74 "MIMICRY RINGS" [ch. vi 



assumed the B pattern. The mortality among the 

 mimetic members of A is six times as great as among 

 those which retain the type form. It is clear therefore 

 that a variation of A which can be mistaken for B is 

 at a great disadvantage as compared with the type 

 form^, and consequently it must be supposed that the 

 Miillerian factor, as the destruction due to experi- 

 mental tasting by young birds is termed, cannot bring 

 about a resemblance on the part of a more numerous 

 to a less numerous species. Further, as Marshall goes 

 on to shew, there can be no approach of one species to 

 the other when the numbers are approximately equal. 

 A condition essential for the establishing of a mimetic 

 resemblance on Miillerian lines, no less than on Bate- 

 sian, is that the less numerous species should take on 

 the pattern of the more numerous. Consequently the 

 argument brought forward in the earlier part of this 

 chapter against the establishing of such a likeness by 

 a long series of slight variations is equally valid for 

 Miillerian mimicry^. 



1 Provided of coiirse that the type form remains in the majority. 

 If the variation occurred simultaneously in more than 50 % of ^ the 

 advantage would naturally be with the variation. 



^ It is possible to imagine an exceptional case though most lonlikely 

 that it would occur. Suppose for example that there were a number 

 of distasteful species, say 20, all of different patterns, and suppose 

 that in all of them a particular variation occiirred simultaneously ; 

 then if the total shewing that variation from among the 20 species were 

 greater than the nvunber of any one of the species, all of the 20 species 

 would come to take on the form of the new variation. In this way it is 

 imaginable that the new pattern wovild gradually engulf all the old ones. 



