282 MISCELLANEOUS OBJECTIONS TO THE [Chap. VH 



ing species, will be favoured only under certain peculiar 

 conditions. A strictly terrestrial animal, by occasion- 

 ally 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 iind on oceanic islands the conditions 

 favourable to their gradual reconversion into a terres- 

 trial 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 disuse; but in this case it would be necessary 

 that they should first have acquired the power of run- 

 ning 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 de- 

 veloped 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 conjectural, and their relative 

 probability cannot be weighed, it would be useless to 



