_Aquatic Mammals._—Many mammals grouped for the present purpose as terrestrial pass a great portion of their lives in brooks, lakes, or rivers, and, being dependent upon such waters for obtaining their subsistence, are necessarily confined to their vicinity; but the truly aquatic mammals, or those living constantly in the water, and unable to move their quarters from place to place by land, are the orders Cetacea and Sirenia, with which may also be grouped the Seals, forming the Pinniped division of the order Carnivora. For the marine Cetacea, animals mostly of large size and endowed with powers of rapid locomotion, there are obviously no barriers to universal distribution over the surface of the earth covered by sea, except such as are interposed by uncongenial temperature or absence of suitable food. Nevertheless it was thought some years ago that the fact of a Whale or a Dolphin occurring in a sea distant from that in which it had usually been found was sufficient justification for considering it as a distinct species and imposing a new name upon it. There are now, however, so many cases known in which Cetaceans from the northern and southern seas, from the Atlantic and Pacific Oceans, present absolutely no distinguishing external or anatomical characters upon which specific determination can be based that the opposite view is gaining ground; and, since some species are undoubtedly very widely distributed, being in fact almost cosmopolitan, there seems little reason why many others should not be included in the same category. The evidence is satisfactory enough in those instances in which the intermediate regions are inhabited by the same forms;—the cases of “continuous areas” of distribution. In those in which the areas of distribution are apparently discontinuous, there may be more room for doubt; but it must not be forgotten that the negative evidence is here of much less value than in the case of land animals, since the existence of Cetaceans in any particular part of the ocean may be easily overlooked. The great Sperm Whale (_Physeter macrocephalus_) is known to be almost cosmopolitan, inhabiting or passing through all the tropical and temperate seas, although not found near either pole. At least three of the well-known species of Rorqual (_Balænoptera_) of the British coasts are represented in the North Pacific, on the South American shores, and near New Zealand, by species so closely allied that it is difficult to point out any valid distinctive characters, though it may perhaps be desirable to wait for a more exhaustive examination of a large series of individuals before absolutely pronouncing them to be specifically identical. There is nothing yet known by which we can separate the “Humpback Whales” (_Megaptera_) of Greenland, the Cape of Good Hope, and Japan. The same may be said of the common Dolphin of the European seas (_Delphinus delphis_) and the so-called _D. bairdi_ of the North Pacific and _D.