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The Sirenians are absent from Notopelagia, but Cetaceans of every kind are abundant. Besides one or more representatives of the true Whalebone Whale (Balmna), Notopelagia has a smaller representative of the group (Neobalmna) entirely restricted to its area. It has also representatives of Megaptera and Balmnoptera, though it is doubtful how far they are even specifically distinct from some of their northern representatives.
Among the Toothed Whales (Odontoceti) we find a large Ziphioid form, Berardius, restricted to the Pacific area, while Ziphius and Mesoplodon also occur there. The Dolphins (Delphinidm) are likewise numerous, and present some distinct species, but not, so far as our present knowledge extends, any generic forms that do not occur elsewhere.
But Notopelagia is sufficiently distinguished from all the five more northern Sea-regions by possessing four genera of Seals and two of Cetaceans entirely restricted to its area.
It has therefore been shown that, for the geography of marine mammals, the ocean may be conveniently divided into six Sea-regions (Plate VIII., p. 216), which are as follows:—
I. Regio Arctatlantica, characterized by its Seals (Phocinm), of which two genera, Halichcerus and Cystophora, are peculiar, whilst Phoca is common to it and Arctirenia; by the absence of Sirenians; and by the possession of a peculiar genus of Cetaceans (Hyperoodon).
II. Regio Mesatlantica, sole possessor of the MonkSeals, Monachus, amongst the Pinnipeds, and of the Sirenian genus Manatus.
III. Regio Indopelagica, characterized by the presence of the Sirenian Halicore and by the absence of Pinnipeds.
IV. Regio Arctirenica, with Phoca like the Regio Arctatlantica, but having Otaria also; the home of the (now extinct) Sirenian Rhytina and of the endemic Cetacean Rhachianectes.
V. Regio Mesirenica, without true Seals (Phocinm), but having Otaria and Macrorhinus from the south; no Sirenian being known there.
VI. Regio Notopelagica, characterized by four endemic genera of Phocidm, and by the presence of many Otarim; without Sirenians, but with two endemic forms of Cetaceans (NeobaUena and Berardius).
In conclusion, attention may be called to some of the more remarkable points in the general distribution of the marine mammals and to their apparent significance.
In the first place it is evident that the Pacific has much more in common with the Notopelagian Region than the Atlantic. Otaria and Macrorhinus, quite unknown in the Atlantic, extend themselves to the northern extremity of the Pacific, the former pervading that ocean up to Behring's Strait, and the latter reaching to the Californian coast. It follows that in former ages there must have been some barrier in the Atlantic which did not exist in the Pacific to stop their progress northwards. The only barrier one can imagine that would have effected this must have been a land uniting South America and Africa, across which they could not travel. Adopting this hypothesis, we have at the same time an explanation of the presence of the Manatee on both the American
and African coasts. The Manatee could hardly live to cross the Atlantic. It is only found close to the coast, in estuaries and rivers, where it browses on sea-weeds and other vegetable food in shallow water. How did it travel from America to Africa (or vice versd), unless there were a continuous shore-line between them? The same may be said of the Monk-Seal (Monachv^i), of which one species lives in the Mediterranean and on the African coast and islands, and another in the West Indies. We can hardly believe that these creatures could easily traverse the whole Atlantic. The hypothesis of a former barrier of land between Africa and America, which we know is supported by other facts of distribution,1 would alone explain the difficulty.
On the other hand, in the Pacific we find no such break between the north and south. The aquatic mammals of Notopelagia have evidently had free access to the whole of the Pacific for a long period, and have well availed themselves of this facility.
Again, while the great Southern Ocean exhibits a considerable uniformity of marine mammalian life, we see the Northern waters divided into two distinctly recognizable Regions by the interposed masses of land. All these facts, with the one exception of the supposed Atlantic barrier, would tend in favour of the now generally accepted doctrine that the principal masses of land and water are not of modern origin, but have existed mainly in their present shapes throughout all ages.
1 Cf. Wallace, Geogr. Distrib., vol. i., p. 156.