214 The Mechanistic Conception of Life 



facts which show that a large number of animals assume, to 

 some extent, the color of the ground on which they are placed. 

 Pouchet found through experiments upon crustaceans and fish 

 that this influence of the ground on the color of animals is 

 produced through the medium of the eyes. If the eyes are 

 removed or the animals made blind in another way these 

 phenomena cease. The second general fact found by Pouchet 

 was that the variation in the color of the animal is brought 

 about through an action of the nerves on the pigment cells of 

 the skin; the nerve action beiag induced through the agency 

 of the eye. 



The mechanism and the conditions for the change in colora- 

 tion were made clear through the beautiful investigations of 

 Keeble and Gamble, on the color change in crustaceans. 

 According to these authors the pigment cells can, as a rule, be 

 considered as consisting of a central body from which a system 

 of more or less complicated ramifications or processes spreads 

 out in all directions. As a rule, the center of the cell contains 

 one or more different pigments which under the influence of 

 nerves can spread out separately or together into the ramifica- 

 tions. These phenomena of spreading and retraction of the 

 pigments into or from the ramifications of the pigment cells 

 form on the whole the basis for the color changes under the 

 influence of environment. Thus Keeble and Gamble observed 

 that Macromysis flexuosa appears transparent and colorless 

 or gray on sandy ground. On a dark groimd their color 

 becomes darker. These animals have two pigments in their 

 chromatophores, a brown pigment and a whitish or yellow 

 pigment; the former is much more plentiful than the latter. 

 When the animal appears transparent all the pigment is con- 

 tained in the center of the cells, while the ramifications are free 

 from pigment. When the animal appears brown both pigments 

 are spread out into ramifications. In the condition of maximal 

 spreading the animals appear black. 



