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. 
65. 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° (, 
