perceive the chlorific and chemic rays, that is to say, the ultra-red and ultra-violet ones, which do not affect our retina? Bert's experiments enable us to answer that they do not. That physiologist is even led to assert that, with regard to light and the different rays, all animals experience the same impressions that man does.
Let us now look at the influence of light upon the color of the skin in animals, noticing first the being which presents the strangest peculiarities in this respect, the chameleon. This animal, indeed, experiences very frequent modifications of color in the course of the same day. From Aristotle, who attributed these changes to a swelling of the skin, and Theophrastus, who assigned fear as their cause, to Wallisniéri, who supposes them to result from the movement of humors toward the surface of the animal's body, the most different opinions have been expressed on this subject. Milne-Edwards, thirty years ago, explained them by the successive inequalities in the proportions of the two substances, one yellowish and the other violet, which color the skin of the reptile, inequalities due to the changes in volume of the very flattened cells that contain these substances. Bruck, renewing these researches, proves that the chameleon's colors follow from the manifold dispersion of solar light in the colored cells, that is to say, from the production of the same phenomenon remarked in soap-bubbles and all very thin plates. Its colors, then, come from the play of sun-light among the yellow and violet 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, determined 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.
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. The gold-and-azure dust that adorns them harmonizes with the tones of colors in surrounding Nature. Night-birds, in the same way, have dark plumage, and the downiness of their coverings contrasts with the stiffness of 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 the torrid zone, is dazzling in its splendor. At the north we find gray tints, dead and of little variety, usually close upon white, by reason of the almost constant reflection from snow.