The extreme span of life of the rat under highly satisfactory conditions of nutrition and hygienic surroundings is about thirty-six months. Probably two-thirds of all animals kept under the best laboratory conditions would die before reaching the age of two years. In the group of animals just described, therefore, the span of life was reduced to about one-fourth to one-sixth of its extreme limit, as the result of faulty nutrition caused by a diet in which no essential was entirely lacking, or lacking to a degree which would cause the development of a specific deficiency disease.

Employing other portions of the basal food mixture described, and in preparing these portions for feeding a series of experimental groups of young rats, we could, in the light of our experience, add such amounts of a growth-promoting fat, and of the several mineral elements in which the food is deficient, as to form a series of diets which would be progressively nearer and nearer to the optimum in the character of each of the essentials for optimum nutrition. When several groups of young rats were confined to these diets we could confidently predict that they would differentiate, one group from another, in their length of life. All would be able to grow to the full adult size, and while they were growing they would appear to be perfectly normal animals with bright eyes and glossy coats. The interval following the completion of growth and preceding rapid decline would be determined by the extent to which, in any single case the faults in the food mixture were corrected by suitable additions. Thus we could make them last six months, ten months or twenty months after they ceased to grow before they would appear old and rough looking and would show the irritability characteristic of most old rats.