THE STRUCTURE AND HABITS OF ANIMALS 37 



superficial, then the red and brown, and the black 

 deepest of all. If the superficial ones contract, 

 the deeper ones will become more apparent, and 

 vice versa. These changes of colour always bear a 

 relation to the surface on which the animal is placed 

 at the time, and are therefore supposed with good 

 reason to be protective. Many attempts have been 

 made to explain this. 



Lister believed that the irritation which excites the 

 action of the chromatophores does not act on them 

 directly, but through the intermediation of the optic 

 nerve. It is essential for this action that the eye 

 and optic nerve should be present and healthy, the 

 action ceasing if the eye is destroyed or the nerve 

 cut. In support of this view he pointed out that 

 blind fish are always dark. Pouchet suggested 

 two possible paths of impulse the spinal cord 

 and the sympathetic system. By dividing the 

 spinal cord the path of impulse was shown to pass 

 along the sympathetic nerves. Again, by dividing 

 some branches of these, he produced zebra-like 

 markings of the skin. Dewar showed that 

 different colours of the spectrum influence the 

 eye differently and cause electric currents of 

 different intensity according to the light employed. 

 These currents are strongest with yellow light, 

 weakest with purple, and nil with black. Semper 

 pointed out that, though electric currents and nerve 

 currents are far from being the same thing, we may 

 not unfairly assume that the nerve current, like the 

 electric current, is greatest with yellow light and nil 

 with black. Thus a black ground, which absorbs 



