Monotony of diet is not incompatible with maintenance of life, and even of health, when the food is restricted to two or three articles only, but for the reasons given in discussing the force value of different foods (p. 7) there is no single food, not even milk, which will support man in ordinary health and vigour for long. There are many primitive races and tribes of man who live comfortably upon a diet so restricted that it would soon prove unbearable for a European or an American. Much, therefore, depends upon custom, and no doubt upon heredity. The Hindu eats boiled rice and ghee, or melted butter, and the low-caste Chinese eats rice to the almost, but not complete, exclusion of other food; the Eskimo lives upon one or two kinds of meat or fish, and a little fat; and the Congo native subsists chiefly upon the plantain. The Central American Indian lives almost entirely upon maize, and some of the Polynesians eat bread-fruit alone for two thirds of the year. The roving Indian of the North American plains originally subsisted all winter upon a diet of salt meat, and the Scotch peasants formerly lived for six days in the week upon oatmeal porridge.

Such a monotony of diet is, however, usually a question of environment and not of choice. The carnivorous Eskimo enjoys canned vegetables when they are offered to him, and the vegetarian African native gorges himself with meat when he can obtain it. As a rule, the more civilised the tribe of man the greater is the variety of his diet, and once accustomed to variety, it is very difficult to subsist upon a too restricted regimen. It is the ability to subsist upon a variety of foods which makes it possible for man to adapt himself so well to his environment when he migrates from one extreme of climate to another.

This adaptation is also possible because the elementary foods possess general nutritive properties for all the organs of the body, rather than special value for individual structures; for example, there is no "brain food " in distinction from food which nourishes other organs as well.

An ideally perfect food combination if made upon purely theoretical considerations of the needs of the body for just so much albumin, fat, starch, sugar, salts, and water would be a compound which in a very short time would become too monotonous and wearisome to be eaten. Even the domestic animals are kept in better condition by occasional slight changes in diet - such, for instance, as are afforded by change of pasturage or the variety which the season of the year produces in their natural food; and it is well known that the flavour of the meat of fish and wild animals depends upon the nature of their diet. For example, canvasback and redhead ducks are much more palatable while feeding upon the wild celery plant than when eating other food, domestic turkeys and capons improve when fed upon grain, swine flesh is made better by feeding the animal with corn than with skimmed milk, and salmon and shad acquire a more delicate flavour when feeding in fresh-water rivers.

In a report of digestion experiments made for the United States Department of Agriculture (Bulletin No. 85, 1900) by Charles D. Woods and L. H. Merrill, the statement is made that "it is a matter of common observation that digestion experiments made with one kind of food material do not give on the whole as reliable results as those in which two or more food materials are used. In other words, it appears that with a mixed diet the same person will digest a larger proportion of nutrients than with a diet composed of a single food material".

Among some peoples the variety of food is considerably restricted by religious observances, custom, and associations. For instance, there is the Buddhist prohibition of meat and the Jewish prohibition of swine flesh. When a variety in food cannot be secured the desired effect in stimulating the appetite and digestive secretions may be obtained by altering the methods of cooking and by modifying the taste and odour of food. On the other hand, too great variety, as well as too elaborate cooking, becomes equally tiresome. Those who eat constantly at restaurants and large hotels, where the table is greatly diversified, often find that a change to a simpler home table agrees with them better.

Woodruff attributes the decrease in drunkenness in the past few years in the United States army to the advantages accruing from a much greater variety in diet since fresh vegetables were made a part of the ration, and since by a system of exchange the soldier has. been enabled to barter an excess of common ration food for a few articles of luxury. He also says: "Variety is necessary in the army for another reason: When the diet is very simple there is apt to be constipation, and in the field this condition is sometimes quite marked. It is recognised by physicians that chronic constipation cannot be properly treated with drugs, and it must be rectified by diet. The field ration, if possible, should overcome this tendency to constipation. Sluggishness of the bowels quite commonly goes along with discontent, homesickness among soldiers, and in such conditions the soldier is not a reliable fighter - he is easily beaten".