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Show 106 MIGRATIONS OF FISH, [Ch. VII. those of the Mediterranean, notwithstanding the proxiii)ity of these seas. The flying-fish are found (some stragglers excepted) only between the tropics,-in receding from the line they never approach a higher latitude than the fortieth parallel. Those inhabiting the Atlantic are said to be different species from those of the eastern ocean *. 'rhe electric gymnotus belongs exclusively to America, the trembler, or Silurus electricus to the rivers of Africa; but the torpedo, or crampfish, is said to be dispersed over all tropical and many temperate seast. All are aware that there are certain fish of passage which have their periodical migrations 1ike some tribes of birds. The salmon, towards the season of spawning, ascends the rivers for hundreds of miles, leaping up the cataracts which it meets in its course, and then retreats again into the depths of the ocean. The herring and the haddock, after frequenting certain shores in vast shoals for a series of years, desert them again and resort to other stations, followed by the species which prey on them. Eels are said to descend into the sea for the purpose of producing their young, which are seen returning into the freshwater by myriads, extremely small in size, but possessing the power of surmounting every obstacle which occurs in the course of a river, by applying theh· slimy and glutinous bodies to the surface of rocks, or the gates of a lock, even when dry~ and so climbing over itt. Gmelin says, that the anseres subsist in their migrations on the spawn of fish, and that oftentimes when they void the spawn, two or three days afterwards, the eggs retain their vitality unimpaired§. When there are many disconnected freshwaterlakes in a mountaino.us region, at various elevations7 each remote from the other, it has often been deemed inconceivable how they could all become stocked with fish from one common source; but it has been suggested, that the minute eggs of * Malte-Brun, vol. i. p. 507. t Phil. Trans. 17471 p. 395. t Ibid. ~ Amoon. Acad., ~ssa.y 75. Ch. VII.] MIGRATIONS OF FJBII. 107 these animals may sometimes be entangled in tho feathers of water-fowl. These, when they alight to wash and plume themselves in the water, may often unconsciously contribute to propagate swarms of fish, whicl1, in due season, will supply them with food. Some of the water-beetles, also, as the dyticidre, are amphibious, and in the evening quit their lakes and pools, and flying in the air transport the minute ova of fishes to distant waters. In "this manner some naturalists account for the fry of fish appearing occasionally in small pools caused by heavy rains. Geographical Distribution and Mig·rations of Tesfacea. The testacea, of which so great ll. variety of species occurs in the sea, are a class of animals of peculiar importance to the geologist, because their remains are found in strata of all ages, and generally in a higher state of preservation than those of other organic beings. Climate has a decided influence on the geographical distribution of species in tliis class ; but as there is much greater uniformity of temperature in the waters of the ocean, than in the atmosphere which invests the land, the diffusion of many marine molluscs is extensive. Some forms, as those of the nautili, volutre, and cyprooro, attain their fullest development in warm latitudes ; and most of their species are exclusively confined to them. Peron and Lesueur remark, that the Haliotis gigantea, of Van Dieman 's land, and the Phasianella, diminish in size as they follow the coasts of New Holland to King George's straits, and entirely disappear beyond them*· Almost all the species of South Amerionn shells differ from those of the Indian archipelaO'o in . 5 the same 1atitudes ; and on the shores of many of the isles of the South Pacific, peculiar species have been obtained. But we are as yet by no means able to sketch out the submarine provinces of shells, as the botanist has done those of the terres. trial, and even of the subaqueous plants. There c~n be little |
