162 MAMMALS OF THE PACIFIC WORLD 



geological time, as has been indicated, some barriers develop, 

 and others disappear. Barriers have been spoken of as chiefly 

 climatic or physiographical but others based upon disease, com- 

 petition with other species, and physiology also occur. All are 

 likely to be by-passed or penetrated sooner or later through 

 temporarily favorable circumstances or because of permanent 

 modification of the affected organisms. 



Even when barriers seem to be complete, as at the edges of 

 wide oceans, accident or the laws of chance eventually permit 

 occasional species to cross to other land masses or to islands. 

 Wherever land and sea meet, animals have repeatedly — not once 

 or twice, but thousands of times during the passage of time — 

 fallen into or been forced into the waters. Trees with climbing 

 mammals clinging to their branches or living in hollows in their 

 trunks fall into rivers and are carried out to sea. Ground-living 

 creatures are sometimes accidentally taken for rides when banks 

 cave in. The debris from such undermined banks and tangled 

 masses of tree-trunks and water weeds sometimes forms ex- 

 tensive rafts. Bats, especially those that migrate, may be blown 

 out to sea by storms. If the distance across a water barrier is 

 short, some of the larger mammals may even swim across ; deer 

 and pigs possibly do this. Natural swimmers, such as otters, 

 can be carried by flooded rivers into the ocean but are likely 

 to swim back to the nearby shore. 



The natural rafts mentioned above have been one of the com- 

 monest means of transportation across seas. Trees and masses 

 of vegetation become undermined by rivers during the rainy 

 season and are carried out to sea. In the East Indies, where 

 the currents between the innumerable islands are changeable, 

 such rafts have good chances to be stranded quickly, but in the 

 open ocean the arrival of one at a remote island must indeed be 

 a rare occurrence. The mammals so transported are likely to be 

 small, tree-living species, and the raft must be large enough and 

 carry sufficient food to preserve life until the accidental landfall 

 is made. 



