234 ORIGIN OF SPECIES 



such islands to forms fitted to live on the land? But seals 

 would necessarily be first converted into terrestrial carnivor- 

 ous 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. 

 Gradations of structure, with each stage beneficial to a chang- 

 ing species, will be favoured only under certain peculiar con- 

 ditions. 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 thoroughly aquatic as to 

 brave the open ocean. But seals would not find on oceanic 

 islands the conditions favourable to their gradual reconver- 

 sion into a terrestrial form. Bats, as formerly shown, prob- 

 ably 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 pur- 

 poses, 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 disuse; 

 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 conjectural remarks have 

 been made merely to show that a transition of structure, with 

 each step beneficial, is a highly complex affair ; and that there 

 is nothing strange in a transition not having occurred in any 

 particular case. 



Lastly, more than one writer has asked, why have some 

 animals had their mental powers more highly developed than 

 others, as such development would be advantageous to all? 

 Why have not apes acquired the intellectual powers of man ? 

 Various causes could be assigned; but as they are conjec- 

 tural, and their relative probability cannot be weighed, it 

 would be useless to give them. A definite answer to the lat- 

 ter question ought not to be expected, seeing that no one can 



