VI] "MIMICRY RINGS" 73 



birds. Further let it be supposed that ^ is a common 

 species of which there are 100,000 individuals in the 

 given area, while B is much rarer, and is represented by 

 5000. The toll exacted by young birds falls relatively 

 more lightly upon A than upon B, for A loses only 

 1 %, whereas 5's loss is 20 %. Clearly if some members 

 of B varied so that they could be mistaken for A it 

 would be greatly to their advantage, since they would 

 pass from a population in which the destruction by 

 young birds was 20 % to one in which it would now be 

 rather less than 1 %. Moreover, as the proportion of 

 B resembling A gradually increased owing to this advan- 

 tage, the losses suffered by those exhibiting the original 

 B pattern would be relatively heavier and heavier until 

 the form was ultimately eliminated. In other words, 

 it is theoretically conceivable that of two distasteful 

 species with different 2:)atterns the rarer could be 

 brought to resemble the more abundant. 



We may consider now what would happen in the 

 converse case in which the more numerous species 

 exhibited a variation owing to which it was confused 

 with the rarer. Suppose that of the 100,000 individuals 

 of A 10,000 shewed a variation which led to their being 

 mistaken for B, so that there are 90,000 of the A 

 pattern and 15,000 of the B pattern of which 10,000 

 belong to species A. A will now lose 1000 out of the 

 90,000 having the A pattern, and | x 1000 out of the 

 10,000 of species A which exhibit the B pattern. The 

 toll of the birds will be J^ of those keeping the original 

 A pattern, and ^-^ of those of species A which have 



