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.]
a. Rhizomorpha canalicularis of Hoffmann, b. Club fungus (Clavaria deflexa) of Hoffmann, found in the mines at Klausthal.]
As regards the subterraneous flora, Dr. Schneider's investigations resulted in showing that the plants which thrive in the dark regions under ground are those which possess no chlorophyl and are sensitive to light. Those which vegetate most luxuriantly there are the _fungi_, and among them especially the _pyrenomycetes_, which are frequent in the waters of mines. Their general aspect is shown in a 480 times magnified form in Fig. 2. They resemble fine threads of delicate structure, and where found are always discovered in great abundance. Most conspicuous by their shape and considerable size are the _rhizomorphæ_, Fig. 3a, and they are remarkable, not only for their brilliant phosphorescence, but also for the peculiar fact that they are only found in places where light does not enter. These _rhizomorphæ_, though this is not easily recognizable from their external appearance, also belong to the fungi and are often seen in strings of the length of over a meter and the thickness of a quill, spreading out in peculiar branches and hanging down from moist beams in dark places. Sometimes they grow like seaweed in the water of the mines, and in this case they give much embarrassment to the miners, because they are apt to obstruct the channels constructed for leading off the superfluous water. In the mines of Freiberg these _rhizomorphæ_ exist in great abundance, and Humboldt already mentions specimens of the length of 4½ feet. Miners in Germany call them _zwirn_ (thread). The student of natural sciences, when encountering these peculiar forms of vegetation, will ask in how far they are the product of their surrounding circumstances (i.e., of the absence of light or the presence of moisture), and in order to find a reply to this question experiments have been made to grow these _rhizomorphæ_ under different conditions of existence. These experiments have shown that from several species of _rhizomorphæ_ other ordinary fungi can be developed, and that the subterraneous specimens therefore may be considered a degeneration and variation of the fungi found above the surface of the ground.