788 The American Naturalist. [October, 
and the large expenditure in the cooling machinery for vapor- 
izing water is an important factor in securing a proper adjust- 
ment in equilibrium of the numerous physiological activities 
of the system. 
The fallacious and misleading theories of nutrition we have 
noticed have been so widely disseminated in recent official re- 
ports and bulletins that a detailed statement of the role of en- 
ergy in animal nutrition, as now recognized by physiologists, 
seems to be required in relation to our present subject. 
The energy required as the motive power in building tissues 
and the elaboration of animal products, as well as that ex- 
pended in muscular work, is all derived primarily from the 
potential energy of the organic substances consumed as food, 
and as an incident of the various metabolic processes of the 
system animal heat is produced. 
The stored, or potential, energy of organic substances is an 
essential element of their constitution, representing the work 
done in their processes of construction, and it can be liberated 
in the form of heat by any of the various methods of disinte- 
gration to which they may be subjected. For example, when 
organic substances are burned the heat produced is a measure 
of the energy stored up in their construction, but this method 
of liberating heat from food constituents has no place in phys- 
iological processes, and the value of foods as fuel is not a legit- 
imate subject of discussion in domestic economy. 
Microbes disintegrate the organic substances on which they 
feed and liberate their stored energy in the form of heat, as in 
the familiar processes of fermentation. The numerous mi- 
crobes in the alimentary canal add their quota to the avail- 
able energy in the form of heat derived from the constituents 
of foods which they tear apart in feeding upon them. The 
digestion of foods by animals is another means of liberating 
the stored energy in the form of heat. 
Destructive metabolism, resulting from the various activi- 
ties of the animal machine, immediately follows the construc- 
tive processes and the stored energy of the tissues originally 
derived from food constituents is liberated as heat which, so 
