The Walrus, the Sea Lions, and the Seals, collectively termed the Pinnipedia,[203] or by some Pinnigrada,[204] constitute the second well-marked group or sub-order of the Carnivora. They are truly inhabitants of the high seas, the land being to them only an occasional resort, when procreation or other causes induce short visits, or temporary residence thereupon. In the previous chapters it has been noted that certain of the so-called Land Carnivora, the White Polar Bear, or the Common Otter (_Lutra_), for example, take freely to the water, and even subsist on finny and other prey derived therefrom, but nevertheless, as a rule, such Carnivora only peradventure are semi-aquatic. The one notable instance to the contrary is the Sea Otter (_Enhydra_), an animal seldom seen on land, though rarely met with far from rocky reefs and islets. Besides mere habit, the Polar Bears and Otters in some points of their organisation--particularly the conformation of the skull of the first, and webbed toes and abundant under-fur in the two last--show a partial gradation and tendency of structure towards their strictly marine brethren, the Seal tribe. The group of the Pinnipedia is one in which considerable interest is centred, and this for several reasons. Their history, as handed down by classical lore, has a shade of the mythical, and well shows how fable has become engrafted on fact. Within the last two centuries their pursuit has been brimful of incident and adventure. As articles of commerce, the oil and the furs of certain kinds of the Seal tribe are of immense importance; whilst the mere hides of all, besides the Walrus tusks, are commodities of great value. Indeed, to the natives of the Arctic regions, Seals are indispensable as a means of every-day existence. But to the naturalist the fact of their being Carnivores peculiarly adapted to an aquatic life, and the study of their habits generally, are subjects of intense interest. Moreover, the gradual, in some instances sudden, diminution of Seal life at the hand of man, points to a possible early period of their extinction, as in the case of the Whales and Manatee tribes, and warns, like the Roman story of the Sybilline books, that if we would read the history of the past, the knowledge must be culled ere the records are swept beyond recall.