It is a remarkable fact that for a time after birth every mammal is incapable of taking the type of diet which suffices for the maintenance of normal nutrition in the adult. It must have milk for a certain period. The duration of dependency on a milk diet is not the same for different species. Among all the mammals the young of the guinea pig appears to have the shortest period of dependence upon the mother. It is born in a very advanced state of maturity and can eat grass or succulent vegetables during the first or second day of post-natal life. The young rat may safely be weaned at the age of twenty-five days, provided the diet of the mother during the time of nursing is satisfactory, and provided the young are put as soon as weaned, upon a diet of good quality. If the mother's diet has been faulty in any respect it may be necessary to prolong the nursing period to thirty to fifty days before the young reach a state of independence. The young pig (swine) is able to eat at six to eight weeks fairly liberally of the normal diet of forage plants and cereals or roots. The human infant must live during the first year of life largely on a milk diet, and cannot thrive without a fairly liberal supply of milk during a considerable part of the entire period of growth. Even eggs cannot entirely replace milk during any part of the nursing period. It is of great importance that we should understand the relationship between the character of the diet of the lactating female and the quality of the milk secreted. Our knowledge of this phase of nutrition has been greatly enhanced during the last few years, and the general principles are now well understood.