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(PLATE VIII., p. 216)
SECTION I.-INTRODUCTORY REMARKS
MOST of the recent writers on geographical distribution have confined their attention to terrestrial Mammals, or at any rate have but casually alluded to the marine groups of that Class. The seven previous chapters having been devoted to the terrestrial Mammals, it is proposed now to examine the principal facts connected with the distribution over the world's surface of the Marine or aquatic members of the Class.
Aquatic Mammals which pass their lives entirely, or for the greater part, in the water are, of course, subject to very different laws of distribution from the terrestrial forms. As regards aquatic Mammals, land is of course an impassable barrier to their extension, and, subject to restrictions in certain cases, water offers them a free passage. Just the opposite is the case with the terrestrial Mammals, to which in most cases land offers a free passage, while seas and rivers restrain the extension of their ranges.
The groups of aquatic Mammals that are represented on the earth's surface at the present time are three in number, viz.: (1) the Sub-order of the Carnivora, containing the Seals and their allies, generally called the Pinni
peds, which are semi-aquatic; (2) the Sirenia, or Manatees, which are mainly aquatic; and (3) the Cetacea, or Whales, which never leave the water, and are wholly aquatic. We will first consider briefly the principal representatives of these three groups, following nearly the arrangement of them employed in Flower and Lydekker's "Mammals Living and Extinct."
SECTION II.-DISTRIBUTION OF PINNIPEDS
The Pinnipeds, which we will take first, comprise three distinct families-the Otariida, the Trichechida, and the Phocida. Beginning with the Otariida, or Eared Seals, commonly known as Sea-lions and Sea-bears, we find the greater number of the species confined to the South Polar Ocean, where they pass most of their time at sea, but, as is well known, resort to the land at certain seasons for breeding purposes. In the Atlantic Ocean, so far as is known, the Eared Seals have never been ascertained to occur much further north than the estuary of the La Plata on the American coast, where the Patagonian Sea-lion (Fig. 41, p. 199) is met with, and the vicinity of the Cape on the African coast, where Otaria pusilla is found. But in the Pacific, on the contrary, three distinct species of Otaria are distributed all over the northern portion of that ocean. Two species of Sea-lions are also met with in the Galapagos, and they likewise occur on the coasts of Peru and Chili. I think therefore we may assume that Otaria was originally an Antarctic form, but has travelled northwards along the West-American coast and is now firmly established in the North Pacific. In a parallel way in the Class of Birds,
the Albatrosses (Diomedea), which is essentially a group of the Antarctic Seas, are represented by three distinct species in the North Pacific.
The second family of the marine Carnivora, on the other hand, the Walruses (Trichechida), are entirely Arctic in their distribution-one species (Trichechus rosmarus)
(see Fig. 42, p. 200) being peculiar to the North Atlantic, a second nearly allied species (T. obesus) takes its place in the Northern Pacific.
The third family of Pinnipeds is more numerous and varied, both in genera and species, than the two preceding, and has a more extended range. The Seals, Phocida,