THE INSTINCTS OF ANIMALS 145 



I drew attention to the ' wood-moth ' (Xyllna), which resembles a 

 broken fragment of half-decayed wood so deceptively, and I pointed 

 out that the colour-resemblance to wood would be in itself of but 

 little use to the insect if it were not combined with the instinct to 

 remain motionless in danger, to ' feign death.' The antennte and legs 

 are drawn close to the body, so that they rather heighten the dis- 

 guise, and, instead of running away, the insect does not move a muscle 

 until the danger is past. This instinct must have evolved hand in 

 hand with the resemblance to a piece of wood, and, just as we sought 

 to interpret the latter from the fact that the moths which most 

 resembled the wood had always the best chance of surviving, so we 

 maintain that those moths would profit most by their resemblance 

 which drew in their legs and antennae closely and lay most perfectly 

 still. Thus the brain-mechanism, which effected the keeping still 

 whenever the senses announced danger, would be more and more 

 firmly established and perfected in the course of selection. 



Even nearly related animals may have quite difierent instincts 

 which secure them against danger. Thus in the group of pocket 

 crabs (Notopoda) there are some species which run away when 

 danger threatens, but others which anticipate the risk of discovery 

 by masking themselves to a certain extent. With the last pair of 

 legs they hold over themselves a large piece of sponge, which then 

 grows till it often leaves only the limbs and frontal region uncovered. 

 Of course there can be no question of consciousness in what the crab 

 does, as is proved by the fact that these crabs will, in case of neces- 

 sity, take a transparent piece of glass instead of the sponge ; but the 

 impulse to cover themselves with something is strong in them, and 

 finds expression not only when they see a really protective substance, 

 but even when they see one which is transparent and therefore 

 wholly useless for the purpose. Crabs from which the sponge has 

 been taken away wander about until they find another ; the impulse 

 is thus set up not only by the sight of the sponge or of a stone, but 

 also by the feeling that their back is uncovered. The large spider- 

 crab of the Mediterranean (Maja squinado) effects its disguise in 

 a somewhat different manner. It has peculiar hooked bristles on the 

 back, and on these it hooks little bunches of seaweed, often many of 

 them, so that it is entirely covered and looks like a bunch of wrack 

 rather than like an animal. Here again a bodily variation has gone 

 hand in hand with the development of the instinct to cover itself: 

 the bristles of the back have become hooked. Many instincts are 

 accompanied by structural modifications, and in the crabs which cover 

 themselves with sponge or stone this is the case, for the last pair of 



