1896.] Relative Efficiency of Animals as Machines. 795 
in fattening animals and cows giving milk with a minimum 
of muscular exercise, as well as in the case of animals engaged 
in severe muscular work. j 
The energy expended in mental activities is of the first im- 
portance in its influence on the efficiency of the animal ma- 
chine in useful work. The nervous system, through which 
mental endowments are manifest, has intimate relations with 
every part of the animal machine and the direction in which 
energy is expended is largely determined through its agency. 
Practical farmers are well aware that animals fail to give ` 
profitable returns for feed consumed when restless and ex- 
cited through any source of disturbance, or when dissatisfied 
with their feed and surroundings. 
The available energy of a liberal supply of nutritious food 
may all be expended, and even the stored energy of the tissues 
drawn upon to carry on the increased physiological activities 
resulting from mental and nervous derangements of the nutri- 
tive machinery without any expenditure in profitable produc- 
tion. In conducting feeding experiments and in the interpreta- 
tion of their results this is one of the most difficult factors to 
deal with, as it may have a dominant influence on the final 
outcome. 
The approximate estimates of the relative efficiency of diff- 
erent animals in utilizing the potential energy of their feed 
in useful work, which have been given in mere outline, will 
require revision and correction as we become better acquainted 
with the specific influence of the variable factors of food and 
environment on the work performed by animal machines. 
Even in their present imperfect form they may, however, 
serve to illustrate the significance of energy as a factor in ani- 
mal nutrition and the futility of formulating diets and nutri- 
tive ratios in terms of their chemical constituents. 
