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 
