BLIND CAVE-ANIMALS. 77 



differences in the mode of life of related forms are entirely 

 unknown, and at the present time it seems impossible even to 

 suggest any hypothesis which would refer such changes in their 

 mode of existence to any sufficient causes. 



By far the larger proportion of nocturnal animals, although 

 they are quite lively even in the darkest night, have eyes 

 quite as good and perfect as those of the diurnal animals. 

 Although, as a fact, here and there as, for instance, in nocturnal 

 birds certain differences have been observed in the structure 

 of the retina (M. S. Schultze) which might be hypothetically 

 connected with its exceptional functions or with the exceptional 

 time at which they are exercised, yet thesa investigations 

 supply us with no answer, not even a hypothetical one, to the 

 question as to why certain animals, provided with organs of 

 sight, fly exclusively by night. If we remember that even in 

 the darkest night a certain amount of light always reaches the 

 earth, we might certainly propound the hypothesis that this 

 minute proportion of light suffices them for seeing clearly by. 

 But this hypothesis would give no true explanation of the 

 observed facts ; this could only be given if it were possible to 

 compare the differences in the structure in the retina of 

 diurnal and noctural animals with direct reference to the scale 

 of intensity of light to which they are exposed. A circum- 

 stance which is more important, because it is directly referable 

 to certain vital relations of animals, is the occurrence of half- 

 blind or wholly blind animals in spots where the light of day 

 cannot penetrate, such as deep caverns, the internal parts of 

 larger animals, and the deepest parts of the ocean or of large 

 fresh-water lakes. The blind crayfish of the mammoth cave 

 in Kentucky is well known, as are also the blind Fishes, 

 Insects, Crabs, Amphibia, and Mammals (moles) of the old and 

 new worlds, and it seems unnecessary to give a complete list of 

 such cases in my text. 19 These familiar facts have hitherto been, 

 and must still be, regarded as so many instances, sufficiently 

 proving the statement that total darkness gradually destroys 

 the eyes of animals originally possessing them ; for, since these 

 organs are absolutely useless in such circumstances, in the 

 course of generations they must gradually disappear, according 



