chap, ii.] DISPERSAL AND MIGRATION. 15 



unusual tidal currents might carry them safely to shores per- 

 haps several hundred miles from their native country. The fact 

 of green trees so often having been seen erect on these rafts is 

 most important ; for they would act as a sail by which the raft 

 might be propelled in one direction for several days in succession, 

 and thus at last reach a shore to which a current alone would 

 never have carried it. 



There are two groups of mammals which have quite excep- 

 tional means of dispersal — the bats which fly, and the cetacea, 

 seals, &c, which swim. The former are capable of traversing 

 considerable spaces of sea, since two North American species 

 either regularly or occasionally visit the Bermudas, a distance 

 of 600 miles from the mainland. The oceanic mammals (whales 

 and porpoises) seem to have no barrier but temperature; the 

 polar species being unable to cross the equator, while the tropical 

 forms are equally unfitted for the cold polar waters. The shore- 

 feeding manatees, however, can only live where they find food ; 

 and a long expanse of rocky coast would probably be as com- 

 plete a barrier to them as a few hundred miles of open ocean. 

 The amphibious seals and walruses seem many of them to be 

 capable of making long sea journeys, some of the species being- 

 found on islands a thousand miles apart, but none of the arctic 

 are identical with the antartic species. 



The otters with one exception are freshwater animals, and we 

 have no reason to believe they could or would traverse any great 

 distances of salt water. In fact, they would be less liable to 

 dispersal across arms of the sea than purely terrestrial species, 

 since their powers of swimming would enable them to regain 

 the shore if accidentally carried out to sea by a sudden flood. 



Means of Dispersal of Birds. — It would seem at first sight that 

 no barriers could limit the range of birds, and that they ought 

 to be the most ubiquitous of living things, and little fitted there- 

 fore to throw any light on the laws or causes of the geographical 

 distribution of animals. This, however, is far from being the 

 case ; many groups of birds are almost as strictly limited by 

 barriers as the mammalia; and from their larger numbers and 

 the avidity with which they have been collected, they furnish 



