Page:Popular Science Monthly Volume 46.djvu/836

816 that they are derived from species that have simply undergone external modification. A careful comparative study of the modifications they have undergone, and of their phases, would be necessary to clear up the doubtful points. The doctrine of the evolution of species is concerned in this investigation. The introduction underground of the ancestors of these fauna may have taken place and may be taking place in two ways: through perfect individuals carried by streams into wide-mouth pits, whence they can not escape to the light, or through eggs or larvae borne into narrow fissures by simple infiltration of water. It is a matter of question whether or not creatures hatched from these germs, which have never lived on the surface, and their descendants,



would be affected by more rapid changes than those which have come underground by accident, but have not been born there. The principal changes undergone are usually albinism, or more or less complete loss of color, and atrophy of the eyes. The organs of vision become, of course, useless in the underground darkness. It is found, on the other hand, that cave-inhabiting animals have the other senses developed to excess; they guide themselves by means of long cirri or long antennae, which are very sensitive; they are put on their guard by means of their hearing, which informs them of distant perils; and by their smell, telling them of invisible game, helps them to their food. Albinism is accounted for as the result of failure to absorb the light-rays. It is generally agreed that cave-dwelling animals have lost through adaptation to the medium the visual organ their ancestors enjoyed.