Remarkable exemplifications of the law of uniformity of organic structure are furnished by the family of the Cetacea, which includes the whale, the cachalot, the dolphin, and the porpoise, and exhibits the most elementary forms of the type of the Mammalia, of which they represent the early or rudimental stage of development. Here, as before, we have to seek these first elements among the inhabitants of the water: for whenever, in our progress through the animal kingdom, we enter upon a new division, aquatic tribes are always found to compose the lowest links of the ascending chain. Here, also, we observe organic development proceeding with more rapidity, and raising structures of greater dimensions in aquatic than in terrestrial animals. The order Cetacea comprises by far the largest animals which inhabit the globe. Whatever may have been the magnitude of those huge monsters which once moved in the bosom of the primeval ocean, or stalked with gigantic strides across antediluvian plains, and whose scattered remains bear fearful testimony of the convulsions of a former world, certain it is that, at the present day, the whales of the northern seas are the most colossal of the living animal structures existing on the surface of this planet. A cursory survey of the organization of the tribes belonging to this semi-amphibious family will impress us with the resemblance they bear to fishes; for they present the same oval outline of the body, the same compact form of the trunk, which is united with the head without an intervening neck; the same fin-like shape of the external instruments of motion and the same enormous expansion and prolongation of the tail, which is here also, as in fishes, the chief agent in progression. With all this agreement in external characters, their internal economy is conducted upon a totally different plan; for although constantly inhabiting the ocean, their vital organs are so constructed as to admit of their breathing only the air of the atmosphere, and the consequences which flow from this difference are of great importance. The necessity of aerial respiration compels them to rise, at short intervals, to the surface of the water; and this air, with which they fill their lungs in respiration, gives their bodies the buoyant force that is required to facilitate their ascent, and supersedes the necessity of a swimming bladder, an organ which is so useful to the fish. With the intent of diminishing still further their specific gravity, Nature has provided that a large quantity of oily fluid shall be collected under the skin, a provision which answers, also, the purpose of preserving the vital warmth of the body. A great accumulation of this lighter substance is formed on the upper part of the head, apparently with a view to facilitate the elevation to the surface of the blowing hole, or orifice of the nostrils, which is placed there.[11]