74 ISLAND LIFE 



Deer also swim well, but there is no reason to believe that 

 they would venture out of sight of land. With the smaller, 

 and especially with the arboreal mammalia, there is a 

 much more effectual way of passing over the sea, by means 

 of floating trees, or those floating islands which are often 

 formed at the mouths of great rivers. Sir Charles Lyell 

 describes such floating islands which were encountered 

 among the Moluccas, on which trees 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 under- 

 stand 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 

 arrive safely on the shores of some land hundreds of miles 

 away from its starting-point. Such small animals as 

 squirrels and field-mice might have been carried away on 

 the trees which formed part of such a raft, and might thus 

 colonise a new island ; though, as it would require a pair of 

 the same species to be thus conveyed at the same time, such 

 accidents would no doubt be rare. Insects, however, and 

 land-shells 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 number 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 



