Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/296

278 DISTRIBUTION [ANIMAL. but they are so widely distributed as to offer few special features of distribution. One or two families as the Selenariadve are almost exclusively tropical ; others as the Catenicellidce and Vinc-ulariadce are confined to the southern hemisphere. The Diastoporidce are mostly northern, while the Celleporidce are found in both north and south temperate seas. But the great mass of the families are either universally distributed or widely scattered over the globe. They range to high northern latitudes, having been found abundantly by the Swedish expedition on the shores of Nova Zembla in 70 N. lat. They inhabit the profound depths of the ocean, having been dredged from near 2000 fathoms in the North Atlantic, 2500 fathoms in the Pacific, and 2650 fathoms in the Southern Ocean during the voyage of the " Challenger." (Busk s Brit. Mas. Catalogue of Marine Polyzoa ; " Challenger lleports," Proc. Hoy. Society, vol. xxiv. pp. 466, 468, 572, C35.) Echinodermata. The best-known groups the starfishes and sea-urchins occur abundantly as fossils, but their ex- is-tiijg distribution does not offer many features of special interest. The Asteroidea, or star-fishes, are most abundant in the Indian and Pacific oceans, while the Ophiuroidea (brittle stars) are better represented in the European and African seas. A few genera are exclusively American, but on the whole star-fishes are far less abundant in the western than in the eastern hemisphere. Although most abundant in shallow seas, they also inhabit the floors of the deepest oceans, some having been obtained by the " Challenger " expedition from a depth of 2700 fathoms in the Pacific. They abound even in the Arctic seas, " hundreds of seastars " having been obtained by a single haul of the swab at 76 N. lat. in Novaya Zembla by the Swedish ex ploring ship "Proven" in 1875. The Echinoidea (sea- urchins) are also very abundant in Eastern seas, while they Tire comparatively scarce in America, Although much larger and more varied in the tropics, they are tolerably abundant in temperate and cold seas ; and they probably reach the greatest depths in the ocean, since some have "been obtained by the " Challenger " from a depth of nearly 3000 fathoms in the North Pacific. (Nature, vol. xii. p. 556; Van der Hoeven, Handbook of Zoology.} Crustacea. The distribution of the higher Crustacea has been discussed in detail by Mr James Dana in the Zoology of the United States Exploring Expedition ; and, considering that most of the species are shore-dwellers, the facts are very interesting and often quite unexpected. We will, therefore, give an abstract of the conclusions of this writer. The marine regions which best represent the distribution of these animals are three in number, termed by Mr Dana the Occidental, the Africo-European, and the Oriental. The first comprises both coasts of the American continent; the second, the eastern shores of the Atlantic both African and European ; and the third comprehends the vast area from the east coast of Africa to the Central Pacific. Each c:f these is of coarse subdivided into climatal and local provinces, but the primary divisions alone are those which we have now to consider. The facts adduced in support of this scheme of distribution are very interesting. No less than 47 genera are exclusively American, and 15 are common to both the east and west coasts; but as 26 genera are said to be confined to the west coast, and 6 to the cast, it will be seen that these two provinces are really very dis tinct, even if they do not form primary regions. The Africo-European region has 19 peculiar genera, and only 8 in common with the American region ; so that the eastern and western shores of the Atlantic are decidedly more dis tinct than the eastern and western coasts of America. The extensive Oriental region is by far the richest, containing no less than 115 peculiar genera, and only 19 in common with the Africo-European region. About 40 genera are said to be found in all three regions. The distribution of Crustacea in relation to temperature also presents some peculiar features. The species are almost equally divided between the tropical and the extra- tropical regions. The highest form of Crustacea the Brachyura are most abundant in the tropics, while the less developed Amphipoda and Isopoda are more numer ous in temperate and frigid zones. This may, however, in part depend on these groups having been less assiduously collected in the tropics. More interesting, and less open to doubt, is the fact that among the four chief types of Crustacea Brachyura, Macroura, Isopoda, and Amphipoda the most highly developed species are extra-tropical. The largest species of the Macroura are found in temperate seas, and though the largest Brachyura are tropical, yet the Maioids the highest group of Brachyura and of all Crustaceans reach their largest dimensions in the temperate zone. Mr Spence Bate adduces the curious fact that in the cosmopolite sub-family Lysianassina, the largest species aie found in Arctic and Antarctic latitudes, while a species from the Straits of Magellan so closely resembles one from Spitzbergen that they may even be identical ; and in the family Caprettidce the same species often occurs in both the northern and southern hemispheres. Mr Dana lays great stress on similar cases of wide and discontinuous distribu tion, which (he considers) necessitate the adoption of the theory of special creations. Thus, two species (Kranssia rugulosa and Galene natalensis} are found at the Ilawaian islands and Natal, but in no intermediate localities. Other identical species occur in the Japan seas and Natal. The same species (Plagusia tomentosa) occurs in South Africa, New Zealand, and Valparaiso ; and another (Cancer Ediuardsii} at New Zealand and Valparaiso. The same species and several identical genera (Latre dlia, Ephyra > Sicyonia) are found in the Mediterranean Sea and Japan, but in no intermediate districts. Closely allied species (of the genera Amphiroidea and Ozius) are found in Australia and Chili ; but perhaps the most singular fact is the occur rence of closely allied or perhaps identical species of Palcemon in New Zealand and the British Seas, and also of certain British or American genera (as Portunws and Cancel-} in New Zealand. Many of these cases, and more especially the last, undoubtedly offer great difficulties on the theory of transmission and specific modification. There are, however, some considerations which afford hints for a possible solution of the difficulty. We now know many cases iu which the distribution of an animal or a group of animals has been rendered discontinuous by its recent ex tinction in intermediate localities. The tapirs, for example, exist only in tropical America and the Malay islands, and it might well be argued that no passage from one of these localities to the other is conceivable for such an animal. But we now know that the South American tapir lived in North America down to post-Pliocene times, that in Europe there were tapirs in the later Pliocene period, while in the Pliocene or Miocene periods allied species inhabited North India and some parts of China. The present remotely isolated forms are therefore seen to be the remnants of a genus which once ranged over almost the whole northern hemisphere. Perhaps more to the point is the case of the genus Panopcta, adduced by Mr Woodward in his Manual of the Mollusca. There are only 1 1 living species, which occur widely scattered in the northern seas, the Cape of Good Hope, Australia, New Zealand, and Patagonia. But of this same genus nearly 150 fossil species are known, dis tributed over many intermediate localities, so that the existing species are seen to be but relics of an ancient form

of life lingering at various points on the outskirts of tha