72 "MIMICRY RINGS" [ch. 



process called regression. This idea involves certain 

 conceptions as to the nature of variation which we 

 shall discuss later. 



Though it is difficult to regard Batesian mimicry 

 as produced by the accumulation of small variations 

 through natural selection, it is perhaps rather more 

 plausible to suppose that such a process may happen 

 in connection with the numerous instances of Miillerian 

 mimicry. For since the end result is theoretically to 

 the advantage of both species instead of but one, it 

 is possible to argue that the process would be sim- 

 plified by their meeting one another halfway, as Miiller^ 

 himself originally suggested. Variations on the part 

 of each in the direction of the other would be favourably 

 selected, the mimicry being reciprocal. 



Difficulties, however, begin to arise when we inquire 

 into the way in which this unification of pattern may 

 be conceived of as having come about. By no one 

 have these difficulties been more forcibly presented 

 than by Marshall^ in an able paper published a few 

 years ago, and perhaps the best way of appreciating 

 them is to take a hypothetical case used by him as 

 an illustration. 



Let us suppose that in the same area live two equally 

 distasteful species A and B, each with a conspicuous 

 though distinct warning pattern, and each sacrificing 

 1000 individuals yearly to the education of young 



^ An English translation of Miiller's paper is given by Meldola, 

 Proc. Ent. Soc, 1879, p. xx. 



2 Trans. Ent. Soc. Lond., 1908, p. 93. 



