ADAPTIVE COLORATION 187 



species may have arrived at a general resemblance to another group 

 without having as yet acquired a likeness to any particular species of 

 the group, the general likeness meanwhile being profitable. 



The second condition named by Wallace is correct for Batesian but 

 not for Miillerian mimicry . . 



The fulfilment of the third condition is requisite for the success of 

 Batesian mimicry. Bates noted that none of the pierid mimics were so 

 abundant as their heliconiid models. If they were, their protection 

 would be less; and should the mimic exceed its model in numbers, the 

 former would be more subject to attack than the latter. Sometimes, 

 indeed, as Miiller found, the mimic actually is more common than the 

 model; in which event, the consequent extra destruction of the mimic 

 would at least theoretically reduce its numbers back to the point of 

 protection. 



In Miillerian mimicry, however, the inevitable variation in abundance 

 of two or more converging and protected species is far less disastrous; 

 though when two species, equally distasteful, are involved, the rarer of 

 the two has the advantage, as Fritz Muller has shown. His lucid ex- 

 planation is essentially as follows: 



Suppose that the birds of a region have to destroy 1,200 butterflies 

 of a distasteful species before it becomes recognized as such, and that 

 there exist in this region 2,000 individuals of species A and 10,000 of 

 species B; then, if they are different in appearance, each will lose 1,200 

 individuals, but if they are deceptively alike, this loss will be divided 

 among them in proportion to their numbers, and A will lose 200 and B 

 1,000. A accordingly saves 1,000, or 50 per cent, of the total number 

 of individuals of the species, and B saves only 200, or 2 per cent. Thus, 

 while the relative numbers of the two species are as i to 5, the relative 

 advantage from their resemblance is as 25 to i. 



If two or more distasteful species are equally numerous, their re- 

 semblance to one another brings nearly equal advantages. In cases of 

 this kind and many are known it is sometimes impossible to dis- 

 tinguish between model and mimic, as all the participants seem to have 

 converged toward a common protective appearance, through an inter- 

 change of features the "reciprocal mimicry" of Dr. Dixey. 



Marshall argues, however, against this diaposematism, maintaining 

 that in the case of two participants in Miillerian mimicry the evolution 

 of the mimetic pattern has been in one direction only toward the more 

 abundant species any variations in the opposite direction being dis- 

 advantageous. 



