16 BUTTERFLIES IN DISGUISE 



examples of a single species found by him (in 

 this instance belonging to the Acraeinae, a closely 

 allied nauseous group) in which the wings had 

 evidently been seized by insectivorous birds, for 

 they show great gaps in their wings, such as a 

 bill would make upon them. By such seizures 

 many of the distasteful butterflies doubtless per- 

 ish, and Meldola shows very clearly by mathe- 

 matical analysis that a resemblance between two 

 species so close that the experimental seizures 

 would be divided between them in the ratio of 

 their numbers gives an advantage decidedly in 

 favor of the scarcer species. Or, as Wallace 

 puts it, *' if two species, both equally distasteful, 

 closely resemble each other, then the number of 

 individuals sacrificed is divided between them in 

 the proportion of the squares of their respective 

 numbers." If the rarer species is only one tenth 

 as numerous, it will benefit in the proportion of 

 one hundred to one. 



Exactly the same argument can be applied 

 to examples of mimicry between two species where 

 neither is distasteful. These cases, though less 

 conspicuous, are probably more numerous than 

 those of which we have been speaking ; for, on 

 the principles that we have laid down, any advan- 



