68 THE FAUNA OF THE DEEP SEA 



the shallow water. This is the case with the fauna 

 of the great caves. There is probably total darkness 

 in these underground lakes and streams, and there 

 is only the remotest possibility of the animals living 

 in them ever seeing, even temporarily, a ray of 

 sunlight or even a glimmer of phosphorescence 

 during the whole of their life-time. We find then 

 that the cave fauna is totally blind. 



The conditions in the deep sea are not quite the 

 same. In some regions there is probably a very con- 

 siderable illumination by phosphorescent light, and it 

 is quite possible that many of the characteristic deep- 

 sea forms may occasionally wander into shallower 

 regions where faint rays of sunlight penetrate, or even 

 that the young stages of some species may be passed 

 at or near the surface of the sea. Taking these points 

 into consideration, then, it is not surprising to find 

 that, in the deep seas, there are very few animals, 

 belonging to families usually provided with eyes, that 

 are quite blind. 



In the majority of cases we find that the eyes are 

 either very large or very small. Only in a small 

 minority of cases do we find that the eyes are recorded 

 to be moderate in size. The relation between the 

 large-eyed forms and the small-eyed forms is not the 



