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 at 

 sea or how many miles they could traverse. Squirrels, rats, and 

 lemmings often migrate from northern countries in bands of 

 thousands and hundreds of thousands, and pass over rivers, lakes 

 and even arms of the sea, but they generally perish in the salt- 

 water. 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 de- 

 scend 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 some- 

 times 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 differ- 

 ent 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 



