THEORY OF NATURAL SELECTION. 201 



ifiere 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 species, will be favored only 

 under certain peculiar conditions. A strictly terrestrial 

 animal, by occasionally hunting for food in shallow water, 

 then in streams or lakes, ni;ght 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 f.om tree to tree, 

 like the so-called flying Lquirrels, for th^ 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 re- 

 duced 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 

 cf man ? Various causes could be assigned ; but as they 

 are conjectural, and their relative probability cannot be 

 weighed, it would be useless to give them. A definite 

 answer to the latter question ought not to be expected, 

 seeing that no one can solve the simpler problem, why, of 

 two races of savages, one has risen higher in the scale of 

 civilization than the other; and this apparently implies 

 increased brain power. 



We will return to Mr. Mivart's other objections. Insects 

 often resemble, for the sake of protection, various objects, 

 such as green, or decayed leaves, dead twigs, bits of lichen, 



