Among the latter, the blind fish of the Mammoth Cave (_Amblyopsis spelacus_) is especially remarkable, because in this being the retrograde development of the organ of vision is accompanied by the production of certain ridges of skin on the body which are endowed with an extreme sensitiveness of touch, and which, according to a work lately published by Professor Von Leydig, are composed of little warts in which the nerve fibers end. Nature, therefore, has in this case compensated the amblyopsis for his loss of sight by endowing him with a highly developed organ of feeling. A similar phenomenon is to be observed in the blind crab (_Cambaras pellucidus_), which is also found in the Mammoth Cave, for in this being, according to Professor Von Leydig, the little warts on the interior feelers, which constitute the organ of smell, have also received an abnormal development. [Illustration: FIG. 1.--THE SEMI-BLIND SALAMANDER (PROTEUS ANGUINEUS) OF THE ADELSBERG GROTTO, IN AUSTRIA.] Better known than the blind fish and the blind crab of Kentucky is the _Proteus anguineus_, a kind of salamander, of a pale rose color, endowed with gills and found in the Adelsberg grotto in Austria. (Fig. 1.) This amphibium has an eye which lies very deep in the body and is almost overgrown by the skin. But this eye is by no means as developed as the organ of vision, for instance, of the water salamander (the triton) or of the so-called axolotl, for it exists only in a kind of embryonic development, and contains neither a vitreous humor nor a lens for the refraction of the rays of light. As, however, the nerve of vision exists, it is possible that this salamander may be able to discern in some manner between light and darkness. The thinking student, when discovering such imperfect organs of sight, will naturally ask how the eye of this salamander, which is so useless for its real purpose, has come into existence, and he will weigh the comparative value of the two following explanations. It may be assumed that there existed once in the Adelsberg grotto a salamander which was absolutely blind, and in which, in consequence of an innate power of evolution, an organ of vision of the lowest kind was gradually formed. But to this assumption the objection may be raised at once, why nature should have produced an organ of vision in an animal living in a grotto, where such an organ is absolutely useless, and where such a development would be quite as paradoxical and improbable as, for instance, the development of fins instead of legs in an animal living on dry land.