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THE POPULAR SCIJSjVCE MONTHLY. 



that they are derived from species that have simply undergone 

 external modification. A careful comparative study of the modi- 

 fications they have undergone, and of theif phases, would be 

 necessary to clear up the doubtful points. The doctrine of the 

 evolution of species is concerned in this investigation. The intro- 

 duction underground of the ancestors of these fauna may have 

 taken place and may be taking place in two ways : through per- 

 fect 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, 



Blind Insect of Caves (Dolicliopoda palpata). 



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 or- 

 gans of vision become, of course, useless in the underground dark- 

 ness. 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 in- 

 forms 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 gener- 

 ally agreed that cave-dwelling animals have lost through adapta- 

 tion to the medium the visual organ their ancestors enjoyed. 



