Diet may be divided into three distinct classes--normal, preventive, and curative. In order to understand the application of diet to these several conditions, it is necessary to observe the following rules: 1 Foods must be selected which contain all the desired nutritive elements. 2 They must be so combined as to produce chemical harmony, or should at least produce no undesirable chemical action. 3 They must be proportioned so as to level or balance their nutritive elements; that is, to prevent overfeeding on some elements of nourishment, and underfeeding on others. Many fine specimens of men and women have been produced without knowledge of these laws, but in nearly every case it may have been observed that the person was normal as to habits, and temperate in eating, therefore led aright by instinct. If one lives an active life, spending from three to five hours a day in the open air, the body will cast off and burn with oxygen much excess nutrition, and will also convert or appropriate certain nutritive elements to one purpose, which, according to all known chemical laws, Nature intended for another. Much better results, however, will be obtained by giving Nature the right material with which to work, thus pursuing lines of least resistance. What foods to select, how they should be combined and proportioned, is determined mainly by laws dependent upon the following conditions: 1 Age. 2 Temperature of environment--time of year or climate. 3 Work or activity. (1) As to age: If we wish the best results we must select and proportion our food according to age, because the growing child or youth needs much structural material--calcium phosphates--with which to build bone, teeth, and cartilage. This is found in cereals and in all grain foods. The middle-aged person needs but little of these--just enough for repair, and the aged person needs practically none. While the growing child needs calcium phosphate, he also needs milk and natural sweets, which named in the order of their preference are honey, maple-sugar, dates, figs, and raisins. This does not mean that a generous quantity of vegetables and fruit cannot be taken, but that the articles first mentioned (cereals and starchy foods) should form a conspicuous part of the child's diet. The adult needs a much less quantity of the heavier starchy foods, because the structural part of the body has been built up. The diet of the adult should consist of vegetables, nuts, and a normal quantity of sweets, a normal quantity of fruits, milk and eggs, with rather a limited amount of cereal or bread products, while the aged, or those having passed sixty, could subsist wholly upon a non-starch diet (non-cereal starch), such as vegetables, milk, nuts, eggs, salads, and fruits, including bananas, which is not a fruit, but a vegetable, and which contains a splendid form of readily soluble starch. (2) As to time of year: