DEVELOPMENT. 219 
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. 
7. The Struggle for Life. 
Every 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 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 Adain and Eve. A pair of Elephants, the 
slowest breeder of all known animals, would become the 
progenitors, in five centuries, of 15,000,000 of Elephants, 
if death did not interfere. In fifteen years, a pair of 
Birds would increase to 2,000,000,000. Evidently a vast 
number must perish, and a far greater host of eggs fail 
to mature. Whatever the average number of individuals 
in any country, twice that number must die annually. 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. 
The total extinction of any one species is exceedingly 
