114 VARIOUS FOOD-PLANTS 
tions, 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 apprecia- 
tion 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. 
41. Food as fuel and building material. Before proceeding 
to compare vegetable with animal foods certain fundamental 
facts regarding food in general must be considered. We know 
that so long as a man is alive and active, the parts of his 
body are wearing out from daily use, and he is losing heat. 
If deprived of food, his weight and strength decrease, while, » 
on the other hand, if he is properly fed, nutritive materials 
become incorporated with the various parts of his body as 
fast as these wear away, and he finds his strength kept up by 
a constant supply of energy. Were it possible to conceive 
of a steam-engine which could derive from the fuel it con- 
sumes not only heat and power but also material to replace 
that used up in action, we should have a machine to which 
we might liken the human body in its use of food. If we 
could imagine, furthermore, a small locomotive able to do 
all this, and also to increase the size of its parts by the addi- 
tion of extra material, so as to grow into a large locomotive, 
such a marvelously endowed machine would be very like 
the body of a child. Thus we see that food answers the 
double purpose of supplying us with building material and 
with fuel. But as already intimated in the last chapter, 
proteids, fats, and carbohydrates are not equally useful as 
sources of substance and energy. 
As the chief wear in our bodies comes upon the muscles 
and other parts that are composed largely of nitrogen, and 
as neither fats nor carbohydrates contain this element, it 
follows that proteids, being nitrogenous, must be of the first 
importance as furnishing building material. This enables 
us to understand why it is that an animal deprived entirely 
