The mole, far back in the past, had eyes, and gained its livelihood above ground in the broad light of day; but, owing to some change in its surroundings, it was forced to burrow beneath the surface of the earth; consequently its organs of sight have degenerated, and are now practically worthless as far as _vision_ is concerned. All moles, however, can tell darkness from light, consequently, are not wholly blind--a certain amount of _sight_ remains. This is due to the fact that, although the optic nerve, on examination, is invariably found to be atrophied or wasted, there yet remain in the shrivelled nerve-cord true nerve-cells; these nerve-cells transmit light impressions to the brain. Even if the optic nerves, and, in fact, all of the structures of the eye, were absent, I yet believe that the mole could differentiate between daylight and darkness. The sensitive tufts and filaments of nerve in the skin, undoubtedly, in many instances, respond to the stimulation of light, so that totally blind animals, animals with no rudimentary organs of vision whatever, and the inception of whose ancestors, themselves wholly blind, probably took place thousands of years ago, show by their actions that light is exceedingly unpleasant to them. Thus, I have seen actinophryans taken from the River Styx in Mammoth Cave (which is their natural habitat), seeking to hide themselves beneath a grain of sand which happened to be drawn up in the pipette and dropped upon the glass slide beneath the object-glass of my microscope. I have repeatedly seen the blind fish of Mammoth Cave seeking out the darkest spots in aquaria. In point of fact, I think it can be demonstrated that light is directly fatal to these fishes; they soon die when taken from the river and placed in aquaria where there is an abundance of light. These fish, although they have rudimentary eyes, never have the slightest remaining trace of nerve-cells in the wasted optic nerve (that is, I have never been able to discover any), thus showing that their appreciation of light is not derived through the agency of their eyes. An eyeless spider (_Anthrobia_) taken from the same cavern showed a like distaste for light, and yet, in this insect, there is absolutely no vestige of an eye or its nerves. Finally, a friend of mine, a youth of eighteen, totally blind since birth, can differentiate between daylight and darkness. On one occasion I carefully blindfolded him and led him into the well-lighted office of a brewery (he had never been in a brewery before), and asked him if it were light or dark. He answered that it was almost as light as day. I then conducted him into the dark beer vaults, and as soon as he passed the door he exclaimed, "How cold and dark it is here!" Thinking that he might possibly associate darkness with coldness, I asked him if this were the case. "No," he replied, "I _see_ the darkness and I _feel_ the cold; they are not the same."