From what has been said in earlier chapters it will be readily understood that students of nutrition during any period previous to the last five or six years must necessarily have had an inadequate understanding of the fundamental principles which underlie the science, and hence were unable to interpret correctly their observations. Several writers were inspired with enthusiasm for a special type of diet and each had, therefore, a polyphemic eye for one or another nutrient principle as overshadowing others in importance. Thus Crichton-Browne (1) had the conviction that meat eating promotes not only physical vigor, but is responsible for many other manly attributes as well. Chittenden (2) was an earnest supporter of the theory that a low protein dietary best promoted physical and mental well-being, while McKay (3) was led by his observations on the peoples of India, to the conclusion that a liberal supply of protein in the food of man was a factor of great importance in determining the success of the human dietary. It is now apparent that the protein moiety is but one of several indispensable components of an adequate food mixture, all of which are indispensable and must be viewed as of approximately equal importance. For each of these components there is an optimum, either of amount or constitution, which will best meet the nutritive needs of a mammal. We are now in a position to reexamine, with a broader viewpoint than the earlier investigators, the whole subject of human nutrition in the light of the results of animal experimentation, with the prospect of arriving at a decision as to the best type or types of diets suited to the development of an imperial race.