The answer is tolerably obvious.  The more the labour of digestion is economised, the more energy is left for the purposes of growth and action.  The functions of the stomach and intestines cannot be performed without a large supply of blood and nervous power; and in the comparative lassitude that follows a hearty meal, every adult has proof that this supply of blood and nervous power is at the expense of the system at large.  If the requisite nutriment is obtained from a great quantity of innutritious food, more work is entailed on the viscera than when it is obtained from a moderate quantity of nutritious food.  This extra work is so much loss—­a loss which in children shows itself either in diminished energy, or in smaller growth, or in both.  The inference is, then, that they should have a diet which combines, as much as possible, nutritiveness and digestibility.

It is doubtless true that boys and girls may be reared upon an exclusively, or almost exclusively, vegetable diet.  Among the upper classes are to be found children to whom comparatively little meat is given; and who, nevertheless, grow and appear in good health.  Animal food is scarcely tasted by the offspring of labouring people; and yet they reach a healthy maturity.  But these seemingly adverse facts have by no means the weight commonly supposed.  In the first place, it does not follow that those who in early years flourish on bread and potatoes, will eventually reach a fine development; and a comparison between the agricultural labourers and the gentry, in England, or between the middle and lower classes in France is by no means in favour of vegetable feeders.  In the second place, the question is not simply a question of bulk, but also a question of quality.  A soft, flabby flesh makes as good a show as a firm one; but though to the careless eye, a child of full, flaccid tissue may appear the equal of one whose fibres are well toned, a trial of strength will prove the difference.  Obesity in adults is often a sign of feebleness.  Men lose weight in training.  Hence the appearance of these low-fed children is far from conclusive.  In the third place, besides size, we have to consider energy.  Between children of the meat-eating classes and those of the bread-and-potato-eating classes, there is a marked contrast in this respect.  Both in mental and physical vivacity the peasant-boy is greatly inferior to the son of a gentleman.

If we compare different kinds of animals, or different races of men, or the same animals or men when differently fed, we find still more distinct proof that the degree of energy essentially depends on the nutritiveness of the food.