The geographical distribution of animals, with a study of the relations ...

14 DISTRIBUTION OF ANIMALS. [part i.take boldly to the water and can swim considerable distances,but we have no evidence to show how long they could live atsea or how many miles they could traverse.Squirrels, rats, andlemmings often migrate from northern countries in bands ofthousands and hundreds of thousands, and pass over rivers, lakesand even arms of the sea, but they generally perish in the saltwater.Admitting, however, the powers of most mammals toswim considerable distances, we have no reason to believe thatany of them could traverse without help straits of upwards oftwenty miles in width, while in most cases a channel of halfthat 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 descendinto the sea, and ofteneven some vegetation on their surfaces ;bear masses of gravel, earth, andand extensive level icefieldsbreak away and float southwards. These might oftencarry with them such arctic quadrupeds as frequent the ice, oreven on rare occasions true land-animals, which might sometimesbe stranded on distant continents or islands.But a moreeffectual because a more wide-spread agent, is to be found inthe uprooted trees and rafts of driftwood often floated downgreat rivers and carried out to sea. Such rafts or islands aresometimes seen drifting a hundred miles from the mouth of theGanges with living trees erect upon them ;and the Amazon, theOrinoco, Mississippi, Congo, and most great rivers producesimilar rafts.Spix and Martius declare that they saw at differenttimes 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 theserafts ; and Admiral W. H. Smyth informed Sir C. Lyell thatamong the Philippine islands after a hurricane, he met withfloating massesof wood with trees growing upon them, so thatthey were at first mistaken for islands till it was found that theywere rapidly drifting along. Here therefore, we have amplemeans for carrying all the smaller and especially the arborealmammals out to sea ; and although in most cases they wouldperish there, yet in some favourable instances strong winds or