PART IV 



ANIMALS IN RELATION TO EACH OTHER 

 AND TO THE OUTSIDE WORLD 



CHAPTER XVII 



THE STRUGGLE FOR FOOD AND ROOM, AND 

 THE SPECIAL MEANS FOR FOOD-GET- 

 TING AND PROTECTION 



The multiplication of animals. — The English sparrow, 

 now a common bird o\'er our \\ hole country, rears five or 

 six broods every year, each brood containing six to ten 

 young. That is, each pair of health}' English sparrows 

 produces from thirty to sixty new sparrows each year. 

 Now if all these young come safely to maturity and each 

 pair maintains the same rate of increase, and every spar- 

 row 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, )-et each pair does produce 

 more than two eggs yearl)-, that is, each pair yearly mul- 



273 



