Chap. VII.] THEORY OF NATURAL SELECTION. 281 



Except by assigning such general and vague reasons, we 

 cannot explain why, in many quarters of the world, 

 hoofed quadrupeds have not acquired much elongated 

 necks or other means for browsing on the higher branches 

 of trees. 



Objections of the same nature as the foregoing have 

 been advanced by many writers. In each case various 

 causes, besides the general ones just indicated, have' 

 probably interfered with the acquisition through natural 

 selection of structures, which it is thought would be 

 beneficial to certain species. One writer asks, why has 

 not the ostrich acquired the power of flight ? But a 

 moment's reflection will show what an enormous supply 

 of food would be necessary to give to this bird of the 

 desert force to move its huge body through the air. 

 Oceanic islands are inhabited by bats and seals, but by 

 no terrestrial mammals ; yet as some of these bats are 

 peculiar species, they must have long inhabited their 

 present homes. Therefore Sir C. Lyell asks, and assigns 

 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 converted 

 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 

 colonise and abound on most oceanic islands. Grada- 

 tions of structure, with each stage beneficial to a changing 

 species, will be favoured only under certain peculiar 

 conditions. A strictly terrestrial animal, by occasionally 

 hunting for food in shallow water, then in streams or 

 lakes, might at last be converted into an animal so 



