CAUSES OF ANIMAL CYCLES 



181 



each female deposits from 3 to 9 million eggs annually, the North 

 Sea is said to contain less than 45 million cod, of which about 23 

 million are probably females (Johnstone, 1908). The eggs must be 



Abra 



Solen 



Nucula 



Corbula 



Recti - 

 naria 



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1911 1912 1913 1914 



Fig. 39. — Some con.?tituents of a marine bottom community in the Thisted 

 Broad belonging to species eaten by plaice. The numbers shown are those 

 occurring in the random samples of 1911-1914, on one-fifth square meter. It 

 shows marked fluctuations of some of the predominants. (After Jensen, 1919.) 



fertilized while floating in the water, and hence bad weather at the 

 time of spawning may cause a large number to fail of fertilization. 

 The eggs of various insects are destroyed by unfavorable weather. 

 Heavy rains prevent hatching of chinchbug eggs; loss of grasshopper 



