In these animals--and I include man--continuous darkness has modified sensibility (sense of touch) to such an extent that it has partially taken on the functions of the useless organs--the eyes; these creatures _see_ with their skins. I do not believe that there is a creature in existence to-day, whether it has eyes or not, which cannot tell the difference between night and day. Professor Semper says that in the Pelew Islands he found a small fresh-water creature, whose generic name is _Cymothoe_, in pools where daylight penetrated, that was absolutely blind.[6] We have fresh-water Cymothoe living in our own waters that are close kin to the Pelew islander mentioned by Semper, and which are not blind. Along the middle of their backs, over the edge of each segment, there is an oblong dark spot. This little collection of coloring-matter is covered by a transparent membrane, the cornea, and has a special nerve leading to the brain, if I may use the word. These spots are primitive eyes, the analogues of which are preserved by many of the true worms. I am inclined to believe that Semper would find primitive eyes of some form or other in the Cymothoe he mentions, if he were again to examine it. The insects, etc., which dwell in caves, and which have eyes, are new arrivals; they have not dwelt long enough in total darkness to have experienced the full effects of changed surroundings. They show, however, that they are beginning to feel such effects, for there is more or less diminution in the color-cells of the eyes and body coverings. My experiments on fish and frogs show, conclusively, that the color-producing function is directly due to light stimulation. The longer fish and frogs are kept in total darkness, the lower is the number of color-cells and the smaller is the amount of coloring-matter. This accounts for the fact that all animals which have dwelt in darkness for untold ages are absolutely colorless. Pigmented or colored fishes, nevertheless, having well-developed organs of vision, have been taken from such depths (over a mile) as to preclude the possibility of a single ray of daylight.[7] These fishes, however, are phosphorescent, and thus furnish their own light. Moreover, I am inclined to believe that the vast depths of the ocean, in certain localities, lie bathed in a continuous phosphorescent glow, so that creatures living there neither lose their color nor their eyes, sufficient light being present to prevent degeneration. Where eyeless and colorless fishes are brought up from great depths, there the ocean is not phosphorescent, but is in absolute darkness. [6] Semper, _Animal Life_, p. 83. [7] Hickson, _The Fauna of the Deep Sea_, p. 150 _et seq._ The preceding observations indicate that the sense of sight is a very old sense, and that it is to be found in a primitive form (ocelli) in animals of exceedingly low organization. That this is true, I will now attempt to demonstrate.