The geographical distribution of animals, with a study of the relations ...

;chap, ii.] DISPERSAL AND MIGRATION. 15unusual tidal currents might carry them safely to shores perhapsseveral hundred miles from their native country.The factof green trees so often having been seen erect on these rafts ismost important ;for they would act as a sail by which the raftmight be propelled in one direction for several days in succession,and thus at last reach a shore to which a current alone wouldnever have carried it.There are two groups of mammals whichhave quite exceptionalmeans of dispersal— the bats which.fly, and the cetacea,seals, &c, which swim. The former are capable of traversingconsiderable spaces of sea, since two North American specieseither regularly or occasionally visit the Bermudas, a distanceof 600 miles from the mainland. The oceanic mammals (whalesand porpoises) seem to have no barrier but temperature; thepolar species being unable to cross the equator, while the tropicalforms are equally unfitted for the cold polar waters.The shorefeedingmanatees, however, can only live where they findfoodand a long expanse of rocky coast would probably be as completea barrier to them as a few hundred miles of open ocean.The amphibious seals and walruses seem many of them to becapable of making long sea journeys, some of the species beingfound on islands a thousand miles apart, but none of the arcticare identical with the antartic species.The otters with one exception are freshwater animals, and wehave no reason to believe they could or would traverse any greatdistances of salt water. In fact, they would be less liable todispersal across arms of the sea than purely terrestrial species,since their powers of swimming would enable them to regainthe shore ifaccidentally carried out to sea by a sudden flood.Means of Dispersal of Birds.—It would seem at first sight thatno barrierscould limit the range of birds, and that they oughtto be the most ubiquitous of living things, and little fitted thereforeto throw any light on the, laws or causes of the geographicaldistribution of animals. This, however, is far from being thecase ; many groups of birds are almost as strictly limited bybarriers as the mammalia ; and from their larger numbers andthe avidity with which they have been collected, they furnish