Thus the severity of the Polar winter remains unfelt at a greater depth of the sea, where myriads of creatures find a secure retreat against the frost, and whence they emerge during the long summer’s day, either to line the shores or to ascend the broad rivers of the Arctic world. Between the parallels of 74° and 80° Scoresby observed that the color of the Greenland sea varies from the purest ultramarine to olive green, and from crystalline transparency to striking opacity--appearances which are not transitory, but permanent. This green semi-opaque water, whose position varies with the currents, often forming isolated stripes, and sometimes spreading over two or three degrees of latitude, mainly owes its singular aspect to small medusæ and nudibranchiate molluscs. It is calculated to form one-fourth part of the surface of the sea between the above-mentioned parallels, so that many thousands of square miles are absolutely teeming with life. On the coast of Greenland, where the waters are so exceedingly clear that the bottom and every object upon it are plainly visible even at a depth of eighty fathoms, the ground is seen covered with gigantic tangles, which, together with the animal world circulating among their fronds, remind the spectator of the coral-reefs of the tropical ocean. Nullipores, mussels, alcyonians, sertularians, ascidians, and a variety of other sessile animals, incrust every stone or fill every hollow or crevice of the rocky ground. A dead seal or fish thrown into the sea is soon converted into a skeleton by the myriads of small crustaceans which infest these northern waters, and, like the ants in the equatorial forests, perform the part of scavengers of the deep. Thus we find an exuberance of life, in its smaller and smallest forms, peopling the Arctic waters, and affording nourishment to a variety of strange and bulky creatures--cetaceans, walruses, and seals--which annually attract thousands of adventurous seamen to the icy ocean. Of these sea-mammalians, the most important to civilized man is undoubtedly the Greenland whale (_Balæna mysticetus_), or smooth-back, thus called from its having no dorsal fin. Formerly these whales were harpooned in considerable numbers in the Icelandic waters, or in the fiords of Spitzbergen and Danish Greenland; then Davis’s Straits became the favorite fishing-grounds; and more recently the inlets and various channels to the east of Baffin’s Bay have been invaded; while, on the opposite side of America, several hundreds of whalers penetrate every year through Bering’s Straits into the icy sea beyond, where previously they lived and multiplied, unmolested except by the Esquimaux. [Illustration: 34. THE WHALE.]