226 COMPARATIVE ZOOLOGY. 



mate, altitude, or depth are commonly inferior to those 

 more restricted : Man is a notable exception. 



There is some relation between the duration of life and 

 the size, structure, and rank of animals. Vertebrates not 

 only grow to a greater size, but also live longer than In- 

 vertebrates. Whales and Elephants are the longest-lived; 

 and Falcons, Ravens, Parrots and Geese, Alligators and 

 Turtles, and Sharks and Pikes, are said to live a century. 

 The life of Quadrupeds generally reaches its limit when 

 the molar teeth are worn down : those of the Sheep last 

 about 15 years; of the Ox, 20; of the Horse, 40; of 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 geo- 

 metrical 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 de- 

 stroyed, the earth would soon be covered by the progeny 

 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 Elephants, 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 allowed 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, 



