794 The American Naturalist. [October, 
The results recorded in table 2 must then be attributed to 
differences in the working machinery of the animals them- 
elves, and the modifying conditions of the relative amount of 
coarse fodder and proteids in the feed consumed, but with our 
present knowledge of vital activities the relative influence of 
these variable factors cannot be determined. 
The expenditures of energy by different animals in differ- 
ent ways, given in table 2 in foot tons of work, are summar- 
ized in percentages of the energy of feed consumed in the fol- 
lowing table for convenience of comparison. 
In the last columns of tables 2 and 4, several well defined 
physiological processes are grouped together from the lack of 
data to discriminate between them, and they undoubtedly 
vary in the relative expenditures of energy required in them 
under different conditions. Without attempting an exhaus- 
tive enumeration of these processes, the following may be no- 
ticed as of the first importance from the work performed in 
their activities. 
Constructive metabolism in the repair of tissues; vaporiza- 
tion of water exhaled by the lungs and skin; work of the in- 
voluntary muscles in respiration and circulation of the blood ; 
energy expended in mental activities and the functions of se- 
cretion and excretion; loss of heat by radiation from the 
body ; and work done by the voluntary muscles. 
It will be seen from table 4 that the largest percentage of 
the available energy of foods is expended in these strictly 
physiological processes concerned in maintaining the integ- 
rity of the animal machine. Under the conditions of feeding 
experiments, with fattening animals and cows giving milk, 
but little mechanical work is done by the voluntary muscles, 
and the last item of our enumeration of physiological processes 
might have been omitted as insignificant in relation to the 
enormous expenditures of energy in the other normal activi- 
ties of the system to which attention has been directed. 
If mechanical work is done by animals, it must be at the 
expense of the energy that might, under other conditions, be 
expended in the manufacture of animal products, as the phys- 
iological processes enumerated above must all be provided for 
