THE PRIMARY CONDITIONS OF ANIMAL LIFE 107 
plant. Animals can not exist without plants. The plants 
furnish all animals with food, either directly or indirectly. 
The amount of food and the kinds of food required by 
various kinds of animals are special conditions depending 
on the size, the degree of activity, the structural character 
of the body, etc., of the animal in question. Those which 
do the most need most. Those with warmest blood, great- 
est activity, and most rapid change of tissues are most 
dependent on abundance, regularity, and fitness of their 
food. As we well know, an animal can live for a longer or 
shorter time without food. Men have fasted for a month, 
or.even two months. Among cold-blooded animals, like the 
reptiles, the general habit of food taking is that of an occa- 
sional gorging, succeeded by a long period of abstinence. 
Many of. the lower animals can go without food for surpris- 
ingly long periods without loss of life. But the continued 
lack of food results inevitably in death. Any animal may 
be starved in time. 
If water be held not to be included in the general con- 
ception of food, then special mention must be made of the 
necessity of water as one of the primary conditions of ani- 
mal life. Protoplasm, the basis of life, is a fluid, although 
thick and viscous. To be fluid its components must be 
dissolved or suspended in water. In fact, all the truly 
living substance in an animal’s body contains water. The 
water necessary for the animal may be derived from the 
other food, all of which contains water in greater or 
less quantity, or may be taken apart from the other 
food, by drinking or by absorption through the skin. 
Sheep are seldom seen to drink, for they find almost 
enough water in their green food. Fur seals never drink, 
for they absorb the water needed through pores in the 
skin. 
64. Oxygen.—Animals must have air in order to live, 
but the essential element of the air which they need is its 
oxygen. For the metabolism of the body, for the chemical 
