Chap. VII.] THEORY OF NATURAL SELECTION. 181 



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, hare 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 hirds 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 power? 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 civilisation 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, excrement 

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

 after recur. The resemblance is often wonderfully close, and is not 

 confined to colour, 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 remarks, "As, 

 according to Mr. Darwin's theory, there is a constant tendency to 

 indefinite variation, and as the minute incipient variations will be 

 in all directions, they must tend to neutralize each other, and at 

 first to form such unstable modifications that it is difficult, if not 

 impossible, to see how such indefinite oscillations of infinitesimal 

 beginnings can ever build up a sufficiently appreciable resemblance 

 to a leaf, bamboo, or other object, for Natural Selection to 3eize 

 upon and perpetuate." 



