8 ANIMAL STUDIES 



prey, when both captor and victim may be destroyed by 

 the expansion of the body. Some fishes die on being taken 

 out of water through the swelling of the air bladder and 

 the bursting of its blood-vessels. If an animal which lives 

 normally on the surface of the earth is taken up a very high 

 mountain or is carried up in a balloon to a great altitude 

 where the pressure of the atmosphere is much less than it 

 is at the earth's surface, serious consequences may ensue, 

 and if too high an altitude is reached death occurs. This 

 death may be in part due to the difficulty in breathing in 

 sufficient oxygen to maintain life, but it is probably chiefly 

 due to the disturbances caused by the removal of the pres- 

 sure to which the body is accustomed and is structurally 

 adapted to withstand. All living animals are accustomed 

 to live under a certain pressure, and there are evidently 

 limits of maximum or minimum pressure beyond which no 

 animal at present existing can go and remain alive. 



But in the case both of temperature and pressure con- 

 ditions it is easy to conceive that animals might exist which 

 could live under temperature and pressure conditions not 

 included between the minimum and maximum limits of each 

 as determined by animals so existing. But it is impossible 

 to conceive of animals which could live without oxygen or 

 without organic food. The necessities of oxygen and organic 

 food (and water) are the primary or essential conditions for 

 the existence of any animals. 



Of course, we might include such conditions, among 

 the primary conditions, as the light and heat of the sun, 

 the action of gravitation, and other physical conditions, 

 without which existence or life of any kind would be im- 

 possible on this earth. But we here consider by " primary 

 conditions of animal life " rather those necessities of 

 living animals as opposed to the necessities of living 

 plants. Neither animals nor plants could exist without 

 the sun, whence they derive directly or indirectly all their 

 energy. 



