256 THE EFFECT OF TEMPERATURE 



trated by another quotation from Lord Lister's paper: 

 " A frog caught in a recess in a black rock was itself al- 

 most black; but after it had been kept for about an hour 

 on white flagstones in the sun, was found to be dusky 

 yellow, with dark spots here and there. It was then 

 placed in the hollow of the rock, and in a quarter of an 

 hour had resumed its former darkness. These effects 

 are independent of changes of temperature." These 

 changes of colour have been shown by Briicke, von Wit- 

 tich, Lister, and others to be due to the pigment gran- 

 ules of certain stellate cells in the skin varying in their 

 degree of concentration towards the centre of the cell, 

 and in their diffusion peripherally through the branch- 

 ing processes. These pigment cells are often of differ- 

 ent colours and are arranged in layers, so that widely 

 different effects may be produced by varying degrees of 

 concentration in them. 



That the reflex mechanism takes its origin in the eye, 

 which is stimulated by the light reflected from the ani- 

 mals' surroundings, was proved by Lord Lister in the 

 following manner: He found that a frog with its 

 eyes removed was totally unaffected by the colour 

 of its surroundings. The nervous system still re- 

 tained the capacity for acting on the pigment cells, 

 however, as the frog, originally dark, became ex- 

 tremely pale after struggling violently to escape. 

 It was then placed in a bright light, but within 

 half an hour became almost coal black again. Occa- 

 sionally protectively coloured animals are found in 

 nature showing a total want of adjustment to the colour 

 of their surroundings. Thus Pouchet* noticed that one 

 * Quoted by Semper, " Animal Life," p. 95. 



