The results of observing animals beyond the completion of growth, to determine their fertility, success in rearing young and the rate at which they developed the characteristics of old age, were so striking that we extended our observations to include restricting single families to a monotonous diet through several generations. This brought to light the fact that a diet may be sufficiently good to enable an animal to grow in a perfectly normal manner, and to exhibit a moderate amount of fertility, yet with each succeeding generation the progeny becomes smaller and less well developed physically, and after two, three or four generations the strain dies out. Several comparable examples of similar experience in man may be cited. Campbell (5) stated that among the poorer classes in the slums of English cities, the family never passes beyond the third generation. Galton (6), in a statistical analysis of one thousand town families and one thousand country families selected from the town of Coventry and the surrounding rural population, found that the town population supplied to the succeeding generation but three quarters of the number supplied by a similar number of country people. In two generations the adult grandchildren of the town dwellers amounted to but little more than half the number of descendants of the country dwellers. The employed urban dwellers are continually being recruited from country stock, and themselves tend to die out. Food is cheaper in rural districts and the selection and quality is distinctly better generally speaking, than it is where the supply is derived almost entirely from the grocery store. The grocer carries a surprising proportion of stock derived from cereal grains, legume seeds, fruits, tubers and muscle meats. It has already been emphasized that food-stuffs from this list cannot be combined so as to constitute a satisfactory diet. The behavior of our rat population when restricted to diets corresponding in their origin to those of the urban dweller of either England or the United States, will be described immediately below. The extent to which the rat has proven to be sensitive to even small modifications of the nature of its diet is truly amazing. It suggests that it is much more important than has been hitherto believed, to have the diet nicely adjusted with respect to all its essential constituents, in order that the development shall be as nearly the optimum as possible. It is under such conditions that the characteristics of youth are preserved as long as possible, and the span of life prolonged.