THE STRUGGLE FOR EXISTENCE 81 



1,048,576 in the twenty-first year. " A bacillus 

 less than j^gD-th of an inch in length multiplies, 

 under normal conditions, at a rate that would 

 cause the offspring of a single individual to fill 

 the ocean to the depth of a mile in five days " 

 (H. E. Crampton). " The cholera bacillus can 

 duplicate every twenty minutes, and might thus 

 in one day become 5,000,000,000,000,000,000,000, 

 with the weight, according to the calculations of 

 Cohn, of about 7,366 tons. In a few days, at this 

 rate, there would be a mass of bacteria as big 

 as the moon, huge enough to fill the whole ocean " 

 (Saleeby). 



The slowest breeder among mammals is the 

 elephant ; it is supposed to rear one young one 

 every ten years, but, as it lives to more than a 

 hundred, Darwin calculated that in 750 years each 

 pair would, if all their offspring lived and bred, 

 be the ancestor of nineteen milhons. The lemmings 

 in the Scandinavian valleys become periodically so 

 numerous that they eat up every plant, and must 

 march or starve. The bands become an army which 

 devastates as it goes, till their problem is solved 

 in the waves of the Baltic or the North Sea. 



A cod has two million eggs, they say ; if these all 

 developed into cods there would soon be no more 

 fishing. An oyster may have sixty million eggs, 

 and the average American yield is sixteen milhons. 

 If all the progeny of one oyster survived and 

 multiphed, its great-great-grand-children would 

 number sixty-six with thirty-three noughts after 

 it, and the heap of shells would be eight times the 

 size of the world. 



Huxley calculated that if the descendants of a 

 single green-fly all survived and multiphed they 



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