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, Ravefis, 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. 
