14 DISTRIBUTION OF ANIMALS. [part t. 
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 hall 
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 Martins declare that they saw at diffei- 
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 
