354 THE STRUGGLE FOR EXISTENCE 



and meet with less unfavorable surroundings. Thousands of 

 the cod's eggs never hatch and thousands of the young cod 

 die from disease, while others are killed; so that but a few 

 finally become full grown. The question naturally arises, 

 how do any reach maturity and live? 



After long years of patient observation, a law known as 

 the "survival of the fittest" has been enunciated and ap- 

 parently established. The tadpole and the young codfish 

 that could swim the fastest and had the strongest jaws were 

 best able to procure food and most likely to survive. All 

 animals that are best fitted to meet their surroundings are 

 the ones that are most apt to live. In other words, these 

 are the fittest animals, and it is these that reach maturity. 



Adaptations to surroundings. — Wherever we may go to 

 observe animals, we shall find that the surviving ones are 

 those best adapted to their surroundings. For example, a 

 tiger with its short hair could not withstand the climate of 

 the polar regions so well as the polar bear with its long, 

 shaggy coat. The common toad could not exist on the 

 deserts of the great Southwest, but the horned toad and the 

 Gila monster prefer these regions because they are adapted 

 to them. 



Moreover, animals are being constantly modified and 

 molded to suit their changing environments. We do not 

 often recognize these changes because they are slight and 

 very gradual ; nevertheless, they amount to a great deal in 

 a long series of years. For example, many insects have 

 been known to change from one kind of food plant to an- 

 other after the former had been destroyed. The same 

 species of insects and reptiles often differ greatly in color 

 when living upon different colored plants or upon earth of 

 different shades, probably to gain protection from their 



