438 



COMPARATIVE ZOOLOGY 



the elephant, 100. Many inferior species die as soon as 

 they have laid their eggs, just as herbs perish as soon as 

 they have flowered. 



8. The Struggle for Life 



Every species of animal is striving to increase in a 

 geometrical ratio. But each lives, if at all, by a struggle 

 at some period of its life. The meekest creatures must 

 fight, or die. 



" There is no exception to the rule that every organic 

 being naturally increases at so high a rate, that, if not 

 destroyed, the earth would soon be covered by the prog- 

 eny of a single pair." If the increase of the human race 

 were not checked, there would not be standing room 

 for the descendants of Adam and Eve. A pair of ele- 

 phants, the slowest breeder of all known animals, would 

 become the progenitors, in seven and one-half centuries, 

 of 19,000,000 of elephants, if death did not interfere. 

 Evidently a vast number of young animals must perish 

 while immature, and a far greater host of eggs fail to 

 mature. A single cod, laying millions of eggs, if al- 

 lowed to have its own way, would soon pack the ocean. 



Yet, so nicely balanced are the forces of nature, the 

 average number of each kind remains about the same. 

 The total extinction of any one species is exceedingly 

 rare. The number of any given species is not deter- 

 mined by the number of eggs produced, but by its sur- 

 rounding conditions. 176 Aquatic birds outnumber the 

 land birds, because their food never fails, not because 

 they are more prolific. The fulmer petrel lays but one 

 egg, yet it is believed to be the most numerous bird in 

 the world. 



The main checks to the high rate of increase are: 

 climate (temperature and moisture), acting directly or 

 indirectly by reducing food ; and other animals, either 



