Several possibilities have been discussed as explanations for the failure of milk of the human species to meet certain nutritional needs of its young under living conditions prevailing in industrialized countries. The thoughts presented are speculative, and, in some instances, more than one explanation has been offered. The welfare of the mother as well as that of the infant must be considered--e.g., the mother's health might be seriously jeopardized if 0.5 to 1.0 mg of iron daily were secreted in milk. Moreover, evolutionary forces may be expected to influence milk production and composition in relation to diets and environmental conditions existing over several hundred thousand years. If we change our diets (e.g., consume less vitamin K) or if we change our environment (e.g., protect infants against sunlight and restrict the infant's contact with iron in dirt), milk that was adequate under prior living conditions may no longer be adequate. Finally, we wish to protect all infants against death or disability, including that resulting from a catastrophic event (e.g., intracranial bleeding as a result of vitamin K deficiency) too rare to be expected to influence evolutionary changes.