GENERAL ZOOLOGICAL FACTS AND THEORIES 283 



(5) The Struggle for Existence 



It has been estimated that if it were possible for oysters 

 to breed unmolested by their enemies, the ocean would be a 

 solid mass of bivalves in four years. Again, if a pair of com- 

 mon robins, which rear on an average four young per year, 

 were allowed to live and produce young year after year, and if 

 all of the offspring were also allowed to live and produce young, 

 the descendants of the first pair would total over one hundred 

 thousand at the end of the tenth year, and over twenty billion 

 at the end of the twentieth year, as shown in Table XVI (256). 



TABLE XVI 



THE. YEARLY INCREASE IN THE NUMBER OF ROBINS IF ALLOWED TO BREED 



UNMOLESTED 



Number of robins at end of loth year 

 Number of robins at end of 20th year 



118,098 

 20,913,948,846 



What really happens, however, is that a few of the birds may 

 succeed in establishing themselves in neighboring regions, but 

 most of the others perish. The final result is a state of practical 

 equilibrium between the number of robins and the factors that 



