RELATIONS AND SURROUNDINGS. 125 



137. The Fundamental Conditions of Life. Chief among 

 the external conditions that influence life are food, 

 oxygen, moisture, and suitable temperature. In the 

 absence of any one of these, life, as we know it, would 

 cease. 



Not only are these conditions essential to life, but 

 variations in them produce important changes in the 

 living animals. The character of the food, the amount 

 of moisture, the degree of heat and cold all play their 

 part in influencing the course of life in every individual. 



138. The Secondary Conditions of Life. In addition 

 to the features mentioned in the preceding paragraph, 

 there are other external influences that aid in molding 

 the animal and its development, even though their influ- 

 ence may not be so profound as those mentioned. Among 

 these are the pull of gravity, the chemical character 

 and the density of the medium in which the animal lives, 

 the atmospheric pressure, light (although not directly 

 necessary to animal life is still a most powerful influence) , 

 electricity, and so forth. Many of the organs and in- 

 stincts of animals are in adjustment to these conditions. 



1384. Practical Exercise. The student should illustrate from his 

 own knowledge of animals some ways in which each of these-factors 

 of the environment can influence the life of the individual animal. 

 What senses or powers or instincts are what they are because of 

 these factors? 



139. The Adaptation of Animals to their Environmental 

 Conditions. The most conspicuous fact about animals 

 is that they are in greater or less degree adjusted or 

 adapted in their structures and instincts and habits 

 to the circumstances in which they live. They are not 

 all adjusted in the same manner nor to the same degree. 



