"A very large number of variables must be considered and either eliminated by matching or allowed for in the plan of any piece of research in . First the animals, including man: to be taken into account are breed, sex, age, size at birth and at completed growth, rate of growth or of production (milk, egg, meat), stage of reproductive life, previous dietary history, physical environment (e.g., temperature and humidity). Second, in diet there is a great complexity of s. Known to be of importance are about 20 s, 14 inorganic elements each in many different compounds, and 17 s of known structure; of s there are 10 sugars and 8 groups of s; 14 s, and as many s, innumerable pigments and aromatic substances, as well as many injurious and inert components, present in countless different foods. Add to which that a complete longitudinal study of human growth must take about 20 years, of growth in a pig at least 3 years and even in a rat 9 months; and that there is the reproductive span also, and old age beyond that. It is obvious that the number of possible questions is enormous and that no one man or one team in the whole of a lifetime can hope to cover by original research more than a minute part of any nutritional field."