THE BACKGROUND OF DARWINISM— ADAPTATIONS 203 



nauseous if eaten, still others, like the skunk and his tribe, that pro- 

 duce offensive odors, have well-defined markings that are classed as 

 examples of warning coloration. 



Immunity coloration. — Professor Reighard has taken exception 

 to the interpretation of conspicuous coloration as warning adaptations. 

 His theory of "immunity coloration" furnishes an alternative interpre- 

 tation which appears less open to criticism. According to this idea 

 well-protected animals are relatively safe from attack and therefore 

 may become conspicuous without endangering themselves. They are 

 immune and therefore their conspicuous markings may be merely the 

 result of color run riot without any check on the part of natural 

 selection. 



Mimicry. — This is a special type of protective coloration in which 

 an otherwise defenseless species has a striking resemblance to some 

 well-protected species with warning coloration. Thus wasps and bees 

 are "mimicked" by flies, beetles, and moths. They enhance the 

 resemblance by similarities of behavior and habitat. Ants are pro- 

 tected by the fact that they contain formic acid, which is distasteful 

 to most animals (though some animals feed very largely on them). 

 Consequently there are many ant mimics belonging to several orders 

 of insects and to spiders. In some cases these ant mimics are inhabit- 

 ants of ant colonies and succeed in passing themselves off as ants even 

 among the ants themselves. The classic cases of mimicry, however, 

 are those in which certain species of edible butterflies are said to 

 mimic other, unrelated, nauseous species of butterflies. 



It is very difficult to distinguish the model from the mimic except 

 by careful anatomical examination which, of course, could not be 

 applied in nature. It should be said about mimicry, however, that it 

 would work only in case the mimic occurs in much smaller numbers 

 than the model, and that the two species occupy the same regions at 

 the same time. Some critics have claimed that these conditions do 

 not prevail in all cases. If their contention is valid, the usual expla- 

 nations of mimicry need revision. Cases of aggressive mimicry are 

 noted among certain predaceous animals, such as spiders which mimic 

 flies and ants, and are therefore able to approach their prey more 

 effectively. 



Signals or recognition marks. — The common cotton-tail rabbit 

 raises its white tail when it runs. This is interpreted as a signal of 

 danger to other rabbits. Some antelopes have a conspicuous white 

 rump which is supposed to be a danger signal. Many distinct specific 



