CHAP. II.] DISPERSAL AND MIGRATION. 15 
unusual tidal currents might carry thei 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 
ycapable 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 
