All food has to answer two distinct purposes: the one to furnish materials for the growth of the body, the other to afford matter for the maintenance of its temperature; and life cannot be supported except on a diet in which the elements of nutrition and those of respiration bear a certain proportion to each other. Now, in milk, the proper food of infants, the elements of the former are to those of the latter about in the proportion of 1 to 2, while in arrowroot, sago, and tapioca they are only as 1 to 26, and in wheaten flour only as 1 to 7. If to this we add the absence in these substances of the oleaginous matters which the milk contributes to supply the body with fat, and the smaller quantity, and to a certain extent the different kind, of the salts which they contain, it becomes apparent that by such a diet the health if not the life of the infant must almost inevitably be sacrificed. But these substances are not only less nutritious, they are also less easy of digestion than the infant's natural food. We all know how complex is the digestive apparatus of the herbivorous animal, of which the four stomachs of the ruminants are an instance, and how large is the bulk of food in proportion to his size which the elephant requires, compared with that which suffices for the lion or the tiger. The stomach of the infant is the simple stomach of the carnivorous animal, intended for food which shall not need to stay long in that receptacle, but shall be speedily digested; and it is only as the child grows older, and takes more varied food, that the stomach alters somewhat in form, that it assumes a more rounded shape, resembling somewhat that of the herbivorous animal, and suited to retain the food longer. The young of all creatures live upon their mother for a certain time after birth; but in all the preparation for a different kind of food, and with it for an independent existence, begins much sooner and goes on more rapidly than in man. Young rabbits are always provided with two teeth when born, and the others make their appearance within ten days. In the different ruminants the teeth have either begun to appear before birth, or they show themselves a few days afterwards; and in either case dentition is completed within the first month, and in dogs and cats within the first ten weeks of existence.