72 



ISLAND LIFE. 



[part I. 



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 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 carried away 

 together, such accidents would no doubt be rare. Insects, how- 

 ever, 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 con- 

 nection, 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 form a 

 broad isthmus connecting Asia and America as far south as the 

 parallel of 60° N. Continuity of land therefore may be said to 

 exist already for all parts of the world (except Australia and a 

 number of large islands, which will be considered separately), 

 and we have thus no difficulty in the way of that former wide 



