oO ClltlOSITIKS ()1<- ENTOMOLOGY. 



but the difiiculties in this view of the case are numerous. 

 One of the strongest arises from the fact, that in those cases 

 where a local variation of the imitated animal exists, the 

 imitator also varies to keep up the resemblance. Others say 

 that as imitator and imitated both inhabit the same district, 

 they are necessarily exposed to the same external conditions, 

 producing the same amount of form and colour. This also is 

 a perfectly untenable argument. It cannot be imagined that 

 an insect resembles a green leaf or the bark of a tree, because 

 both are exposed to the same physical conditions. By the 

 Darwinian Theory of Natural Selection, the explanation of 

 these remarkable analogies is sufficiently easy. All animals, 

 without exception, are liable to variation in form, colour, and 

 in size. Among insects in particular, variations of marking 

 and form are most frequent. Let us now imagine a race of 

 insects like those of the genus Leptalis, that have no special 

 means of defence, and are consequently liable to be devoured 

 by predaceous animals. When a variety of Leptalis arose 

 which happened to resemble in any slight degree the oifensive 

 ]Ieliconidae, it would be much less liable to be pursued by 

 predaceous animals than the unchanged original to which they 

 were accustomed, hence that variety would have a greater 

 chance of being propagated. Similar variations would occur 

 in subsequent generations, those imitations that most closely 

 resemble the model always being left, until at last this 

 remarkable result would follow, that two insects, belonging to 

 distinct families, would so closely resemble one another 

 as only to be distinguished by a close inspection of their 

 structural jjceuliarities." 



