ANIMAL LIFE 177 



The great economic fisheries of Northern 

 Europe are limited to the continental shelf 

 and continental slope, and the causes which 

 lead to the fluctuations in these fisheries have 

 been the subject of many recent investiga- 

 tions, especially by the Norwegians. There 

 are indications that in some years the occur- 

 rence of abundant food at the time of the 

 hatching of the eggs leads to a great develop- 

 ment in the quantity and quality of the fish 

 for a certain year, and the fish of that par- 

 ticular year dominate the character of the 

 catches over a long series of years. 



It is probable that the animal assemblages 

 in tropical regions, as in northern waters, are 

 distributed in somewhat similar zones of depth. 



Beep-Sea Benthos. — We have stated that 

 plants can only function within the photic 

 zone, but still their dead remains, falling 

 through the intermediate waters to the bottom, 

 may on the way down and on the bottom fur- 

 nish food for animals. It has been pointed 

 out that a great change in the physical condi- 

 tions, such as temperature, viscosity, penetra- 

 tion of light, takes place, especially in the 

 tropics, at about 400 or 500 fathoms, and here 

 many of these falling particles may be retarded 

 in their descent, and may furnish at that level 

 a rich feeding-ground for bathypelagic animals 

 — a kind of artificial bottom, or extension of 



