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Show 10~ MIGRATIONS OF BIRDS. [Ch. VI. Humboldt, of many American water-fowl, from ono part of tho tropics to another in a zone where there is the same tern .. perature throughout the year. Immense flights of ducks leave the valley of the Orinoco, when the increasing depth of its waters and the flooding of its shores prevent them from catching fish, insects, and aquatic worms. They then betake themselves to the Rio Negro and Amazon, having passed from the eighth and third degrees of north latitude, to the first and fourth of south latitude, directing their ·course south southeast. In September, when the Orinoco decreases and re-enters into its channel, these birds return northwards*· The insectivorous swallows which visit our island would perish during winter, if they did not annually repair to warmer climes. It is supposed, that in these aerial excursions the average rapidity of their flight is not less than fifty miles an hour, so that when aided by the wind they soon reach warmer latitudes. Spallanzani c~;tlculated that the swallow can fly at the rate of ninety-two miles an hour, and conceived that the rapidity of the swift might be three times greater t· The rate of flight of the eider-duck (Anas rnollissima) has been ascertained to be ninety miles an hom•; and that of hawks and several other tribes, to be one hundred and fifty miles. When we reflect how easily different species, in a great lapse of ages, may be each overtaken by gales and hurricanes, and, abandoning themselves to the tempest, be scattered at random through various regions of the earth's surface, where the temperature of the atmosphere, the vegetation, and the animal productions, might be suited to their wants, we shall be pre· pared to find some species capriciously distributed, and to be sometimes unable to determine the native countries of each. Captain Smyth informs me, that when engaged in his survey of the Mediterranean, he encountered a gale in the gulf of Lyons, at the distance of between twenty and thirty leagues from the coast of France, which bore along many land bil'ds * Voy11ge aux Regions Equinoxiales, tome vii. 11. 429. t Fleming, Phil. Zool., vol. ii. p. 43, Ch. VI.] GEOGRAPHICAL DISTRIBUTION OF REPTILES. lOS of various species, some of which alighted on the ship, while others were thrown with violence against the sails. In this manner islands become tenanted by species of birds inhabiting the nearest main land. Geographical Distribution and Dissemination of Reptiles. A few facts respecting the third great class of vertebrated animals, will ~uffice to show that the plan of nature, in regard to their location on the globe, is perfectly analogous to that already exemplified in other parts of the organic creation, and has probably been determined by similar causes. . Of the great saurians, the gavials which inhabit the Ganges differ from the cayman of America, or the crocodile of the Nile. The monitor of New Holland is specifically distinct from the Indian species ; these latter again from the African and all from their congeners in the new world. So itt regtt;d to sna~es ; we find the boa of America, represented by the python, a dtfferent though nearly allied genus, in India. America is the country of the rattle-snake, Africa of the cerastes, and Asia of the hoode~ snake ot· cobra di capello. There is a legend ~h~t St. Patnck expelled all reptiles from Ireland, and certain It Is that none of the three species of snakes common in Encrland, nor the toad, have been observed there by naturalis~. !hey have our common frog, and our water-newt, and accordmg to Ray (Quad. ~64.) the green lizard (Lace1·ta viridis ). Schultes the botanist observed, a few years since, in his tour in England, that there were two great islands in Europe of which th'e floras were unknown, Sardinia and Ireland; we believe he mt~·ht also have added the fauna of the latter country. The range of the large reptiles is, in general, quite as limited as .that of some. orders of the terrestrial mammalia. The great saurians s~mettmes cross a considerable tract, in order to pass from one rtver to another; but their motions by land are generhally slower than those of quadrupeds. By water however t ey may tran spor t th emse1 v es to distant situations m' ore easily' ~he larg~r alligator of the Ganges sometimes descends beyond t e braclush water_of the Delta into the sea, and in such cases >I< |