186 THE ORIGIN OF SPECIES 



fresh -water productions are low in the scale of nature, 

 and we have reason to believe that such beings become 

 modified more slowly than the high; and this will give 

 time for the migration of aquatic species. We should 

 not forget the probability of many fresh-water forms 

 having formerly ranged continuously over immense areas, 

 and then having become extinct at intermediate points. 

 But the wide distribution of fresh-water plants and of 

 the lower animals, whether retaining the same identical 

 form or in some degree modified, apparently depends in 

 main part on the wide dispersal of their seeds and eggs 

 by animals, more especially by fresh- water birds, which 

 have great powers of flight, and naturally travel from one 

 piece of water to another. 



On the Inhabitants of Oceanic Islands 



We now come to the last of the three classes of facts 

 which I have selected as presenting the greatest amount 

 of difficulty with respect to distribution, on the view that 

 not only all the individuals of the same species have 

 migrated from some one area, but that allied species, 

 although now inhabiting the most distant points, have 

 proceeded from a single area — the birthplace of their 

 early progenitors. I have already given my reasons for 

 disbelieving in continental extensions within the period 

 of existing species, on so enormous a scale that all the 

 many islands of the several oceans were thus stocked 

 with their present terrestrial inhabitants. This view re- 

 moves many difficulties, but it does not accord with all 

 the facts in regard to the productions of islands. In the 

 following remarks I shall not confine myself to the mere 

 question of dispersal, but shall consider some other cases 



