Mimetic Analogy. 313 



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 expla- 

 nation 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, varia- 

 tions of marking and form are most frequent. Let us now 

 imagine a race of insects, like those of the genus Leptalis, tha,t 

 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 de- 

 gree the offensive Heliconida?, 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, as in the cases that we have illustrated, 

 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 

 peculiarities. 



The use of the terms mimetic analogy and mimicry, as 

 descriptive of these undeniable phenomena, has been strongly 

 objected to by certain writers, who imagine that the words imply 

 that the animals have power to change or alter their own condi- 

 tion. No supposition can be possibly more absurd. The change 

 is not effected by the will of the individual animal, but occurs in 

 the species ; variations are involuntary, and at present even their 

 very cause is unknown ; all that is known is that they do occur 

 both in wild and domestic animals, and that they are capable of 

 hereditary transmission. And it is evident that those varie- 

 ties that are protected in the most complete manner from 

 their natural enemies, are the most likely to survive and per- 

 petuate their race. 



