The Struggle for Existence 143 



talion of these sleeping prawns, and if we turn the motley 

 into a dish and give a choice of seaweed, each variety after 

 its kind will select the one with which it agrees in colour, and 

 vanish. Both when young and when full-grown, the ^Esop 

 prawn takes on the colour of its immediate surroundings. 

 At nightfall Hippolyte, of whatever colour, changes to a 

 transparent azure blue : its stolidity gives place to a nervous 

 restlessness; at the least tremor it leaps violently, and often 

 swims actively from one food-plant to another. This blue 

 fit lasts till daybreak, and is then succeeded by the prawn's 

 diurnal tint. 



Thus, Professor Gamble continues, the colour of an animal may 

 express a nervous rhythm. 



The Case of Chameleons 



The highest level at which rapid colour-change occurs is 

 among lizards, and the finest exhibition of it is among the chame- 

 leons. These quaint creatures are characteristic of Africa; but 

 they occur also in Andalusia, Arabia, Ceylon, and Southern In- 

 dia. They are adapted for life on trees, where they hunt insects 

 with great deliberateness and success. The protrusible tongue, 

 ending in a sticky club, can be shot out for about seven inches in 

 the common chameleon. Their hands and feet are split so that 

 they grip the branches firmly, and the prehensile tail rivals a mon- 

 key's. When they wish they can make themselves very slim, con- 

 tracting the body from side to side, so that they are not very read- 

 ily seen. In other circumstances, however, they do not practise 

 self-effacement, but the very reverse. They inflate their bodies, 

 having not only large lungs, but air-sacs in connection with them. 

 The throat bulges ; the body sways from side to side ; and the crea- 

 ture expresses its sentiments in a hiss. The power of colour- 

 change is very remarkable, and depends partly on the contraction 

 and expansion of the colour-cells (chromatophores) in the under- 

 skin (or dermis) and partly on close-packed refractive granules and 

 crystals of a waste-product called guanin. The repertory of pos- 



