148 The Outline of Science 



than elusive, and the theory of Alfred Russel Wallace was that 

 the flaunting conspicuousness serves as a useful advertisement, 

 impressing itself on the memories of inexperienced enemies, who 

 soon learn to leave creatures with "warning colours" alone. In 

 any case it is plain that an animal which is as safe as a wasp or a 

 coral-snake can afford to wear any suit of clothes it likes. 



Masking 



The episode in Scottish history called "The Walking Wood 

 of Birnam," when the advancing troop masked their approach by 

 cutting down branches of the trees, has had its counterpart in 

 many countries. But it is also enacted on the seashore. There 

 are many kinds of crabs that put on disguise with what looks like 

 deliberateness. The sand-crab takes a piece of seaweed, nibbles 

 at the end of it, and then rubs it on the back of the carapace or on 

 the legs so that it fixes to the bristles. As the seaweed continues 

 to live, the crab soon has a little garden on its back which masks 

 the crab's real nature. It is most effective camouflaging, but if 

 the crab continues to grow it has to moult, and that means los- 

 ing the disguise. It is then necessary to make a new one. The crab 

 must have on the shore something corresponding to a reputation ; 

 that is to say, other animals are clearly or dimly aware that the 

 crab is a voracious and combative creature. How useful to the 

 crab, then, to have its appearance cloaked by a growth of inno- 

 cent seaweed, or sponge, or zoophyte. It will enable the creature 

 to sneak upon its victims or to escape the attention of its own 

 enemies. 



If a narrow-beaked crab is cleaned artificially it will proceed 

 to clothe itself again, the habit has become instinctive; and it 

 must be admitted that while a particular crab prefers a particular 

 kind of seaweed for its dress, it will cover itself with unsuitable 

 and even conspicuous material, such as pieces of coloured cloth, 

 if nothing better is available. The disguise differs greatly, for 

 one crab is masked by a brightly coloured and unpalatable sponge 



