v] THE SOURCES OF FOOD 135 



seas. It contains an excess of organic matter which 

 it has removed from the abyssal levels of water. 

 Even in shallow seas, near the land, or at the 

 junction of warm and cold currents, there are also 

 these vertical currents from the bottom towards the 

 surface. Seasonal changes of temperature must also 

 produce them, for in the winter the surface waters 

 must cool and sink to the bottom. Wherever there 

 is a vertical current coming up from the sea-bottom 

 there should be an increase in the abundance of 

 plant life, for this vertical current should contain an 

 excess of ultimate food-stuffs. For the same reason 

 there should be a greater amount of life in the 

 shallow water near the land. 



We do actually find this abundance of plant life 

 near the land, and it is pretty certain that local 

 abundances of plankton are sometimes due to up- 

 welling currents bringing abundant food-stuff from 

 the bottom. The richness of plant life in the 

 plankton of the circumpolar seas is also due in part 

 to the up welling currents. There are great differences 

 from year to year in the abundance of fish life in the 

 Norwegian seas, and also in the time of spawning, and 

 in the condition of the fish ; and it is remarkable 

 that it is the years when the temperature of the sea 

 is lowest which yield the greatest abundance of life. 

 Now this has been explained by attributing the 

 lower temperature of those years to a stronger drift 



