52 The Study of Animal Life parti 



6. Rapid Change of Colour. — For ages the chamaeleon 

 has been famous for its rapid and sometimes striking 

 changes of colour. The members of the Old World 

 genus Chajnaleo quickly change from green to brown 

 or other tints, but rather in response to physical irrita- 

 tion and varying moods than in relation to change of 

 situation and surrounding colours. So the American 

 " chamseleons " {Atiolts) change, for instance, from emerald 

 to bronze under the influence of excitement and various 

 kinds of light. Their sensitiveness is exquisite ; " a pass- 

 ing cloud may cause the bright emerald to fade." Some- 

 times they may be thus protected, for " when on the broad 

 green leaves of the palmetto, they are with difficulty per- 

 ceived, so exactly is the colour of the leaf counterfeited. 

 But their dark shadow is very distinct from beneath." Most 

 of the lizards have more or less of this colour-changing 

 power, which depends on the contraction and expansion 

 of the pigmented living matter of cells which lie in layers 

 in the under-skin, and are controlled by nerves. 



In a widely different set of animals — the cuttle-fishes — 

 the power of rapid colour-change is well illustrated. When 

 a cuttle-fish in a tank is provoked, or when one almost 

 stranded on the beach struggles to free itself, or, most 

 beautifully, when a number swim together in strange unison, 

 flushes of colour spread over the body. The sight suggests 

 the blushing of higher animals, in which nervous excitement 

 passing from the centre along the peripheral nerves influ- 

 ences the blood-supply in the skin ; but in colour-change the 

 nervous thrills affect the pigment-containing cells or chroma- 

 tophores, the living matter of which contracts or expands 

 in response to stimulus. It must be allowed that the colour- 

 change of cuttle-fish is oftenest an expression of nervous 

 excitement, but in some cases it helps to conceal the 

 animals. 



More interesting to us at present are those cases of 

 colour- change in which animals respond to the hues of 

 their surroundings. This has been observed in some 

 Amphibians, such as tree-frogs ; in many fishes, such as 

 plaice, stickleback, minnow, trout, Gobius ruthensparri. 



