea-squirts, which in adult