48 THE GENESIS OF SPECIES. [CHAP. 



and explicable only on the theory of descent" But this at 

 the best is but a partial and very incomplete explanation. 

 It is one, moreover, which Mr. Wallace does not accept. 16 

 It is very incomplete, because it has no bearing on some of 

 the most striking cases, and of course Mr. Darwin does not 

 pretend that it has. We should have to go back far indeed 

 to reach the common ancestor of the mimicking walking- 

 leaf insect and the real leaf it mimics, or the original pro- 

 genitor of both the bamboo insect and the bamboo itself. 

 As these last most remarkable cases have certainly nothing 

 to do with heredity, 17 it is unwarrantable to make use of that 

 explanation for other protective resemblances, seeing that 

 its inapplicability, in certain instances, is so manifest. 



Again, at the other end of the process it is as difficult 

 to account for the last touches of perfection in the mimicry. 

 Some insects which imitate leaves extend the imitation 

 even to the very injuries on those leaves made by the at- 

 tacks of insects or of fungi. Thus, speaking of one of the 

 walking-stick insects, Mr. Wallace says : 18 " One of these 

 creatures obtained by myself in Borneo ( Ceroxylus lacerar 

 tus) was covered over with foliaceous excrescences of a 

 clear olive-green color, so as exactly to resemble a stick 

 grown over by a creeping moss or jungermannia. The 

 Dyak who brought it me assured me it was grown over 

 with moss, although alive, and it was only after a most mi- 

 nute examination that I could convince myself it was not 

 so." Again, as to the leaf-butterfly, he says : 19 " We come 

 to a still more extraordinary part of the imitation, for we 

 find representations of leaves in every stage of decay, vari- 

 ously blotched, and mildewed, and pierced with holes, and 

 in many cases irregularly covered with powdery black dots, 



16 Loc. cit., pp. 109, 110. 



17 Heredity is the term used to denote the tendency which there is in 

 offspring to reproduce parental features. 



18 Loc. cit,, p. 64. 19 Loc. cit., p. 60. 



