The craving for change of food depends, as a rule, on the fact that the fodder habitually consumed is not sufficient for the requirements of the system. In this case, the condition of the food - its being too dry, too hard, or too moist - or the proportion or number of its constituents or elements will probably be at fault; for in health, the nearer the food is to perfection, the less will be the desire for change. Thus we find that the craving for vegetable food evinced by a foal increases with age; and that many species of animals live almost exclusively on one particular kind of diet. Although the horse in a state of nature might appear to be a one-food (a solely grass-feeding) animal, grasses differ from each other in their nature and composition. A study of the tables on pages 62 and 63 shows that deficiency of constituents in one food may be supplied by feeding on other foods. As regards the food of the horse, the transition from a state of nature to life under civilised conditions is chiefly one from green food to dry food, and from food poor in nitrogenous matter to one rich in that constituent. The means for remedying any ill effects to health from such a transition are self-evident. It goes almost without saying that, as a rule, the less the desire shown by a horse to change his food, the more suitable will it be to meet his requirements, supposing that he is in strong work.