190 TEOPICAL NATURE, AND OTHEH ESSAYS. 



creatures. But there are also a number of species of 

 Leptalis, which are brilliantly red, yellow, and black, 

 and which, band for band and spot for spot, resemble 

 some one of the Danaidse or Heliconidse which inhabit 

 the same district and which are nauseous and uneatable. 

 Now the usual difficulty is, that a slight approach to 

 one of these protected butterflies would be of no use, 

 while a greater sudden variation is not admissible on the 

 theory of gradual change by indefinite slight variations. 

 This objection depends almost wholly on the supposi- 

 tion that, when the first steps towards mimicry occurred, 

 the South American Danaidae were what they are now ; 

 while the ancestors of the Leptalides were like the ordi- 

 nary white or yellow Pieridse to which they are allied. 

 But the danaioid butterflies of South America are so 

 immensely numerous and so greatly varied, not only in 

 colour but in structure, that we may be sure they are of 

 vast antiquity and have undergone great modification. 

 A large number of them, however, are still of compara- 

 tively plain colours, often rendered extremely elegant 

 by the delicate transparency of the wing membrane, but 

 otherwise not at all conspicuous. Many have only 

 dusky or purplish bands or spots ; others have patches 

 of reddish or yellowish brown — perhaps the commonest 

 colour among butterflies ; while a considerable number 

 are tinged or spotted with yellow, also a very common 

 colour, and one especially characteristic of the Pieridse, 

 the family to which Leptalis belongs. We may there- 

 fore reasonably suppose that in the early stages of the 

 developmeut of the Danaidse, when they first began to 

 acquire those nauseous secretions which are now their 

 protection, their colours were somewhat plain ; either 



