LIGHT AND LIFE. 121 
come from the play of sunlight among the yellow and vio- 
let substances distributed very curiously under its wrinkled 
skin. It passes from orange to yellow, from green to blue, 
through a series of wavering and rainbow-like shades, de- 
termined by the state of the light’s radiation. Darkness 
blanches it, twilight gives it the most delicate marbled 
tints, the sun turns it dark. A part of the skin bruised or 
rubbed remains black, without growing white in the dark. 
Bruck satisfied himself, moreover, that temperature does 
not affect these phenomena. 
The influence of light and of the surrounding color on 
the tint of fish and shell-fish was long ago observed. These 
creatures change their shade with that of the bottom they 
live on. Georges Pouchet, a late student of these phenom- 
ena, found that in such a case light does not act on the 
skin directly, but on the retina of the eye, which, through 
the great sympathetic nerve, transmits the modifying in- 
fluences of luminous vibrations from without to the colored 
cells of the epidermis. Turbots, for instance, placed on a 
white and a black bottom alternately, become dark or light. 
But, if their eyes are put out, they do not change color. 
All animals having fur or feathers are darker and more 
highly colored on the back than on the belly, and their 
colors are more intense in summer than in winter. Night- 
butterflies never have the vivid tints of those that fly by 
day, and among the latter those of spring have clearer, 
brighter shades than the autumn ones. Night-birds, in the 
same way, have dark plumage, and the downiness of their 
coverings contrasts with the stiffness of that in those that 
fly by day. Shells secluded under rocks wear pale shades, 
compared with those that drink in the light. We have 
spoken above of cave-animals. What a distinction between 
those of cold regions and those of equatorial countries! 
The coloring of birds, mammals, and reptiles, peopling the 
vast forests or dwelling on the banks of the great rivers in 
