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 occupied by their whole lives. (By this rule how long ought man to live?) Animals, moreover, move from place to place, and work with their muscles. The energy for this comes from the food they eat. Plants do not use food for this purpose. Another need for food comes from the necessity for heat in all living things. The activities of animals cause the tissues to wear out, or break down, and food furnishes material with which new living matter is built up by the cells and the tissues repaired. We have already stated the rôle of oxygen in setting free energy in the living substance of the cell by oxidizing it. There is no furnace in the body as in an engine, but the oxidation occurs in the cells themselves and the fuel is built up into living matter by the cells before it is oxidized. Plants must lift mineral from the inorganic to the organic world before it can be food for animals. Plants can assimilate minerals; animals cannot. The body cannot make bone out of limewater. The iron in iron tonics cannot be used. Iron makes the grain brown, and the peach red. There is ten times as much iron in our food as the body needs.

State four reasons why animals need food. Which of these reasons is very powerful with plants? Least powerful? Absent altogether? Why is constant breathing necessary for life? When is breathing more rapid? Why? People who lead what kind of lives usually have poor appetites? Good appetites? Why? What was the first distinct organ evolved by animals? (Animal Biology, Chap. IV.)

The Body is a Machine for transferring Energy.—Energy cannot be destroyed, but it can be transferred and changed in form. When a coin is rubbed on the table, muscular energy, supplied by oxidation in the muscle, produces the motion. Friction may change motion into heat, and the coin will become very hot. The uniting of food and