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 

 hard parts, as bones and teeth and nails, are not really liv- 

 ing. 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 



