On the other hand, one may suppose, and this is the more probable explanation, that the _Proteus anguineus_ is descended from a kind of salamander, which possessed perfectly developed eyes in the beginning, and that the imperfect organ of vision in the descendants living in the dark caves is the result of gradual degeneration. This is the more likely to be true as in many other cases, also, we find that organs which become useless and cannot be employed have gradually degenerated. Our common mole furnishes an example. Its eyes also have become small and are deeply hidden in the muscles, although they are by no means as much degenerated as in the _Proteus anguineus_, and are still possessed of a lens and a retina. Their nerve of vision, however, has become very imperfect, and its connection with the brain is interrupted, so that the animal for this reason can have no perception of light. Notwithstanding the above, however, it is doubtful whether the degeneration and gradual disappearance of the visual organ is in all cases the result of their being no longer employed, since there exists in dark caves a kind of beetle, the _Machaerites_, in which species the female only is blind, while the male has a well developed organ of sight. In this case it cannot be maintained that the absence of light has been the cause of the blindness of the female beetle, because it would have acted equally upon the male. Nevertheless, no other explanation can be found for the blindness. The problem, therefore, is hitherto unsolved. Of late the investigations of naturalists have been extended to the animal life existing not only in grottoes and caves, but also in mines and pits created by the action of man, and this has led to many interesting discoveries and remarkable results. A naturalist who has especially enlarged our knowledge with regard to the subterraneous fauna and flora is Dr. Robert Schneider, of Berlin, who made his studies in the coal mines near Waldenburg and Altwasser, in Silesia, the salt mines of Stassfurt and the metal mines of Klausthal, in the Upper Harz Mountains. [Illustration: FIG. 2.--GROUP OF PYRENOMYCETES, A SPECIES OF FUNGI FREQUENT IN THE WATERS OF MINES. a. Found in the mines of Waldenburg; b. In the brown-coal pits near Westeregeln; c. In the Dorothea pit at Klausthal.] [Illustration: FIG. 3. a. Rhizomorpha canalicularis of Hoffmann, b. Club fungus (Clavaria deflexa) of Hoffmann, found in the mines at Klausthal.]