ually 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. The fore-limbs are reduced to the condition of flattened ovoid paddles, incased in a continuous integument, showing no external sign of division into arm, fore-arm, and hand, or of separate digits, and without any trace of nails. There are no vestiges of hind-limbs visible externally. The general surface of the body is smooth and glistening, and devoid of hair. In nearly all species a compressed median dorsal fin is present. The nostrils open separately or by a single crescentic valvular aperture, not at the extremity of the snout, but near the vertex.
Animals of the order Cetacea abound in all known seas, and some species are inhabitants of the larger rivers of South America and Asia.
Fig. 1.—Common Dolphin.
Their organization necessitates their life being passed entirely in the water, as on g the land they are absolutely helpless; but they have to rise very frequently to the surface for the purpose of respiration. They are all predaceous, subsisting on living animal food of some kind. One genus alone (Orca) eats other warm-blooded animals, as seals and even members of its own order, both large and small. Some feed on fish, others on small floating Crustacea, pteropods, and medusa?, while the staple food of many is constituted of the various species of Cephalopods. With some exceptions the Cetacea generally are timid, inoffensive animals, active in their movements, sociable and gregarious in their habits.
Among the existing members of the order there are two very distinct types the toothed whales, or Odontoceti, and the baleen (whale-bone) whales, or Mystacoceti, which present throughout their organization most markedly distinct structural characters, and have in the existing state of nature no transitional forms.
The problem of the origin of the Cetacea and their relations to other forms of life is at present involved in the greatest obscurity. They present no more signs of affinity with any of the lower classes of vertebrated animals than do many of the members of their own class. Indeed, in all that essentially distinguishes a mammal from one of the oviparous vertebrates, they are as truly mammalian as any, even the highest, members of the class. Any supposed signs of inferiority are simply modifications in adaptation to their peculiar mode of life. In the present state of our knowledge, the Cetacea
