40. Vegetable foods in general. In the foregoing sections, it has been shown that a classification of vegetable foods based upon the manner and degree of their usefulness is at the same time a fairly accurate grouping according to chemical peculiarities, the reason being that their use depends largely upon their composition. Guided by this principle we may now profitably compare vegetable foods with those of animal origin, so as to gain a better appreciation of their relative value in supplying human needs. A satisfactory understanding of the uses of food-plants clearly involves the study of this larger question.