808 THE ORIGIN OF SPECIES 



})resent homes. Therefore Sir 0. Lyell asks, and as- 

 signs certain reasons in answer, why have not seals and 

 bats given birth on such islands to forms fitted to live 

 on the land? But seals would necessarily be first con- 

 verted into terrestrial carnivorous animals of considerable 

 size, and bats into terrestrial insectivorous animals; for 

 the former there would be no prey; for the bats ground- 

 insects would serve as food, but these would already be 

 largely preyed on by the reptiles or birds, which first 

 colonize and abound on most oceanic islands. Gradations 

 of structure, with each stage beneficial to a changing spe- 

 cies, will be favored only under certain peculiar condi- 

 tions. A strictly terrestrial animal, by occasionally hunt- 

 ing for food in shallow water, then in streams or lakes, 

 might at last be converted into an animal so thoroughly 

 aquatic as to brave the open ocean. But seals would 

 not find on oceanic islands the conditions favorable to 

 their gradual reconversion into a terrestrial form. Bats, 

 as formerly shown, probably acquired their wings by at 

 first gliding through the air from tree to tree, like the 

 so-called flying squirrels, for the sake of escaping from 

 their enemies, or for avoiding falls; but when the power 

 of true flight had once been acquired, it would never be 

 reconverted back, at least for the above purposes, into 

 the less efficient power of gliding through the air. Bats 

 might, indeed, like many birds, have had their wings 

 greatly reduced in size, or completely lost, through dis- 

 use; but in this case it would be necessary that they 

 should first have acquired the power of running quickly 

 on the ground, by the aid of their hind legs alone, so as 

 to compete with birds or other ground animals; and for 

 such a change a bat seems singularly ill-fitted. These 



