iv INFLUENCE OF NERVES ON COLOUR 143 



among the causes of the modification of colours in animals, 

 since the influence of its action in this respect is well known. 

 But it is clear that this is in most cases only an indirect 

 influence determined by other causes, especially light and 

 warmth, so far as it affects the question of the permanent 

 modification of animal forms. 



The nerves may influence the colouring of the skin in two 

 ways : either by changing the supply of blood, and the de- 

 position of pigment depending upon it that is, indirectly, or 

 directly by changing the shape of the pigment -cells. The 

 first action depends more upon temperature, the second more 

 upon light. Warmth increases the flow of blood to the skin, 

 cold diminishes it the former by dilating, the latter by con- 

 tracting the blood-vessels; both, however, must act directly by 

 expanding and contracting the pigment-cells, like illumination. 



Finally, electric stimulation, or division of nerves, acts 

 upon the colour of the skin. After removal of the cerebellum 

 in a frog the skin becomes strikingly variegated. As excite- 

 ment produces temporary paleness or redness in the human 

 skin, and as irritable, malicious persons are always distin- 

 guished by a pale yellowish complexion, in consequence of 

 constant excitement of the nervous system, so the colours of 

 animals vary with excitement, sometimes to a striking 

 degree. I may mention the familiar case of the chameleon, 

 which when angry and after death turns yellow. The tree- 

 frog also alters in colour in captivity, apparently through 

 nervous excitement, as is usually said from discontent. 



But in the latter case the direct influence of the colour of 

 the surroundings must evidently also be taken into account, 

 for the tree-frog, like many other animals, assimilates its 

 colour to that of its temporary environment. 1 



It is well known that many other animals likewise change 



1 This process is usually described by the inappropriate expression : sympa- 

 thetic colouring. I would substitute for this : stimulation-colouring. 



