108 ANIMAL LIFE 



changes which take place in the body of every living ani- 

 mal, a supply of oxygen is required. This oxygen is de- 

 rived directly or indirectly from the air. The atmosphere 

 of the earth is composed of 79.02 parts of nitrogen (includ- 

 ing argon), .03 parts of carbonic acid, and 20.95 parts of 

 oxygen. Thus all the animals which live on land are en- 

 veloped by a substance containing nearly 21 per cent of 

 oxygen. But animals can live in an atmosphere containing 

 much less oxygen. Certain mammals, experimented on, 

 lived without difficulty in an atmosphere containing only 

 14 per cent of oxygen ; when the oxygen was reduced to 7 

 per cent serious disturbances were caused in the animal's 

 condition, and death by suffocation ensued when 3 per 

 cent of oxygen was left in the atmosphere. Animals which 

 live in water get their oxygen, not from the water itself 

 (water being composed of hydrogen and oxygen), but from 

 air which is mechanically mixed with the water. Fishes 

 breathe the air which is mixed with or dissolved in the 

 water. This scanty supply therefore constitutes their at- 

 mosphere, for in water from which all air is excluded no 

 animal can breathe. Whatever the habits of life of the 

 animal, whether it lives on the land, in the ground, or in 

 the water, it must have oxygen or die. 



6,5. Temperature, pressure, and other conditions. Some 

 physiologists include among the primary or essential gen- 

 eral conditions of animal life such conditions as favorable 

 temperature and favorable pressure. It is known from ob- 

 servation and experiment that animals die when a too low 

 or a too high temperature prevails. The minimum or 

 maximum of temperature between which limits an animal 

 can live varies much among different kinds of animals. It 

 is familiar knowledge that many kinds of animals can be 

 frozen and yet not be killed. Insects and other small ani- 

 mals may lie frozen through a winter and resume active 

 life again in the spring. An experimenter kept certain 

 fish frozen in blocks of ice at a temperature of 15 C. 



