« AnteriorContinuar »
to the Narwhal in many points of its general structure (Fig. 50). These may be looked upon as quite isolated
Fig. 50.—THE BELUGA.
forms characteristic of the Arctic portion of the Atlantic and Pacific.
SECTION V.—DIVISION OF THE MARINE AREA OF THE
GLOBE INTO SEA-REGIONS
From what has been already said, it will be evident that although many of the marine mammals have a wide distribution, others are very definitely localized; and a study of the latter will enable us to divide the oceanic portion of the globe into six Sea-regions, corresponding to a certain extent with the six Land-regions already discussed. It is proposed to name these Sea-regions, which are shown in the map (Plate VIII., p. 216), as follows:
(1) The North Atlantic Sea-region, or Arctatlantis (äpktos and 'Atlavtis = the daughter of Atlas), consisting of the northern portion of the Atlantic down to about 10° N. lat.
(2) The Mid-Atlantic Sea-region, or Mesatlantis (uecos and 'Atlavtis), consisting of the middle portion of the Atlantic down to about the Tropic of Capricorn.
(3) The Indian Sea-region, or Indopelagia ("Ivdos and tréiayos), containing the Indian Ocean down to about the same degree of S. lat., and extending from the coast of Africa on the west to Australia and the great Oriental islands on the east.
(4) The North Pacific Sea-region, or Arctirenia (άρκτος and ειρήνη = pax), containing the northern portion of the Pacific Ocean down to about the Tropic of Cancer.
(5) The Mid-Pacific Sea-region, or Mesirenia (uéoos and eipnun), containing the inter-tropical portion of the Pacific Ocean; and finally,
(6) The Southern Sea-region, or Notopelagia (vótos and térayos), containing the whole of the South Polar Ocean all round the globe south of the above-mentioned limits.
We will now proceed to consider shortly the characteristic mammals of these six Sea-regions.
SECTION VI.—THE NORTH ATLANTIC SEA-REGION,
Amongst the Pinnipeds two well-marked generic forms, the Grey Seal (Halichorus) and the Bladder-Seal (Cystophora), are exclusively confined to Arctatlantis. The True Seals (Phoca) and the Walrus (Trichechus) are found in this region and in Arctirenia; and of the former genus three species (P. vitulina, P. grænlandica, and P. barbata) are actually common to both these Sea-regions, while the Walruses (Trichechus rosmarus and T. obesus) of the two Sea-regions are perhaps somewhat doubtfully distinguishable. It may be easily understood how this has come to pass, because the Seals and Walrus in the course of time, during unusually mild summers, may have extended themselves along the north coast of the American continent into the Northern Pacific. But Arctirenia, as we shall presently show, is markedly distinguishable from Arctatlantis by the presence of Eared Seals (Otaria), which are utterly unknown in the whole of the Atlantic area. Otaria is in fact, as regards Arctatlantis, a lipomorph.
The Sirenians are entirely absent from the North Atlantic and constitute another lipomorph of that area.
Coming to the Whales, we find the Mystacoceti well · represented in the North Atlantic by Balæna, Megaptera,
and Balenoptera; but of these the two latter are almost universally distributed over the ocean, and Balæna recurs again in the North Pacific as well as in more southern latitudes, so that there is no genus of Whalebone Whales peculiar to Arctatlantis.
Proceeding to the Odontoceti, the case is different. Amongst the Physeteridæ, Hyperoodon is confined to Arctatlantis. Arctatlantis therefore may be said to be well characterized by the possession of at least three genera of marine mammals not found elsewhere, viz., Halichorus, Cystophora, and Hyperoodon, and by the complete absence of the Eared Seals (Otariidx).
SECTION VII.—THE MID-ATLANTIC SEA-REGION,
Mesatlantis has certainly not so many forms of marine mammals confined to its area as Arctatlantis, but there seem to be good grounds for its separation. As we descend
towards the tropics the true Seals (Phocinæ), which are constituted to live in colder water, gradually fall off in number, and in Mesatlantis are no longer met with. But in their place we find the genus Monachus, or Monk-Seal, restricted to Mesatlantis, one species (M. albiventer) occurring in the Mediterranean and on the North African coast, and a second (M. tropicalis) being found in the West Indies. Mesatlantis is likewise the true home of the wellmarked Sirenian genus Manatus, one species of which (M. americanus) frequents the coast of America and another (M. senegalensis) that of Africa.
As regards the Cetaceans, we are not at present able to say that Mesatlantis, although well furnished with many generic types of this Order, has any one peculiar to it. We must therefore rest content with assigning two genera of marine mammals, Monachus and Manatus, as characteristic forms or topomorphs of the Sea-mammal-life of Mesatlantis.
SECTION VIII.—THE INDIAN SEA-REGION,
OR INDOPELAGIA The marine Carnivora, so far as we know, are entirely foreign to Indopelagia, but the Sirenians are well represented by the Dugong (Halicore), which pervades all its northern coasts from North Australia to India and the Red Sea and down the African coast to the confines of British East Africa. Whether the species of Halicore found at different points within this area are the same or different is still a matter of discussion, but there can be no doubt that Halicore is an exclusive inhabitant of
Indopelagia. As regards the Whales of Indopelagia, we know that Physeter, Cogia, and Ziphius, and numerous forms of Delphinidæ occur there, but are not aware of any Cetacean that is entirely restricted to this Sea-region.
SECTION IX.—THE NORTH PACIFIC SEA-REGION,
As was pointed out when speaking of Arctatlantis, Arctirenia has one genus of Phocidæ (Phoca) in common with the North Atlantic, and three of the species of this genus appear to be actually identical in these two Searegions, whilst a fourth Phoca (P. fasciata) is only found in the North Pacific. The Walrus (Trichechus) is again a form of marine mammals common to both the great northern Sea-regions. But the feature of Pinnipedian life that absolutely distinguishes Arctirenia from Arctatlantis is the presence in the former of three (if not four) wellmarked species of the Eared Seals (Otariidæ), which are absolutely unknown in the vast extent of the Atlantic down at least to 30° S. lat.
Arctirenia has unfortunately lost its Sirenian, Steller's Sea-cow (Rhytina stelleri), the largest modern representative of this formerly prevalent group, which since the days of the Pleistocene has greatly diminished in numbers, but I think we may still treat Rhytina as one of the characteristic forms of the Arctirenian Sea-region. The North Pacific is also, even at the present day, the sole possessor of a remarkable genus of Whalebone Whales which combines the long head and elongate form of Balaonptera with the absence of the dorsal fin of Balæna.