CHAP. VII.] THEORY OF NATURAL SELECTION. 169 



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 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 hereafter 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 seize upon and perpetuate." 



But in all the foregoing cases the insects in their original state 

 no doubt presented some rude and accidental resemblance to an 

 object commonly found in the stations frequented by them. Nor 

 is this at all improbable, considering the almost infinite number 

 of surrounding objects and the diversity in form and colour of 

 the hosts of insects which exist. As some rude resemblance is 

 necessary for the first start, we can understand how it is that the 

 larger and higher animals do not (with the exception, as far as 

 I know, of one fish) resemble for the sake of protection special 

 objects, but only the surface which commonly surrounds them, 



