CRUSTACEA OF FRESH WATERS 187 



settle some of the most fundamental problems of the 

 evolution theory. 



At all events, we do not find in any truly subter- 

 ranean species large and peculiarly modified eyes 

 like those of many deep-sea animals, and this may 

 be associated with the complete darkness of their 

 habitat, not lighted by phosphorescent organisms as 

 the deep sea is. In another respect these animals 

 differ from those of the deep sea, for they are all 

 colourless or nearly so ; while many of the inhabitants 

 of the deep sea, as we have already seen, are brilliantly 

 coloured. 



