In the case of those groups where the state of their nutrition was sufficiently good to enable them to show a moderate fertility, it would be observed that the infant mortality would be higher the more deficient the food mixture. On such diets partly corrected by suitable additions the infant death rate has been uniformly 100 per cent. Another group with additions more nearly approaching the optimum will rear 40 to 60 per cent of their young, while the same basal mixture, fed supplemented with the right amounts of the substances in which it is deficient will produce a group of adult rats which will scarcely lose one litter in a hundred.

Chart 11. - The three curves shown illustrate the lack of fat-soluble A and the low quality of the proteins in a mixture of milled cereal products and legume seeds. In each experiment the chief mineral deficiencies were corrected by the addition of sodium chlorid and calcium carbonate. Lot 2152 had a diet which contained proteins of poor quality and was deficient in fat-soluble A. Note that the animals grew but little, and began very early to fail as shown by loss of weight. Their entire lives were not longer than the period during which a well fed rat normally grows.

Lot 2153 had a diet essentially like that of Lot 2152 except that its proteins were enhanced by the inclusion of round steak. The diet was still deficient in fat-soluble A although by no means lacking in it. Growth was rapid for a time but the full adult size was never reached, and the animals early developed the characteristics of old age.

Lot 2070 had essentially the same diet as Lot 2153 except that cabbage and whole milk powder (Merrill-Soule) were included. These served to further enhance the proteins and to furnish fat-soluble A in moderate amount. The result is very spectacular. The animals grew rapidly to large size and maintained for a long period the characteristics of youth.

Formerly in our colony the terminal event in the lives of many of the animals restricted to such deficient diets as have just been described, was a lung infection caused by streptothrix. As the animals lost their vitality through restriction to faulty diets they became more and more a prey to the disease. With selection of vigorous breeding animals and with cleanliness we have succeeded in greatly reducing the occurrence of this infection.

When the diet is made sufficiently good to enable the rats to rear only a fair amount of their young, there is a strong tendency for the animals to be smaller with each succeeding generation, until within three, four or five generations they fail to reproduce and the strain dies out. All the experimental groups described, it should be remembered, would have been sufficiently well nourished to enable them to grow to two-thirds to full adult size, and to appear normally nourished while they were growing, yet dietary deficiencies of no greater magnitude than those we are here discussing, may profoundly influence the life history of the individual or the history of the family.

Using the degerminated cereal, and whole cereal, and legume seed mixture described as the basis of the experimental diets just discussed, it would be necessary to add about 1.5 per cent of calcium carbonate; 1.0 per cent of sodium chlorid; perhaps enough of some phosphate to furnish about half of one per cent of the phosphate radical, to make the diet complete with respect to its mineral content. The addition of 3 to 5 per cent of butter fat or 2.0 per cent cod liver oil, and 15 per cent of a purified protein, casein for example, would complete the organic portion of the ration and would constitute a highly satisfactory food mixture through many generations with no noticeable deterioration of the race on account of monotony of food supply. How strikingly similar is the history of such a series of animals on carefully graded diets, to the experience of the urban dwellers of England, which Galton describes. (See Chart 12.)