THEORY OF NATURAL SELECTION. 215 



to this latter point I shall hereafter recur. Tlie resein- 

 blauce is often Avonderfully close, and is not confined to 

 color, but extends to form, and even to the manner in 

 which the insects hold themselves. Tlie caterpillars which 

 project motionless like dead twigs from the bushes on 

 which they feed, olfer an excellent instance of a resem- 

 blance 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 diffi- 

 cult, if not impossible, to see how such indefinite oscilla- 

 tions of infinitesimal beginnings can ever buikl up a suffi- 

 ciently appreciable resemblance to a leaf, bamboo, or other 

 object, for natural selection to seize upon and perpetuate." 



But in all the forgoing cases the insects in their original 

 state no doubt presented some rude and accidental resem- 

 blance to an object commonly found in the stations fre- 

 quented by them. Nor is this at all improbable, consider- 

 ing the almost infinite number of surrounding objects and 

 the diversity in form and color 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 sur- 

 rounds them, and this chiefly in color. Assuming that an 

 insect originally happened to resemble in some degree a 

 dead twig or a decayed leaf, and that it varied slightly in 

 many ways, then all the variations which rendered the in- 

 sect at all more like any such object, and thus favored its 

 escape, would be preserved, while other variations would 

 be neglected and ultimately lost; or, if they rendered tlie 

 insect at all less like the imitated object, they would be 

 eliminated. There would indeed be force in Mr. ^livart's 

 objection, if we were to attempt to account for the above 

 resemblances, independently of natural selection, through 

 mere fluctuating variability; but as the case stands there is 

 none. 



Nor can I see any force in Mr. Mivart's difficulty wuth 



