Chromatic function. 419 



Red-admiral butterfly is not so affected. The colors of the epidermal 

 products, as the chitinized skin of insects, also hairs and feathers, are 

 often rich and deep and do not fade away after death. They may be 

 considered the ripened products of the soft pigment la} r er below the epi- 

 dermis. In this la} y er the colors are lighter and more vivid and usually 

 fade after death. 1 



Green the green of plants, must be the color that solar action alone 

 would develop in an organism, the other colors, white, black, &c. , be- 

 ing deteriorations from this. Copper-colored people retain the primitive 

 coloring most nearly, while the yellow races are next. In the white and 

 black the normal coloring is lost. In black the unassimilated (unburnt) 

 carbon carried to the sweat glands in the blood and not oxidized there, 

 leaves the pigment coat- and its products ( hair ) of that color. 



CHAPTER XLIV. 



CHROMATIC- FUNCTION. 



Certain fish, as the fresh water Stickleback, Perch, Salmon, &c. , 

 change color to correspond with the color of their surroundings, a cir- 

 cumstance which makes them harder to be seen, and thus protects them. 

 The power to make this change is called the chromatic function. The 

 coloring matter, or pigments, in the skin of these animals, is found to 

 lie in several strata at different depths. In the epidermis are pigment 

 cells, which remain always the same size and so do not directly change 

 the color of the animal. But in the cutis below are usually two or 

 three layers of pigment cells, the top one immediately under the epider- 

 mis is a layer of light-colored yellow cells, beneath them red or brown, 

 and in the deepest layer the black. < < In some spots the pigment cells, 

 of one kind or the other, may be wholly wanting, sometimes the black 

 ones form a close mass in one spot, while in others the red or yellow 

 predominate, but very few spots are devoid of pigment altogether. " 

 These pigment cells, like all protoplasm, are endowed with contractility 

 under stimulus, and it is to this contractility that the change of color is 

 due. These cells are called chromatophores. If they are all relaxed, 

 brown or black will predominate. If the light colored ones remain re- 

 laxed while the dark ones are contracted, the pattern will be lighter, &c. 

 It is now ascertained that the change of color is effected by the color of 

 the medium in which the animal is, through the eye and optic nerve. 

 If tho nniniMl is blind, or the optic nerve is injured, the change of color 

 does not take place. The irritation causing the contraction of the chro- 

 matophores is found to proceed from the brain by way of the sympa- 

 1 Sup. Pop. Sci. Mo., I to VI, p. 630. 



