We may safely say, in the words of Alexander Agassiz, that the abundance of life in the many favoured localities of the ocean far surpasses that of the richest terrestrial faunal districts. The most thickly populated tropical jungle does not compare in wealth of animal or vegetable life with certain portions of the continental shelf on the western border of the Gulf Stream. In this connection we may also cite Humboldt, who before the marvellous revelations in reference to the life of the sea made by recent dredging expeditions wrote: "Upon surfaces less varied than we find on continents, the sea contains in its bosom an exuberance of life of which no other portion of the globe could give us an idea." The distribution of life in the sea is analogous to the distribution of life on land, but in a reverse direction with reference to sea-level. A traveller passing from the tropical plains of eastern Mexico and ascending Orizaba, for example, crosses successive belts of vegetation, each with its indigenous animals, but merging one with another so as to make a gradation in the luxuriance of the flora and the abundance of animal life from the wonderfully rich plains adjacent to the Gulf coast to the snow-capped mountain top. In the sea, the tropical plains with their tangled vegetation and plentiful animal life are represented by the still more uniform plain forming the submerged continental shelf with its strange forests of flowerless plants, the seaweeds. These submarine jungles shelter hosts of animal species, many of which swarm in countless myriads. This life embraces all grades of invertebrates, such as the microscopic protozoa, sponges, radiate animals like the coral-polyps, starfishes, sea-urchins, etc., and crustaceans in vast variety, and, among vertebrates, includes fishes, reptiles, and mammals. Even birds might be included in this category, since many of them are more at home on the sea than on the land. The struggle for food among this multitude is intense. As with many animals on the land, adaptive coloration is here a means of escape from enemies, and many of the animals assume the brilliant hues of the surrounding vegetation. The water is less transparent than air, and in the deep sea it is always night. Counteracting to some extent this diminution or absence of sunlight, many marine animals are luminous and shine with phosphorescent light of many different tints. This property is shared also by the animals of the sunny, shallow sea as well as by those always living in the cold midnight of the great deep and in the polar oceans.