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 
-Tesult 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 
