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 
larvee 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, 
