Reviewing briefly the food used by young plants, birds, and mammals, we find: Plants living upon proteins, starch, sugar, fat, and salts. Birds living upon proteins, fat, and salts. Mammals living upon proteins, sugar, fat, and salts.

All possess proteins, all possess salts, all possess either fat or sugar or starch, or a combination of these. Starch, sugar, and fat are the fuel foods. Proteins and salt are the growth and repair foods.

So we see that all of these living organisms are provided by nature with material for growth and repair on the one hand, and materials for fuel on the other, Whether this fuel consists of sugar, or of fat, or of starch, or a combination of all, seems to be a matter that is adjusted according to the conditions of life of the individual organism. The organisms which have been chosen as examples represent the whole realm of living nature, so that the principles derived from the study of these organisms represent the fundamental principles of life.