PROTECTIVE COLORATIOX. 133 



the present state of our knowledge be explained in this way. 

 There is no obvious relation, for example, between the green 

 colour of the Iguana or Tree Snake and its food. And it is 

 still less easy to explain the wonderfully close likeness between 

 the larvEe of certain Geometers and the twigs of their food 

 plant, and between the spider discovered by Mr. Forbes and 

 the droppings of a bird. 



To explain these phenomena by natural selection demands, 

 of course, as an essential preliminary, that the resemblances 

 should be useful. Admitting for the moment that this is 

 proved, we have still the apparent objection that comparatively 

 few creatures have succeeded in availing themselves of this 

 means of protection. Mr. Poulton has grappled with this 

 difficulty, and has suggested* that "the antagonistic principle 

 would be found in the too complete success of the method 

 itself." If all animals acquired a perfect resemblance to their 

 surroundings, the senses of their enemies would have to be 

 proportionately sharpened, or they would get no food. 



Combination of Many Methods of Defence. 



Mr. Poulton quotes the Puss Moth caterpillar as being well 

 defended from its enemies in more than one way. This cater- 

 pillar (fig. 11), being green, is inconspicuous ; but when it is 

 discovered it retracts the head, the effect being " an intensely 

 exaggerated caricature of a vertebrate face, which is probably 

 alarming to the vertebrate enemies of the caterpillar." Besides 

 this, it can eject, from glands near the mouth, a liquid which 

 has been shown to be a mixture of formic acid and water, and 

 is therefore naturally irritant ; it is also stated by De Geer 

 (quoted by Kirby and Spence) that this insect can bite very 



* Mr. Poulton has iised this argument in relation to warning colours, 

 but it must hold good in the present case if it does in the other. 



