248 SECRETS OF ANIMAL LIFE 



Mole are closed. Practically complete blindness is 

 illustrated by a North American cave-crayfish, a 

 cave-prawn from Carniola, and some other Crusta- 

 ceans. In some of the cave-beetles the degeneration 

 has affected not only the eye but the optic ganglion 

 as well. The cave-fishes are of special interest, for 

 they show all grades of optic degeneration down to 

 complete blindness. In some of them, moreover, 

 the eyes of the young forms are less degenerate 

 than those of the adults a state of affairs which has 

 been observed in some other types besides fishes. 

 It suggests that there may be a process of individual, 

 as distinguished from racial, degeneration. 

 ! There is no doubt as to the " naturalness " of the 

 old theory that the " blindness " of cave-animals is 

 the cumulative hereditary result of the disuse which 

 living in darkness involves. Thus we cannot over- 

 look the fact that some of the blind forms have very 

 near relatives which live in the light of day and have 

 well-developed eyes. To take an analogous case, 

 that of deep and shallow water : Doflein found in 

 Sagami Bay two varieties of a small crab, one living 

 in the darkness of deep water and with very rudi- 

 mentary eyes, the other living in shallow water 

 and with the eyes well developed. An interesting 

 feature of this case was that the practically blind 

 mother-crab of the deep water was carrying about 

 larvae with darkly pigmented eyes, showing all the 

 essential parts. This again points to the conclusion 

 that the darkness may hinder the development of 

 the eye in the individual lifetime. In other words, 



