184 THE ORIGIN OF SPECIES 



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 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 

 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 bene- 

 ficial, 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 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 ^j 

 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, flowers, spines, ex- 

 crement of birds, and living insects; but to this latter point I shall 

 hereafter recur. The resemblance is often wonderfully close, and 

 is not confined to color, but extends to form, and even to the 

 manner in which the insects hold themselves. The caterpillars 

 which project motionless like dead twigs from the bushes on which 

 they feed, offer an excellent instance of a resemblance of this 

 kind. The cases of the imitation of such objects as the excrement 

 of birds, are rare and exceptional. On this head, Mr. Mivart re- 

 marks, "As, according to Mr. Darwin's theory, there is a con- 

 stant tendency to indefinite variation, and as the minute incipient 



