DRIFTING LIFE 127 



hundred fathoms it is already so dim that few plants can 

 live in a healthy condition. More will be found on the 

 subject of the light beneath the sea surface in Chapter X. 

 Although there is a depth limit at which the normal life of 

 the plant becomes impossible, yet at times collections may 

 show their presence at still greater depths, the likelihood 

 is however that these plants will be in a dying condition 

 and will have sunk to those depths under their own weight. 



Now animals, as we know, are not so dependent on light 

 directly ; in fact there are many animals on land that seem 

 to prefer darkness and are nocturnal in their habits, 

 shunning the light of day. 



The same applies to the sea animals, there are many that 

 live in the dark deep layers ; in fact, recent research has 

 shown that there is no depth at which some plankton 

 organisms do not exist. 



All the animals are not, however, found evenly distributed 

 in the water layers from top to bottom. Each animal 

 seems to show a definite preference for some particular 

 depth region ; for instance, there are some that live always 

 in the upper fifty fathoms or so, rarely penetrating to 

 deeper levels ; others again will never be caught between 

 one hundred fathoms and the surface, but always live 

 deeper. On account of these differences in depth distribu- 

 tion, exhibited by the various animals of the plankton, we 

 find that catches made from different levels are each quite 

 characteristic and distinct one from another in their 

 composition, in the same way that collections from different 

 geographical regions are each characterized by the presence 

 of those organisms that are prevalent in the locality in 

 which the catch was made. 



It is a general fact that in the daytime the animals that 

 live in the layers quite near the surface are very few, this is 

 especially the case in the summer months. For instance, 



