There is no kind of alimentary substance of which it can be said absolutely that it is always proper for the sustenance of man. To be serviceable, _the food must be adapted to the age, constitution, state of health, and mode of life, of the individual, and to the climate and season of the year_. The same diet which, administered to an adult, is healthful and nutritious, may prove irritating and injurious to a child; and, in like manner, the stimulating animal diet which in winter is highly grateful to the system of a hard-working unexciteable labourer, may prove utterly destructive of health when indulged in during summer by an inactive and exciteable female. It becomes, therefore, an object of deep interest to determine the principal causes and states of the system which render modifications of diet necessary. Among these modifying causes, _the varying state of the body at different periods of life_ ought certainly to be considered as one of the most influential. The differences observed in the constitution of the same person at different ages, are at least as great as those subsisting between different individuals at the same age. In infancy the lymphatic and nutritive functions greatly predominate over the muscular; and if the highly animalized food which suits the latter were then freely administered, it would infallibly induce disease, and fail to afford the requisite nourishment. In manhood, again, when muscular activity and nutrition are at their height, the lymphatic system in abeyance, and the person subjected to laborious exertion, the mild food of infancy would be equally misplaced. In advanced age, another change of constitution occurs. The soft tissues of early life have disappeared, and the whole frame is dry and wasted--a condition which, in its turn, requires food of a different kind from that fitted for either of the preceding stages.