Let us illustrate the principle we have in mind by the results of animal experimentation. McCollum, Simmonds and Parsons (10) restricted a large number of groups of young rats to diets which were faulty in different degrees with respect to the quality of their protein content. The experiments included the period of growth, and of adult life at least to the point of marked development of senile characters. The fertility and success with the nursing of young, the stability or instability of the nervous system as shown by indifference to being handled, which is characteristic of the well-nourished rat, or the apprehensiveness and timidity of the malnourished one, were carefully observed, etc. Successive generations of the same families, where young were reared, were maintained on the family diet. These experiments will be discussed more in detail in a later chapter, but it may be stated here that the deviation of the quality of the protein moiety of the diet even in slight degrees from the optimum, exerts a profound influence on the life history of the animals and on the functioning of the nervous system.

It may be added that the faults of a diet need be no more pronounced than are encountered in a mixture consisting of cereal grains, legume seeds, tubers, fleshy roots and muscle meats, to produce profound physical deterioration when such a regimen is adhered to over any considerable period.