Colour Change 



in turn is connected with the central nervous system of the 

 animal. Therefore the cells can act in harmony and as the result 

 of a common stimulus. This stimulus may be brought to the cells 

 by reflex action from outside influences (such as light and heat) 

 acting directly on the skin, or by outside influences (such as the 

 colour of the surroundings) acting through the eye.* Or, on the 

 other hand, the stimulus may be sent from within the animal by 

 reflex action, and may have its origin in the character of the 

 food or some item of the physiological condition of the creature. 



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Fig. II. When the pigment cells send out interlacing branches 

 the resulting color is dark. After Lister. 



It is easily proved that certain definite stimuli bring about 

 definite results. Bright light or heat causes the cells to contract 

 and the skin to become lighter in colour. Darkness or cold causes 

 expansion of the cells and resulting dark colour of the skin. Thus 

 hibernating frogs are dark-coloured; the same frogs sitting in 

 bright light are so light-coloured that they would seem a different 

 species to the casual observer. Absence or presence of moisture, 

 other conditions being equal, seems also to play a part in causing 

 the pigment cells to contract or expand. Toads kept in a dry 



* Blind specimens change colour much less rapidly than do those that are normal. 



23 



