Cave and a few adjoining caves—the only species in this country of the genus—is blind, but possesses rudiments of the outer eye, several corneal lenses surviving. On the other hand, the species of this or the closely allied representative genus Bathyscia, to which they are now referred by Dr. Horn, are very numerous in Europe, and are scavengers in habit. Bedel, in his list of the cave insects of Europe (1875), states that sixty-five species are known, and that several others were known but not described, and that probably further explorations in the region of the Pyrenees, both in France and Spain, will lead to further discovery of species. It appears that not all the species live in caves, but occur in the open air under large stones, moss, vegetable detritus, or at the entrances to caves. It is apparent, then, that the cave animals are emigrants from out of doors, and that the cave species, by isolation from the light and from interbreeding with out-of-door forms, as well as by adaptation to total darkness, have become fixed species with separate generic characters.
Equally instructive and explanatory of the origin of cave animals in general is the genus Ariophthalmus. In the caverns of the central United States there are only eight species, and none occur elsewhere in America, though we have two or three species of Trechus, one at least not infrequent, and Trechus micans is common to both hemispheres. Not alone loss of sight and eyes, but other modifications of the body, legs, and antennae, evidently the result of loss of sight, occur, so universal is the modification of the organism. It is evident that southern Europe is the zoögeographical center of this subgenus, for sixty-four species of completely eyeless beetles referred to this genus have already been discovered in the caves of Austria, Italy, France, and Spain. Lately, however, owing to the studies of Putzeys, and especially of De Perrin, the genus Anophthalmus has been united to Trechus, since there is a series of forms with more or less rudimentary eyes connected with the eyed species of Trechus. Bedel also tells us that in all the species of Trechus there is a natural tendency to penetrate into grottoes, even when ordinarily they live in the open air buried in the earth under stones.
It seems reasonable to conclude that the cave species, which are without optic ganglia, optic nerves, and any traces of eyes, had originally, by adaptation to total darkness, become isolated, and that their characteristics after being fixed by heredity have been transmitted for generations, becoming as unchanging in their way as the physical conditions of darkness and uniform temperature surrounding them. Those living in the open air in the soil under stones, or at or just within the entrances to caves, vary most as regards the eyes, as we have found to be the case with the other forms previously mentioned.