iv SHIFTS FOR A LIVING 57 



and when adult, and can change from one colour to 

 another with ease. It is often difficult to detect among 

 the brightly coloured seaweeds. 



7. Special Protective Resemblance.- -The likeness be- 

 tween animals and their surroundings is often very pre- 

 cise, and includes form as well as colour. Thus some 

 bright butterflies, e.g. Kallima, are conspicuous in flight, 

 but become precisely like brown withered leaves when 

 they settle upon a branch and expose the under sides of 

 their raised wings ; the leaf-insects (Phyllium) have leaf- 

 like wings and legs ; the " walking-sticks ' (Phasmidse), 

 with legs thrown out at all angles, resemble irregular 

 twigs ; many caterpillars (of Geometra moths especially), 

 sit motionless on a branch, supported in a strained atti- 

 tude by a thin thread of silk, and exactly resemble twigs ; 

 others are like bark, moss, or lichen. Among caterpillars 

 protective resemblance is very common, and Prof. Poulton 

 associates its frequent occurrence with the peculiarly 

 defenceless condition of these young animals. The 

 body is a tube which contains fluid under pressure ; a 

 slight wound entails great loss of blood, while a moderate 

 injury must prove fatal." " Hence larvae are so coloured 

 as to avoid detection or to warn of some unpleasant 

 attribute, the object in both cases being the same to 

 leave the larva untouched, a touch being practically 

 fatal." Among backboned animals we do not expect to 

 find many examples of precise resemblance to surround- 

 ing objects ; but one of the sea-horses (Phyllopteryx 

 eques) is said to be exceedingly like the seaweed among 

 which it lives. Organisms are often variable in shape 

 and coloration, and it is theoretically conceivable that 

 in conditions of life where the struggle for existence was 

 keen and likewise subtle, variants in the direction of the 

 garment of invisibility may have defined the surviving 

 type, while variants in the direction of conspicuousness 

 were speedily eliminated year after year, decade after 

 decade, century after century, as they cropped up. It 

 should be noted that some of the striking protective 

 resemblances are little more than exaggerations of 

 peculiarities of form and colouring which are seen in 



