EFFACING COLORATION 183 



projecting ridges, the flanks, the belly, and the face, 

 and throws up the animal against the background. 

 A rough clay-bird model is, owing to these lines of 

 rotundity, clearly visible a long way off. But if we 

 balance the shadow by painting those parts white 

 in proportion to the depth of shadow, the result is 

 an effacing gradation, i.e. the white breast melting 

 gradually into the flanks. Such a bird is invisible at 

 a few yards. 



The same principle that applies to the mass 

 applies also to the shading of separate patches of 

 colour. A striped creature is thus rendered trans- 

 parent, as it were, when at rest, and a grey, evasive 

 patch when in motion. 



But there is no need to search beyond our own 

 country for examples of sympathetic colouration 

 among land animals. Insects carry all their under- 

 takings to a high degree of perfection, and there are 

 no closer resemblances between animals and their 

 surroundings than are to be met with amongst cater- 

 pillars. The stick-caterpillars, for example, in form, 

 posture, and colouring resemble so closely the branches 

 or thorns of their food-plant that they can only be 

 distinguished with great difficulty. Clasping a twig 

 with the strong hand, into which the tail is modified, 

 and supporting the head by a transparent girdle, 

 they extend the body at the proper angle, stiff and 

 straight. 



Their skin, dark and gnarled, resembles to a nicety 

 the bark of the surrounding twigs ; and the resemblance 



