The line of study which was pursued by the author led to the formulation of the above described working hypothesis which has so effectively served as a guide to the interpretation of experimental data since that time. This clear-cut statement of the essential factors which the diet must furnish in order to maintain normal nutrition served to enable anyone to study the dietary properties of any food substance or mixture, using animals as test organisms, rather than through the use of chemical methods. The result of this change in the method of attack on nutrition problems was that there has been more advance in this field since 1915 than in many years before. It made possible the development of the biological method for the analysis of a food-stuff. This method gives exact information as to the quality of every factor which contributes to the success of the nutrition of an animal. This method has been described in Chapter II (A Biological Method For The Analysis Of A Foodstuff. 17. Mccollum'S Experiments Not Verified By Osborne And Mendel), p. 27. As the result of the accumulation of data of this type we have come to look upon foods in a new way. Instead of classifying them on the basis of their protein, or fat, or carbohydrate content, as was formerly done, we classify them on the basis of their biological properties as foods. Those which are faulty in similar respects are classed together, irrespective of their chemical composition. Thus the bean with twenty-three per cent of protein, is logically classed with the potato which has but two per cent, because the bean proteins are of very poor quality and are of little use for the promotion of growth or for the repair of tissue waste. The classification referred to, however, was the outcome of the discovery that all vegetable foods which have the function of storage organs (seeds, tubers, fleshy roots) have similar dietary properties and similar deficiencies regardless of their chemical composition as revealed by the standard food analysis. The highly specialized muscle tissue falls in the same class with the cereals in respect to its dietary properties, and differs very markedly from the glandular organs just as do the seeds, tubers and fleshy roots from the vegetative parts of the plants, the leaves.