NATURAL SELECTION IN ACTION 373 



this model, the birds tended to avoid two other species which mimicked 

 it in markings. These latter two species, however, were completely pal- 

 atable to birds which had not had experience with the unpalatable model. 



Mimicry is a widespread phenomenon among insects. In view of ex- 

 periments such as those cited we may conclude that resemblances evident 

 to human eyes are also effective in deceiving such predators as birds. 

 Hence development of resemblances of this kind will be favored by natu- 

 ral selection. While, as we have seen, some mimicry is remarkably de- 

 tailed, more general resemblances and partial similarities will also have 

 selective value insofar as they reduce the chance of attack by predators. 

 There is experimental evidence (e.g., Brower, 1958c) that birds have some 

 ability to generalize — once having learned to avoid an unpalatable model 

 they will avoid other species that to our eyes have some, but not striking, 

 resemblance to that model. 



So far we have been discussing protective mimicry, mimicry which pro- 

 tects from predators. There is also the possibility that mimicry may be 

 aggressive. If, for example, a predator resembles its prey, that predator 

 may be able to approach its victim more easily than it could otherwise do. 

 Some predatory flies lay their eggs in colonies of bees; when the larval 

 flies emerge they feed upon the immature stages of the bees. If, as is some- 

 times the case, the fly resembles the bees, its entrance into the bee colony 

 to lay its eggs may be facihtated. The bees may not be "suspicious" of the 

 beelike fly. Brower, Brower, and Westcott (1960) have discussed this ques- 

 tion, citing a probable example. In this case the adult fly attacks and feeds 

 upon adult bumblebees. The authors concluded that the resemblance of 

 the fly to the bee, extending even to the tone of its buzz, probably makes 

 capture of the victim easier than it would otherwise be. The authors dem- 

 onstrated that protective mimicry of the Batesian variety is also involved 

 in this instance. Toads which had learned to avoid bumblebees (unpalat- 

 able because of their stings) also avoided the mimicking flies, whereas 

 toads lacking experience with bumblebees usually ate the flies readily. 



Conclusion 



In this chapter we have summarized investigations which demonstrate 

 that protective coloration, including mimicry, does protect. The results 

 demonstrate that when predators utilize the sense of sight, prey organisms 

 which, for example, blend with the background are afforded sufficient 

 protection so that their greater success in survival and reproduction can 

 account for origin of evolutionary changes, as postulated by the theory of 

 natural selection. And in the case of industrial melanism we see differential 



