The peculiar colouring 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 colour 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, green, 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. According to Scoresby, they are about one-fourth of an inch asunder, and in this proportion a cubic inch of water must contain 64, a cubic foot 110,592, a cubic fathom 23,887,872, and a cubic mile nearly twenty-four thousand billions! From soundings made in the situation where these animals were found, the sea is probably more than a mile deep; but whether these substances occupy the whole depth is uncertain. Provided, however, the depth to which they extend be about 250 fathoms, the immense number of one species mentioned above may occur in a space of two miles square; and what a stupendous idea must we form of the infinitude of marine life, when we consider that those vast numbers, beyond all human conception, occupy after all only a small part of the green-coloured ocean which extends over twenty or thirty thousand square miles! It is here that the giant whale of the north finds his richest pasture-grounds, which at the same time invite man to follow on his track. A small red crustacean (_Cetochilus australis_) which forms very extensive banks in the Pacific, and in the middle of the Atlantic about 40° S. lat., affords a similar supply of food to the whales frequenting those seas, and exposes them to the same dangers.