At the Iowa Experiment Station, Evvard has conducted extensive experiments of a character which were intended to demonstrate that the appetite and instincts of the hog serve to enable it to make such an adjustment of the relative amounts of the several food-stuffs offered it, as may induce better results in the rate of growth than can be generally secured when the adjustment is made by the feeder, and when the mixture of the ingredients of the ration are offered in a form admitting of no choice by the animal (7). The data secured seem to show that there is some basis for the belief that this element of selection by the animal itself is worth taking advantage of. It should be mentioned that as a rule in all these trials the animals were given a choice of only three foods, one of these being a cereal grain, another, a protein-rich food, and a third a plant leaf. In some experiments a salt mixture was made available. The reasons for the employment of the leaf as a never-failing component of the food supply of the growing pig could not have been explained before the studies of McCollum and his associates with simplified diets and with diets restricted to a single food-stuff. Feeding such diets with single and multiple food additions led to a knowledge of the exact nature of the dietary faults of each. In connection with the types of diets employed by Evvard it should be mentioned that in case the animal ate fairly liberally of all the food-stuffs offered it, a serious mistake would be hardly made, since the proportions of the several components eaten could be varied to a considerable degree without preventing growth. In the case of the mixture of maize, 50 per cent, alfalfa leaves, 30 per cent, and peas, 20 per cent, described above (Chart 7), it has been found that for the rat these are the best proportions in which these three ingredients can be mixed for the promotion of growth and reproductive processes. It has been further established that with the use of these three food-stuffs a moderate rate of growth may be secured, but that few, if any, young will ever be produced if the mixture fed contains more than 50 per cent or less than 10 per cent of alfalfa leaf. The importance of combining the natural foods in the right proportions is easily seen from these results. It is interesting to note further, that shifting the proportions of maize, peas and leaf in this mixture over a range of 20 per cent does not materially change the protein content, or indeed, the chemical composition of the food mixture to a degree that could be expected to make so great a difference in the state of nutrition of the animals as is actually observed.

There are now available the results of a very extensive series of feeding trials in which the rations were made up of one seed, one leaf and one legume (pea, bean) in various proportions (8). These have failed to reveal any mixture which is quite the equal of the first ration of this type ever employed, that composed of maize 50 per cent, alfalfa leaf 30 per cent, and peas 20 per cent. It is, of course, easily possible that better mixtures of vegetable foods may be found by further effort but these results show very definitely that for the omnivorous type of animal, whose digestive tract is so constituted that the consumption of large volumes of leafy foods is not possible, it is by no means a simple matter, to derive the diet entirely from the vegetable foods, and secure the optimum of well-being. The data afforded by the experiments described form a demonstration of the fact that wide variety is of little value as a safeguard to nutrition. Chemical analysis, no matter how accurate and thorough, fails to throw much light upon the dietary value of a food-stuff. The only way in which the problems of nutrition can be solved is through numerous and properly planned feeding experiments. Such studies, however, were not possible before the solution of the problem of successfully feeding mixtures of purified foodstuffs. These led to the formulation of an adequate working hypothesis regarding what factors operate to make an adequate diet, and made possible the interpretation of the cause of success or of failure with diets of the complexity employed in daily life. It will be shown later that the consumption of milk and its products forms the greatest factor for the protection of mankind in correcting the faults in his diet.