“When animals are viewed from the standpoint of machines they are wonderful mechanisms. Not only are they self-feeding, self-controlling, self-maintaining and self-reproducing, but they are far more economical in the energy they are able to develop from a given weight of fuel material, than any other existing form of motor.
While they are like the steam engine in requiring carbonaceous fuel, oxygen and water for use in developing energy; these are made to combine in the animal body at a much lower temperature than is possible in the steam engine, and a much smaller proportion of the fuel value is lost in the form of heat, when work is being done.”
Collins, E. V. and Caine, A. B.
"Testing draft horses,"
Bulletin (Iowa Agricultural Experiment Station): Vol. 20
, Article 1.
Available at: http://lib.dr.iastate.edu/bulletin/vol20/iss240/1