CHAP, in THE THEOEY OF HUMAN NUTKITION 89 



rule, guided by the instinct of self-preservation, by the need of food, 

 which is stimulated by special internal sensations called appetite 

 or hunger, according to the degree of intensity (Vol. IV. 

 Chap. II. 4). Herbivorous or frugivorous animals derive their 

 nourishment from vegetables; carnivorous from herbivorous or 

 other carnivorous animals ; man is usually regarded as omnivorous, 

 because he lives sometimes on vegetable, sometimes on animal 

 food, more often on both, according to the climate, the kind of 

 food most easily obtainable, and the flora and fauna of the region 

 in which he lives. 



In tropical countries he lives almost entirely on vegetables, 

 in the frigid zones on meat and animal fat, and in temperate 

 regions on a mixed diet. 



The nitrogenous and non-nitrogenous food substances necessary 

 for the nutrition of men and animals, are found in varying quantities 

 in the different natural foods, both animal and vegetable. Proteins 

 and fats predominate in animal food, carbohydrates in vegetable 

 food, as will readily be seen from a glance at the diagram of the 

 percentage composition of the groups of food substances found in 

 the complex animal and vegetable food most used by man in our 

 temperate climate (Fig. 16). 



In order to solve the practical problem of the choice of foods 

 best suited to meet the alimentary requirements of man, or, in 

 other words, to ascertain the normal diet of adult men, physio- 

 logists, hygienists, and doctors have usually had recourse to 

 statistics. After examining the quality and quantity of the 

 animal and vegetable food taken daily by a large number of 

 persons in good health, they have found the average quantities of 

 protein, fat, and carbohydrate, and considered these to represent 

 the normal daily diet of adult man. 



The value of the statistical method rests entirely on the 

 assumption that in the choice and quantity of his food, man, like 

 animals in general, is guided by instinct and experience, and 

 that he has learned to desire and take what is good for him and 

 to reject or refuse what is harmful. Yon Noorden, in his recent 

 treatise on the pathology of metabolism (1900), expresses this idea 

 in the following words : " If the knowledge deduced from the 

 systems of nutrition selected by different races of their own accord, 

 be regarded as a biological law, we are undoubtedly justified in 

 assuming that mankind, in the course of about four thousand 

 years of struggle for existence, has established a suitable diet and 

 the proper standard of protein." 



The most reliable statistics for our purpose are those derived 

 from careful inquiry into the food of persons free to choose it for 

 themselves. The results of inquiries into the diet of persons 

 living in a community, such as soldiers and sailors, school children, 

 pupils in art or technical institutions, and prisoners, though far 



