DARKNESS OF THE DEPTHS 39 



that life for it is impossible at the ordinary barometric 

 pressure. 



Everyone knows what happens when on a sunny 

 day the sky suddenly becomes overcast with rain- 

 clouds : the sunlight is changed to gloom. We can, 

 therefore, imagine what things would be like if, instead 

 of a layer of cloud, a layer of water lOO fathoms thick 

 were to interpose between us and the sun : the gloom 

 would deepen into darkness. At 200 fathoms, even 

 if the water is quite clear, there is not enough light 

 to affect a photographic plate, and this is the con- 

 dition of affairs in the depths of the ocean. There 

 can therefore be no plant life, because plants can only 

 grow in sunlight. We should also expect to find — if 

 we believe in natural selection, and look upon eyes 

 as organs that can only be kept up to the mark 

 where there is useful work for them to do — that the 

 eyes of the animals would be in some way impaired, 

 and this we do indeed find to be the case with many 

 animals that live actually at the bottom in great depths. 

 There are, for instance, certain animals related to the 

 crabs and lobsters (crustacea), certain mollusks and 

 a few fishes which undoubtedly pass the whole of 

 their life at, or close to, the bottom of water that is 

 over 500 fathoms deep, and among these it is rare 

 to find any that have eyes in any way comparable 

 with those of their shallow-water relatives. Among 



