Eggs and milk occupy a unique place in the catalog of foods. The purpose for which they were produced in nature throws much light upon their value as food.

As will be learned from the lesson, "Evolution of Man," no living creature exists for the sole benefit of other creatures, but because once created, the inherent struggle of all living matter to survive and to reproduce itself has evolved wonderful and various adaptations. Every organic substance is primarily produced in nature for a specific purpose in the life of its species. The lumber in our houses owes its existence to the plant's struggle for sunlight, which made it necessary for the tree to possess a strong storm-withstanding stem to hold aloft its leaves above the shade of other foliage.

The leaves and the stems of grass are primarily an essential part of the life of the plant, and not food for animals. The greater part of the human food of plant origin represents in nature the nutrient material supplied by the parent plant for the early life of the seedling. All grains, nuts, fruits and roots, and tubers are merely modified forms of food material adapted to the rapid nourishment of the young plant.

Every form of life exists for itself alone.

The starch and the oil of seeds, the sugar of fruit, and the lesser quantities of nitrogen contained in all seeds, are in a more available form for cell-nourishment than would be the original mature portions of plant life.

Milk and eggs in the animal world occupy a position identical to that of seeds and fruit in the plant world; that is, they are created for the first nourishment of the offspring.

In the process of evolution, a fundamental distinction between birds and mammals is in the manner in which the young are nourished. The egg of the bird supplies sufficient nourishment to develop the young bird to a point where it can exist upon the ordinary food of the adult bird.

The hen's egg must contain all food material necessary to form all portions of the body of the chick, and to supply it for a time with heat and energy.

An average egg weighs two ounces; of this weight about 10 per cent is shell, 30 per cent yolk, and the remainder white. The white of the egg is composed of albumin and water. The yolk consists of globulin, egg-fat, and lecithin; this latter substance contains a considerable proportion of phosphorus, and is one of the essential contingents of brain and nerves. The egg-shell contains 13 per cent protein, 10 per cent fat, and one per cent ash.

The younger the animal, the more rapid is the growth of the animal body compared with the amount of energy expended. For this reason the percentage of nitrogen in milk and in eggs is much too great to form a balanced adult diet, and should be supplemented by articles containing larger proportions of heat-producing materials, preferably carbohydrates. The proteid material of eggs is in a form especially adapted to the construction of new cells. For this reason it is one of the best known foods for use in cases of emaciation, where new tissue is to be added rapidly to the body. An egg contains about fourteen decigrams of nitrogen. Ten eggs, therefore, would supply an ample amount of nitrogen for the daily needs of the average body, were no nitrogen taken from other sources. In feeding patients who are convalescing from fevers or other wasting diseases, it is sometimes necessary to prescribe a diet of from ten to twelve eggs daily for a limited time.

Composition of eggs.

Milk and eggs not a balanced adult diet.

Eggs for emaciation and convalescents.

The consumption of five eggs a day, when we rely wholly upon this article for animal proteids, is quite sufficient for one performing ordinary labor, when supplemented by one succulent and one tuber vegetable.