62 "MIMICRY RINGS" [ch. 



mutation, and that natural selection is responsible 

 merely for its survival and the elimination of the less 

 favoured form from which it sprang. 



There is a serious difficulty in the way of accepting 

 the former of these two views. If our two species, 

 model and would-be mimic are, to begin with, markedly 

 different in pattern, how can we suppose that a slight 

 variation in the direction of the model on the part 

 of the latter would be of any value to it ? Take for 

 example a well-known South American case — ^the 

 resemblance between the yellow, black, and brown 

 Ithomiine, Mechanitis saturata (PI, X, fig, 7) and 

 the Pierine, Dismorphia praxinoe (PL X, fig. 3). 

 The latter belongs to the family of the "whites," and 

 entomologists consider that in all probabihty its 

 ancestral garb was white with a little black like the 

 closely allied species D. cretacea (PI, X, fig. 1), Can 

 we suppose that in such a case a small development 

 of brown and black on the wings would be sufficient 

 to recall the Ithomiine and so be of service to the 

 Dismorphia which possessed it? Such a relatively 

 slight approach to the Ithomiine colouring is shewn 

 by the males of certain South American "whites" 

 belonging to the genus Perrhyhris (PI, X, figs. 4 and 5), 

 But the colotu- is confined to the under-surface and 

 the butterflies possessing it could hardly be confused 

 with a Mechanitis more than their white relations 

 which enthely lack such a patch of colour. If birds 

 regarded white butterflies as edible it is difficult to 

 suppose that they would be checked in their attacks 



