Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/297

279 ANIMAL.] DISTRIBUTION 279 vast area it once occupied. Such cases as these occur in all classes of animals where our knowledge of the extinct fauna is sufficiehtly extensive, and we are therefore justified in believing that a large proportion of the existing instances of anomalous and discontinuous distribution are to be explained in a similar way. In the case of the Crustacea we must also take into account our comparative ignorance of many parts of the globe, and especially our ignorance of the powers of dispersal of the ova, and of the young animals during their earlier larval condition. This dis persal may systematically occur to a far greater extent than we are yet aware of ; though only in rare and exceptional instances may a species succeed in maintaining itself beyond the normal limits of its race. The certainty we are now acquiring of the long duration and wide-spread influence of the glacial period must also materially affect such questions as these ; for although the equatorial low- lauds may never have suffered from its influence, it is highly probable that during the period of greatest cold the temperature of the entire ocean may have been lowered, while in certain directions cold currents may have afforded a passage for temperate forms of marine animals from the northern to the southern hemisphere. While admitting, therefore, that the distribution of Crustacea presents to us some problems of extreme difficulty, we must deny that they are such as to justify us in resorting to a solution such as &quot; special creation,&quot; which is negatived by the evidence afforded by almost every other class of animals. The reports of the &quot; Challenger &quot; expedition already published afford valuable information on the distribution of Crustacea iu the oceanic depths. The higher forms (Decapoda) have been found living at a depth of 1875 fathoms in the North Pacific, 2600 fathoms under the equator, and 2385 fathoms in the South Pacific. In the North Atlantic, at a depth of 1900 fathoms, was found a cray-fish allied to tha Astacidw, but deprived of even the rudiments of eyes, while others equally blind (from both the Atlantic and Pacific) are believed to have their nearest allies among the extinct Eryonidae of the Jurassic period. The higher Crustacea, which are most abundant at great depths, and which have afforded the greatest variety of new and interesting forms, belong to the Schizopoda. They have been found at depths of more than 2000 fathoms in the Pacific, and down to 2550 fathoms in the Atlantic Ocean. Some of them are blind, but a more remarkable fact is, that many of them are brightly coloured, though living in absolute and perpetual darkness. Among the Edriopthalmata (sessile-eyed Crustacea) examples of the remarkable blind family Munopsidue have been found at a depth of 2175 fathoms in the Atlantic, and at nearly 2000 fathoms in the Southern Ocean. Other forms usually found in shallow water (Scrolls) also occurred at great depths more than 2000 fathoms in the Pacific ; and one of these, obtained near the southern ice-barrier at a depth uf over 1900 fathoms, was of &quot;a fine blue colour with a red spot over the middle of the body.&quot; Entomostraca also occur at great depths, the most remarkable being a gigantic Ostr;icod found at 1600 fathoms in the Southern Ocean. At 1375 fathoms, near the Crozets, a Pycnogonoid (sea-spider) was obtained, measuring 2 feet across the legs. But besides these bottom-dwellers, the trawl nets at different depths showed that the ocean is inhabited by peculiar tribes of free-swimmers principally Copepoda, Amphipoda, and Cypridinas, often of a bright orange colour. These occurred in all parts of the Pacific to a depth of 2000 fathoms, but they were never found in the surface nets. (United States Exploring Expedition, vols xiii. and xiv. ; Spence Bate on &quot; Geographical Distribution,&quot; in Spsnce Bate and West wood s British Sessile- Eyed Crustacea; Dr lludolf von Willemb es-Suhin s &quot; Pieport on the Crustacea of the Challenger, &quot; Proc. of the Royal Society, vol. xxiv. p. 585.) Cirrkipedia. The barnacles are a tolerably extensive group of anomalous Crustacea, whoso distribution differs somewhat from that of the more typical portion of the class. The genera are almost all widely or universally distributed, from 74 18 N. lat. to Cape Horn, and some of the species have an equally wide range. No genus having more than a single species is confined to the torrid zones, and only two genera are limited to the southern hemisphere. Although the temperate zones have a smaller area than the torrid, they possess rather more species of Cirrhipeds, which Mr Darwin imputes to the fact of these zones being two, w-hile the torrid is but one. As in some groups of the higher Crustacea, large species are most abundant in the temperate zones. Owing to the wide range of the genera the Cirrhipedal regions can only be determined by the distribution of species. These, according to Mr Darwin, are as follows : 1. The North Atlantic, comprising North America and Europe down to N. lat 30 ; 2. The West American, from Behring Straits to Tiefra- del-Fuego ; 3. The Malayan, from India to New Guinea ; 4. The Australian, comprising Australia and New Zealand. The Malayan and Australian regions are the richest in Cirrhipeds. During the voyage of the &quot; Challenger&quot; these animals were found to inhabit the deep seas, the most remarkable being a gigantic Scandium from a depth of 2850 fathoms in the North Atlantic, while other forms occurred at almost the same depth in the Pacific. (Darwin s &quot; Monograph of Cirrhipedia,&quot; Ray Society, 1854.) Molhisca. The marine Mollusca, from their great abund ance in all seas, the ardour with which they have been collected and studied, and the frequency of their occurrence as fossils, offer an extensive field for the study of distribu tion. But many causes have combined to render the results yet arrived at unsatisfactory. Their classification has been for some time undergoing a progressive change, owing to the greater attention paid to the organization and development of the animals, but there is still much uncer tainty as to the limits of genera and sub-genera. Owing to their being in many cases articles of commerce, either on account of their uses or their beauty, the place where they were originally obtained has often been confused with the place from which they were exported. Their numbers, too, have so rapidly increased that few persons have been found to devote themselves to the great labour of geographical tabulation. And, lastly, the genera are so often of great extent and world-wide distribution that the range of species alone has generally been attended to. The late Dr Woodward established a series of eighteen marine provinces, founded professedly on the fact of one- half of the species being peculiar. The distribution of the genera is only casually mentioned, and it is almost certain that a large number of these provinces have no claim to rank as primary retgions as regards the distribution of the Molluscan fauna. Dr Woodward himself states that his Arctic province is comparatively small and exceptional, while the three southern faunas of America, Africa, and Australia differ extremely. All the warmer provinces may, he says, be naturally grouped into three great divisions the Atlantic, the Indo-Pacific, and the West American ; and these are perhaps the only true Molluscan regions. The Indo-Pacific extends from the Red Sea and east coast of Africa to the easternmost Pacific islands, and exactly corresponds to Mr Dana s Oriental region for Crustacea. About 100 species are said to range over nearly the whole of this vast area. The Atlantic region unites the fauna of the east coast of America with that of West Africa and South Europe, but it also has considerable affinity for that