€H. IV] LIFE IN THE SEA 101 



iishes have had their stomachs full of herring eggs. If by any 

 circumstance one species receives an advantage then some other 

 necessarily suffers. Thus it is maintained by fishermen that the 

 operation of the Wild Birds Protection Acts has been to cause a 

 diminution in the abundance of cockles in some localities, since 

 increased numbers of gulls were spared to eat these molluscs. 

 Fluctuations in the abundance of planktonic organisms in the sea, 

 which are smaller than the great annual periodic changes which 

 we have been considering, are often to be traced to such minor 

 disturbances in the " balance of life." In the sea there is always 

 a certain relation between the numbers in groups of species, and 

 though at any one time there is an equilibrium, still this 

 ■equilibrium is a continually changing one ; and changes in the 

 physical conditions of the sea — which changes are fundamental 

 ones — must disturb the balance. 



