842 MODIFICATIONS OF DIET. [BOOK 11. 



mammal, we find that the former is almost as clearly distinguished 

 from the latter by its frequent and abundant faeces as by the 

 anatomical features of its organization. We have already urged 

 that, since the faeces serve as a means of excretion of the real 

 waste products of metabolism, a certain amount of vehicle to 

 carry these away is of advantage or even necessary ; but there are 

 no facts at present known to us, which shew that the larger 

 intestinal current of the purely vegetable diet effects any such 

 good as can compensate for the obvious waste of labour incurred 

 in its transport and management, to say nothing of the opportuni- 

 ties of mischief offered by a mass of material more subject to the 

 dominion of foreign organisms than even to that of the body 

 itself, though these opportunities are less than with a corresponding 

 mass of animal origin. With respect to these three features then, 

 the strictly vegetarian diet seems, on physiological grounds inferior 

 to one of a mixed nature. There are as we said other aspects, 

 still of a strictly physiological kind, to be considered, such as the 

 relative digestibility of vegetable articles of food, the relative 

 metabolic value of the food-stuffs of vegetable origin, and the 

 influence of animal extractives ; but any fuller discussion of these 

 points would be out of place here. 



554. We have treated the diet discussed above as a normal 

 diet, suitable for man under ordinary or general circumstances. 

 Ought such a diet to be modified for the various exigences of life 

 such as labour, age, climate, and the like ? 



We shall discuss the influence of age in the concluding portions 

 of this work. 



We may be inclined at first sight to assume that the total 

 amount of the diet should vary with the weight, that is the 

 size of the individual; and indeed in discussions on nutrition, 

 statements concerning metabolism and amount of food are often 

 given in terms of per kilo of body weight. In a broad sense it 

 may be true that a small man needs less food than a large one ; 

 but it must be remembered that, as we saw in speaking of animal 

 heat, the smaller organism, having the relatively larger surface, 

 carries on a more rapid metabolism per unit of body weight, 

 and so needs relatively more food. And moreover the influence 

 of size is probably far less than the influence exerted by the 

 inborn individual characters of the organism, giving rise to what 

 we may call the personal equation of metabolism. The smaller 

 metabolism of woman, leading to the use of a scantier diet, as 

 compared with that of man, is to be regarded in this light rather 

 than with reference to the average lesser weight of woman. The 

 relative metabolism of the two sexes may be illustrated by the 

 case of an active man and his wife, both of about the same age 

 and weight, the man being rather the heavier and the woman 

 rather the older, who, in carrying out together an experiment on 

 the relative values of vegetable and animal food, both lived for 



