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Show 72 ISLAND LIFE. [PART I. floatinO' trees or those floating islands which are often formed at the ~outh's of great rivers. Sir Charles Lyell describes such floatinO' islands which were encountered among the Moluccas, on which ~rees and shrubs were growing on a stratum of soil which even formed a white beach round the margin of each raft. Among the Philippine Islands similar rafts with trees growing on· them have been seen after hurricanes; and it is easy to understand how, if the sea were tolerably calm, such a raft might be carried along by a current, aided by the wind acting on the trees, till after a passage of several weeks it might arriYe safely on the shores of some land hundreds of miles away from its starting-point. Such small animals as squirrels and mice might have been carried away on the trees which formed part of such a raft and miO'ht thus colonise a new island ; though, as it ' 0 would require a pair of the same species to be carried away toO'ether such accidents would no doubt be rare. Insects, how-o ' ever, and land-sh~lls would almost certainly be abundant on such a raft or island, and in this way we may account for the wide dispersal of many species of both these groups. Notwithstanding the occasional action of such causes, we cannot suppose that they have been effective in the dispersal of mammalia as a whole; and whenever we find that a considerable number of the mammals of two countries exhibit distinct marks of relationship, we may be sure that an actual land connection, or at all events an approach to within a very few miles of each other, has at one time existed. But a considerable num her of identical mammalian families and even genera are actually found in all the great continents, and the present distribution of land upon the globe renders it easy to see how they have been able to disperse themselves so widely. All the great land masses radiate from the arctic regions as a common centre, the only break being at Behrings Strait, which is so shallow that a rise of less than a thousand feet would form a broad isthmus connecting Asia and America as far south as the parallel of 60° N. Continuity of land therefore may be said to exist alr.eady for all parts of the world (except Australia and a number of large islands, which will be considered separately), and we have thus no difficulty in the way of that former wide CHAP. V.] DISPERSAL OF ANIMALS AND PLANTS. 73 diffusion of many groups, which we maintain to be the only explanation of most anomalies of distribution other than such as may be connected with unsuitability of climate. The Dispersal of Birds.-Wherever mammals can migrate other vertebrates can generally follow with even greater facility. Birds, having the power of flight, can pass over wide arms of the sea, or even over extensive oceans, when these are, as in the Pacific, studded with islands to serve as resting places. Even the smaller land-birds are often carried by violent gales of wind from Europe to the Azores, a distance of nearly a thousand miles, -so that it becomes comparatively easy to explain the exceptional distribution of certain species of birds. Yet on the whole it is remarkable how closely the majority of birds follow the same laws of distribution as mammals, showing that they generally require either continuous land or an island-strewn sea as a means of dispersal to new homes. The Dispersal of R eptiles.-Reptiles appear at first sight to be as much dependent on land for their dispersal as mammalia, but they possess two peculiarities which favour their occasional transmission across the sea-the one being their greater tenacity of life, the other their oviparous mode of reproduction. A large boa-constrictor was once floated to the island of St. Vincent, twisted round the trunk of a cedar tree, · and was so little injured by its voyage that it captured some sheep before it was killed. The island is nearly two hundred miles from Trinidad and the coast of South America, whence it almost certainly came.1 Snakes are, however, comparatively scarce on islands far from continents, but lizards are often abundant, and though these might also travel on floating trees, it seems more probable that there is some as yet unknown mode by which their eggs are safely, though perhaps very rarely, conveyed from island to island. Examples of their peculiar distribution will be given when we treat of the fauna of some islands in which .they abound. The Dispersal of Amphibia and Fnsh-water Fishes.The two lower groups of vertebrates, Amphibia and freshwater fishes, possess special facilities for dispersal, in the fact of 1 Lyell's P1·inciples of Geology, II., p. 3G9. |