As illustrating how foods that have, superficially at least, approximately the same chemical composition may react differently in the animal body we have the observations of Watson on rats fed with porridge, made by boiling oatmeal with water and skim milk, as contrasted with a diet of bread and skim milk, the two diets having essentially the same composition. Of fourteen young rats fed exclusively on porridge, all, with the exception of two that were withdrawn, succumbed within five months, while the bread and milk-fed animals thrived as usual. Adult rats, however, can live for prolonged periods and maintain their weight on a porridge diet. It is believed that the difference in the behavior of young rats to these two closely allied forms of diet, is due to a difference in the digestibility of the food, the porridge being presumably less readily digested by the young animals than bread. With the more fully developed digestive powers of the adult animals, however, this difference in availability practically disappears as a potent factor in their nutrition. Finally, mention may be made of the fact that a pure rice diet, notably deficient in proteid, arrests the growth of young rats and leads to a fatal issue within three months, while adult rats placed on such a diet lose weight rapidly and die in about the same time. All of these facts bearing on the nutrition of animals quite remote from man have significance as showing how any wide departure from a physiological diet, for that particular species or type, may lead to very undesirable results, and they warn us not to be too hasty in drawing far-reaching conclusions and sweeping deductions from a few experiments with a given species of animal.