THE STRUGGLE FOR EXISTENCE 113 



The first fact to which attention may be drawn is 

 one not generally appreciated, viz., the enormous 

 powers of increase possessed by organisms, if con- p ow ers 

 sidered as living under ideally favourable condi- of in - 

 tions. A numerical example will bring this fact home of on- 

 to US. ganisms. 



Let us suppose that an organism, say a plant, can 

 produce fifty seeds in one year. Let us suppose that 

 all these are sown, and that all grow to adult life, 

 each in turn producing fifty seeds in the second year ; 

 suppose that each of these fifty plants again produces 

 fifty seeds, developing into seedlings in the following 

 year, and so on. A simple arithmetical calculation 

 will show us that in the tenth year, if every plant 

 survived, we should have the prodigious number of 

 1953 millions of millions of plants then existing 

 derived from the original one ! 



To take actual cases, shepherds' purse one of 

 our commonest weeds is calculated to produce not 

 fifty but 12,000 seeds annually. Burdock is believed 

 to produce over 40,000, whilst purslane may give rise 

 to 2,000,000 ! Our estimate of fifty, therefore, is 

 immensely under the actual facts. 



Man himself is calculated to be capable of doubling 

 his numbers every twenty-five years. Few wild birds 

 produce less than six young per annum. Let us 

 suppose that each pair produces young four times 

 in their lives. Each pair may therefore, if all live, 

 give rise in fifteen years to many millions of birds 

 like themselves, including the offspring produced 

 in turn by these descendants of the original pair. 

 A carrion fly can produce 20,000 larvse, and each 

 larva is mature in about five days. In three 

 months, therefore, if all of them lived and produced 

 eggs and larvse in turn, the original carrion fly would 



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