Animals, taking one kind with another, nourish themselves on an immense variety of food. The flesh and the blood of other animals of all kinds, warm or cold, the leaves, twigs, fruits, juices of plants, putrid carcases, hair, feathers, skin, bran, sawdust, the vegetable mould or "humus" of the earth's surface, the sand of the sea, with its minute particles of organic detritus, all serve as food to different kinds of animals. Some are very little fettered in their tastes, and are called "omnivorous," others are bound in the strictest way to a diet consisting of the leaves of some one species of plant or the juices of one species of animal. Some of the latter class, under stress or privation, can accommodate themselves to a new food very different in character and origin from that which is habitual to them; others have no elasticity in this respect, and must have their exact habitual food-plant or food-animal, unless they are to die of starvation. Man exhibits his great powers of accommodation to changed circumstances in respect of food as well as in other matters. If we are to suppose, as is probable, that our original ape-like ancestors fed exclusively upon fruits and an occasional egg or juicy grub, how vast are the changes in diet to which man has habituated himself! Man is sometimes said to be omnivorous, but this is not a sufficient description of the state of things which has grown up as he has spread over the earth's surface. Every race--and even many a small group of men--has its accustomed diet, to depart from which is a pain and a difficulty, even though new kinds of food may be gradually accepted and even become popular. Man has in this, as in so many other things, a large range of possible accommodation, but he has at the same time habits the continuance of which are necessary for the healthy working of the nervous system. The psychical element in the matter of food-habit is important in all higher animals, but most of all in man. The digestive organs are controlled by the nervous system, and the brain acts upon the latter in such a way as to favour or to restrain the "appetite" and the secretion of the elaborate digestive juices, so that fear, surprise, disgust, and "nausea" (that strange product of mental and physical reactions) may destroy appetite and inhibit the digestive process. There are vast populations of men who live on rice, or beans, or meal, and never eat animal food, not even milk (after babyhood), nor cheese, and would be, at a first attempt to eat it, "put off" and disgusted by a mutton chop. There are others who subsist almost entirely on fish, others who live on dried beef, others who live on the fat of whales and seals, and would be for a generation or two injured, half starved, and some of them even killed, by a change of diet. Again, there are others who consider that they must have and will be "ill" unless they had the cooked flesh of an ox or sheep as part of their daily food.