150 ELEMENTARY ZOOLOGY. 



suitable to the surroundings than others. Those that 

 have inherited qualities that are more in accord with the 

 needs of the organism will have an advantage over the 

 others in the struggle for existence, and will be more 

 likely to survive, have offspring, and transmit the qualities 

 that made them successful. In this way nature merely 

 weeds out the animals that do not become adjusted to the 

 conditions. This is called natural selection. 



It is quite possible that both of these views are true: 

 that the surroundings do have the power to force varia- 

 tion in accordance with the needs of the animal ; and that 

 the environment may select those that are in best adjust- 

 ment to itself, no matter how the qualities arise . One thing, 

 however, is clear : the environment can, in the long run, 

 force adjustment by selection, even if it cannot make an 

 organism vary in the right direction. This is because it 

 has a kind of final veto power over characteristics that 

 are out of harmony with it. 



164. Summary. 



1. The environment influences the development of the 

 individual quite as really as ancestors do. 



2. Food, oxygen, moisture, and temperature are the 

 factors in the surroundings which have the most immedi- 

 ate influence on life. Following these, gravity, light, and 

 the chemical condition and density of the water and 

 atmosphere are important to animals in many ways. 



3. The harmony which we see between animals and 

 their surroundings we call adaptation. The structure, 

 organs, habits, instincts, which show the harmony are 

 called adaptations. All important and useful possessions 

 are adaptations to some real features of the environment, 

 to some actual need of the organism. Animals cannot 



