Protoplasm can only live under certain narrow conditions, of which the presence of oxygen, a moderate degree of warmth and of moisture, and a more or less regular supply of food are the most obvious. In the highly developed mammalia, and especially in man, these conditions are far more stringent than in lower creatures, many of which are possessed of great powers of resistance, whereas man can only survive the absence of oxygen for a few minutes, of water for a few days and food for a few weeks. On the other hand the range of temperature in which man can exist and be active is wider than that of any animal, owing partly to the capacity which his body has of automatically maintaining itself at a constant temperature, a capacity shared with birds and mammals, and partly to the artificial protections against extremes of cold with which he has learned to provide himself.
In the matter of food man is entirely dependent upon animals and plants. Plants, by means of their chlorophyll, use the energy of the sun's rays to build up their tissues from inorganic matter, but, if we exclude oxygen, water and salts, all the essential constituents of a man's dietary are the products of the protoplasmic activity of other organisms. When we consider the almost universal distribution of man upon the face of the earth, and that the plants and animals which flourish in different regions vary to an enormous degree, it is clear that the races of mankind are able to exist upon dietaries which must differ widely from one another. The difference is, however, more apparent than real, for the animals and plants consumed all owe their value as food to the fact that they are composed of one or more of three substances, protein, fat and carbo-hydrate. So that the contrast between the diets of different peoples, though very obvious when we regard the source and the appearance of their food, is less marked if its actual composition be ascertained. Yet when this has been done, and the diets of various races have been expressed as containing on the average such and such proportions of the three main food-stuffs it will still be found that these proportions, as well as the total amounts of food taken, are far from identical. The variations are caused by numerous factors, such as the nature of the plants and animals available for food, and the degree of intelligence shown in securing a regular supply. The quality and sufficiency of the diet has, conversely, a far-reaching influence upon the development of the race, an influence which is directly seen in the physical well-being associated with an adequate supply of suitable food. The progress of any nation will be hampered if its citizens are ill fed, for upon food depends not only life itself, but the power to work and to resist disease. Further, the development of the race is indirectly affected by diet through the sharpening of the wits and the social co-ordination which arise in the course of evolution from the necessity for the provision of food. For the supply of food depends upon the exertions of the individual, and its regular distribution upon the organization of the race.
In this chapter we shall pass in review some of the experimental data upon which the science of dietetics is founded. In so doing we must not neglect the evidence already provided. In the experience of mankind we have spread before us the results of the experiment of ages. That experiment shows that the human race can live a healthy life under dietetic conditions of great variety. This single consideration indicates with what care conclusions should be drawn. Wisdom does not find her dwelling place exclusively in this or in that system. Only by a careful and open-minded examination of the teachings of experience, as well as of the results of scientific experiment, can principles be enunciated which will form a basis upon which dietaries may be constructed for the varying conditions of health and of disease.
The problems of dietetics are of great intricacy, for no definite food or combination of foods can be regarded as suitable for different people, or for the same person under different circumstances. The application of the experimental method has, however in this, as in other branches of science, yielded some insight into the general principles which govern the nutrition of the body.
Innumerable examples could be gathered from the vegetable and animal worlds to illustrate the great effect of food variations upon growth and development, of which we may quote two or three. Brennan's experiments show the influence of a nutritive soil upon the evolution of a plant. In 1872 that observer set some plants of Tradescantia Virginia in good soil and carefully tended them. The ovary of this flower is trimerous, there being three petals, three sepals and six stamens. In seven years a number of the blossoms showed five segments, and in fourteen years an ovary with eight segments was detected. On the other hand, a bad soil, as every farmer knows, will yield a poor growth. The dwarf plants of the Japanese have been produced in the course of time by keeping them in poor soil and pruning the roots. A good instance of the effect of under-nutrition upon the development of animals is furnished by de Varigny, who kept tadpoles in the gill-bearing stage for two years by withholding suitable food. When flesh was given the limbs were evolved in three weeks, the long tail gradually disappeared, and the tadpoles became toads at last. But perhaps the most striking example of the effect of food upon development in the animal world is afforded by the queen bee, which grows from a larva in all respects similar to that of the worker bee, but is differently treated in the one respect that it is assiduously fed upon a rich diet.
The state of nutrition may also influence the determination of sex, particularly in many of the lower organisms. Prantl observed that the spores of ferns developed chiefly prothallia, the male elements, if placed in a soil poor in nitrogenous material, especially if there was much crowding; with better soil and fewer spores, most of them were found to contain the female archegonia. Yung found that frogs fed upon meat produce a greater percentage of females. Many quadrupeds and birds multiply more freely upon a plentiful diet, although a change from a natural supply to an excess of food may have the opposite effect, and even produce sterility.
The nature of the food may modify the structure of the alimentary tract. John Hunter demonstrated, in a specimen preserved at the Royal College of Surgeons, that in a gull which had been fed for a year upon grain the muscular coat of the stomach became thickened. It has since been shown that in certain gulls this takes place annually; for half the year grain forms the dietary, and at this time the stomach wall takes on gizzard-like characters, whilst when the animal is feeding on fish these features disappear. It is not within our scope to discuss here the effects which changes of diet produce in the domestic animals, in the composition, for instance, of the wool of the sheep, of the flesh of the ox and pig, and in many other respects. The annals of institutes of experimental agriculture in this country, in Canada, the United States, and on the Continent, record a great number of such observations and abound in material of the greatest interest, to some of which we shall have occasion to refer.