PART V 



ANIMALS IN RELATION TO EACH 



OTHER, TO PLANTS, AND TO 



THE OUTSIDE WORLD 



CHAPTER XXX 



THE STRUGGLE TO LIVE, ADAPTA- 

 TION AND DISTRIBUTION 



The multiplication of animals. — The English sparrow, 

 now a common bird over our whole country, rears five or 

 six broods every year, each brood containing six to ten young. 

 That is, each pair of healthy English sparrows produces 

 from thirty to sixty new sparrows each year. Now if all 

 these young come safely to maturity and each sparrow 

 maintains the same rate of increase, and every sparrow lives 

 to its normal age, how long will it take to cover the face of 

 the land with these pugnacious, noisy, little birds? As a 

 matter of fact a professor of mathematics has solved this 

 problem, and finds that at the normal rate of increase, 

 and if no sparrows were to die save naturally of old age, 

 it would take about twenty-five years to give one sparrow 

 to every square inch in the United States. 



But English sparrows are not the only birds in the country, 

 and although the robins, bluebirds, woodpeckers, and the 

 scores of other kinds do not lay so many eggs nor lay so 

 many times a year, yet each pair does produce more than 



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