ENERGY RELATIONS OF PLANTS AND ANIMALS 27 



organism was indicated. It represents both building mate- 

 rial and fuel. If we liken the living body to a power-house, 

 we see clearly how both kinds of supplies are required. 

 Coal is the most bulky supply of the power-plant and the 

 one on which its operation most immediately depends. 

 But there must also be new parts to replace those which 

 wear out. The up-keep of the building calls for new 

 wood-work, for paint, etc. A certain difficulty is encoun- 

 tered in the attempt to show parallel conditions in the 

 case of the body because the separation of the two func- 

 tions is here much less sharp. Protein food, on the whole, 

 has a peculiar title to be regarded as building material, 

 but it is also an entirely available fuel. It is as if planks 

 and beams designed primarily to repair the structure of the 

 power-house were fed into its furnaces. The suggested 

 comparison must not be pressed too far, for it conveys 

 an impression of wanton destructiveness which we cannot 

 assume to be just in the case of the organism. There are 

 some minor supplies brought into the power-house which 

 are not fuels nor precisely materials for repair. The oil 

 is an example. Some of the extractives of the diet occupy 

 an analogous position, being neither sources of energy nor 

 of construction, but nevertheless favorably affecting the 

 course of events. This comes near to our conception of a 

 drug in relation to the processes of life. 



