sea, but they generally perish in the saltwater. Admitting, however, the powers of most mammals to swim considerable distances, we have no reason to believe that any of them could traverse without help straits of upwards of twenty miles in width, while in most cases a channel of half that distance would prove an effectual barrier. _Ice-floes and Driftwood as Aiding the Dispersal of Mammals._--In the arctic regions icebergs originate in glaciers which descend into the sea, and often bear masses of gravel, earth, and even some vegetation on their surfaces; and extensive level ice-fields break away and float southwards. These might often carry with them such arctic quadrupeds as frequent the ice, or even on rare occasions true land-animals, which might sometimes be stranded on distant continents or islands. But a more effectual because a more wide-spread agent, is to be found in the uprooted trees and rafts of driftwood often floated down great rivers and carried out to sea. Such rafts or islands are sometimes seen drifting a hundred miles from the mouth of the Ganges with living trees erect upon them; and the Amazon, the Orinoco, Mississippi, Congo, and most great rivers produce similar rafts. Spix and Martius declare that they saw at different times on the Amazon, monkeys, tiger-cats, and squirrels, being thus carried down the stream. On the Parana, pumas, squirrels, and many other quadrupeds have been seen on these rafts; and Admiral W. H. Smyth informed Sir C. Lyell that among the Philippine islands after a hurricane, he met with floating masses of wood with trees growing upon them, so that they were at first mistaken for islands till it was found that they were rapidly drifting along. Here therefore, we have ample means for carrying all the smaller and especially the arboreal mammals out to sea; and although in most cases they would perish there, yet in some favourable instances strong winds or {15}unusual tidal currents might carry them safely to shores perhaps several hundred miles from their native country. The fact of green trees so often having been seen erect on these rafts is most important; for they would act as a sail by which the raft might he propelled in one direction for several days in succession, and thus at last reach a shore to which a current alone would never have carried it.