vi] "MIMICRY RINGS" 73 



birds. Further let it be supposed that A 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 patterns 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 f X 1000 out of the 

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

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

 A pattern, and 3% of those of species A which have 



