1896.] Relative Efficiency of Animals as Machines. - 785 
in the exhalation of water by plants, and evaporation from the 
soil; and at the Madison meeting similar estimates were given 
of the potential energy of an acre of corn, and of a fat ox, as 
representing the work done in the constructive processes of 
growth. The substance of the last mentioned paper, with 
some additional matter was published in the AMERICAN NAT- 
URALIST of July, 1894. 
Some further illustrations of the same general piiiiijlte are 
now presented in an inquiry as to the relative efficiency of dif- 
ferent classes of animals, as machines, in utilizing the potential 
energy of their food in useful work. 
From the imperfect data now available, there are many 
questions relating to this subject that cannot be definitely 
answered, but the approximate quantitative estimates we are 
able to make must be of interest in suggesting the lines of re- 
search required for a satisfactory solution of the problems in- 
volved in discussing the economy of foods and diets, and espe- 
cially in the interpretation of the results of feeding experi- 
ments. 
The chemical theories of nutrition have been so generally 
accepted in popular expositions of alimentary processes that it 
may be well to recapitulate the leading facts relating to energy 
as a factor in physiology in order to clear up the field of view 
and give due prominence to the principles we have to deal 
with. 
The food consumed by animals serves two distinct purposes 
which should be clearly distinguished. The materials re- 
quired in building tissues, and in the manufacture of animal 
products (meat, milk, wool, etc.), have alone been noticed in 
popular essays on the subject of nutrition, while the quite as 
important expenditures of the energy supplied in foods, as the 
motive power required in the constructive processes involved 
in converting the food constituents into animal tissues and 
products, have been misinterpreted or entirely ignored. 
As pointed out in the papers above noticed, but a limited 
amount of the constituents of foods are stored up by animals 
in their processes of growth—in their increase when fattened 
—or in the animal products they manufacture. With refer- 
