I So THE DOCTRINE OF DESCENT. 



posed to the persecutions of insectivorous birds and 

 other animals; but this is not the case, for they have a 

 disagreeable smell,' which, in all likelihood, renders them 

 obnoxious. Their smell and flavour are thus a protec- 

 tion, as the birds and lizards who have once seized them 

 by mistake are certain, ever after, to leave them unmo- 

 lested. Now, as the insectivora do not test the individual 

 case, but have adopted a general repugnance to the as- 

 pect of the Heliconidse, if other butterflies resembled the 

 .Heliconidae without possessing the bad smell, they would 

 participate in the security to life enjoyed by the Heli- 

 conidse in proportion as they approach their external 

 appearance. The case has actually occurred, for Bates 

 discovered a number of species of the otherwise very 

 different genus, Leptalis, of which each almost undis- 

 tinguishably resembles one of the Heliconidae both in 

 colour and form. The Leptalids have also adopted the 

 flight of the Heliconids, share their habitats, and, al- 

 though without the offensive smell, fly about with im- 

 punity. This state of things would be impossible if the 

 Leptalidse were not considerably in the minority, so as 

 to be in a measure hidden by the Heliconidae. 



Wallace has proved that species protected by mimicry 

 of other animals are invariably in the minority, and often 

 very rare in comparison with the species which they imi- 

 tate. Neither the explanation that like conditions of life 

 produced like results, nor the hypothesis that, in some 

 cases at least, the mimicry consists in reversion to a 

 common original species, is in any way satisfactory. 

 Many cases can be interpreted only by natural selec- 

 tion, those, namely, where from the first, before the imi- 

 tation had begun, such a resemblance already existed 



