The early experiments of McCollum and his co-workers yielded results which seemed to warrant the generalization that, when estimated on the basis of the nutritive needs of the rat, all seeds, of plants, tubers and fleshy roots were decidedly deficient in this substance. This judgment rested on their observation that all foods they examined, which were functionally storage tissues of plants, could be enhanced in their dietary properties by the addition of butter fat. This generalization still appears to be justified, although with certain reservations which at that time could not be discerned. There is room for difference of opinion among students of nutrition concerning what constitutes an adequate amount of any dietary essential. One worker may make the basis of his judgment the ability of an animal to grow in an apparently normal manner to approximately the full adult size and may thus be led to the belief that a certain intake of fat-soluble A is adequate. Another worker, planning his experiments so as to cover the entire life history of his animals, may find that the minimum amount of the factor in question which will just serve for the support of normal growth and the apparent maintenance of health during several months, is inadequate when the entire span of life is taken into account, and that a degree of deficiency which did not make itself apparent in the shorter period of observation is easily detected in the appearance of early deterioration and aging. The fertility may be affected as may infant mortality, or there may ensue defective development of the bones or teeth or both. Much depends upon the standards of the investigator concerning what constitutes "normal" nutrition. As will be pointed out in a later chapter, we have come to hold the view that "normal" is not a good word to use in nutrition studies. Optimal well-being, the best condition which can be reached, should serve as the standard of comparison.