The posterior extremity, in all the Cetacea, is either absolutely deficient, or else rudimentary. If rudimentary, its sole vestige consists of certain small bones, the imperfect representation of a pelvis, suspended, as it were, in the flesh, and unconnected with the spinal column. Here we may observe a remarkable difference between the whale and the seal: in the latter, as we have seen, there is a short tail, and the posterior extremities perform the office of a true caudal fin; in the former this important organ of progression consists, to use Mr. Bell’s words, of “an extremely broad and powerful horizontal disc, varying in figure in the different genera, but in all constituting the principal instrument of locomotion.” In fishes the tail is set vertically, but in whales horizontally; and it has been well said that the admirable adaptation of such a peculiarity in its position to the requirements of the animal forms a fresh and beautiful illustration of the infinite resource and foresight of the Creative Wisdom. Thus: the fishes, respiring only the air contained in the dense liquid medium in which they live, require no access to the atmosphere; and, therefore, their progression is chiefly confined to the same region. But the whales, breathing atmospheric air, must necessarily come to the surface for each respiration; and hence they need a powerful instrument or lever, the position of which shall apply its impulse in a vertical direction, so as to impel their colossal bulk from the lowest depths of ocean to the surface every time the lungs require to receive a fresh supply of atmospheric air. The greatest rapidity of motion is effected by alternate strokes of the tail against the water, upwards and downwards; but the usual progression is accomplished by an oblique lateral and downward impulse, first on one side and then on the other, just as a boat is propelled by a man with a single oar in the art of “sculling.” The extent of the tail in some of the larger species is really immense; the superficies being no less than about a hundred square feet, and its breadth considerably exceeding twenty feet. The common, right, or Greenland whale (_Balæna mysticetus_) has been, for centuries, the object of man’s systematic pursuit, on account of its valuable oil and scarcely less valuable baleen. [Illustration: THE GREENLAND WHALE.]