THE SELECTIVE AGENTS 235 



between species of Planema and their mimics, forms of 

 Pseudacraea. 



Coincidence has proved itself unable to produce such 

 cases as those in South America described by Bates, 

 where the colouration is a comparatively complicated 

 mixture of red and yellow on a dark background. These 

 cases, indeed, almost seem to be an exception to the rule 

 stated above, that aposematic patterns are extremely 

 simple. 



The geographical variations of a mimetic species, 

 according to the different species serving as models in 

 the various subdivisions of its habitat, such as in the 

 case of Acraea alciope ^ or Pseudacraea eurytus (see next 

 chapter), with the transitional forms at the meeting 

 points of two areas, are extremely difficult to explain, 

 except by the theory of a causal connection between 

 model and mimic. 



One of the great difficulties felt by many to militate 

 against acceptance of the theory of mimetic resemblance 

 produced by natural, selection is lack of definite evidence 

 of a selective agent. It is claimed that it has not yet 

 been proved that birds eat butterflies in sufficient numbers 

 to produce selective action, and, moreover, that they have 

 in certain cases been seen to devour just those species 

 which are claimed to be exempt. ^ This latter point has 

 been already dealt with. I think that few would claim, 

 for instance, that the sting of a bee is of no protection 

 to it because the Bee-eaters appear to be especially fond 

 of them. A mimetist would account for such cases as 

 the ashy wood swallow devouring Danaines in this way. 



Regarding the general question, there is plenty of 

 indirect evidence of attacks by birds upon butterflies. All 

 collectors must have noted the ^-shaped gaps in the hind 

 wings, symmetrical on both sides, that could only have 



» Bedrock, No. 1, April 1912, pp. 57-64. 



* Punnett, Bedrock, vol. ii, July 1913, pp. 159-60. 



