VAKIABLE RESEMBLANCE IN VERTEBRATA, ETC. 89 



yellowish-green in a very short time, the change 

 beginning in a few seconds. At night when it was 

 asleep it became light and straw-coloured. In the 

 winter it died, probably on account of the scarcity 

 and monotony of the only insect diet which could be 

 obtained for it. For many days before its death it 

 became almost black and lost all power of changing its 

 colour. Its weakened nervous system either ceased to 

 respond to the influence of light, or was unable to pro- 

 duce any effect upon the pigment cells, which were thus 

 paralysed, with their pigment permanently diffused. 

 Green frogs also generally become dark before they die. 



Explanation of darkness of blind animals 



Some authorities have maintained that an animal 

 of a kind which possesses the power of altering its 

 colour should, when blind, become light- instead of 

 dark-coloured. When the skin is light-coloured the 

 pigment in the cells is strongly contracted, so that the 

 coloured surface contributed by each cell occupies but a 

 small space, and produces but little effect ; when the skin 

 is dark the coloured parts of the cells are relaxed, and 

 stretch out into the long branching processes, so that 

 each dark surface becomes as large as possible. The 

 latter is evidently the condition of rest, while concen- 

 tration is the state of activity. It is therefore to be 

 expected that when the coloured parts of cells are 

 cut off from all stimuli, they will be permanently 



