Few natural groups present so many remarkable illustrations of several of the most important general laws which appear to have determined the structure of animal bodies as that of the whales.

The term "whale" is commonly but vaguely applied to all the larger and middle-sized Cetacea, and, though such smaller species as the dolphins and porpoises are not usually spoken of as whales, they may to all intents and purposes of zoölogical science be included in the term. Taken all together the Cetacea constitute a distinct and natural order of mammals, characterized by their aquatic mode of life and external fish-like form. The body is fusiform, passing anteriorly into the head without any distinct constriction or neck, and posteriorly tapering off gradually toward the extremity of the tail, which is provided with a pair of lateral pointed expansions of skin supported by dense fibrous tissue, called "flukes," forming together a horizontally placed, triangular propelling organ.