20 THE PROBLEMS OF EVOLUTION 



depth. Although pressure is not the only physical 

 factor which affects animals, its effects are among 

 the most striking. 



The animals and such plants as depend upon 

 other organisms for food give striking evidence of 

 another phase of their surroundings which may 

 be called the organic environment. Green plants 

 are not independent of organic associations, but 

 their dependence seems less vitally associated with 

 other organisms than that of animals. Many green 

 plants live normally in woods, but any environ- 

 ment which provides shade, moisture, and proper 

 soil conditions is favorable to their existence. 

 Trees are not essential beyond the fact that in 

 nature forests alone usually guarantee the proper 

 conditions. Animals and colorless plants, on the 

 other hand, must have organic food. They are 

 utterly dependent upon other organisms, ulti- 

 mately on the green plants, and so far even man 

 has failed to abolish this dependence in any case. 

 A host of adaptations are associated with this 

 dependence. Aggressive and defensive structures, 

 concealing adaptations, and even many adapta- 

 tions directly associated with the physical environ- 

 ment have to do with an animal's food-securing 

 habits or his defense against those of other crea- 

 tures. Organic environment also leads to the bene- 

 ficial associations, as symbiosis and social organi- 

 zation, and to the one-sided benefits of parasitism. 



