Animals have been looked upon as machines for converting the vegetable products of the farm into animal products of greater value, and this in the light of the law of the conservation of energy may be interpreted as the conversion of the potential energy of field crops into the useful work of an animal machine. Considered as machines for the manufacture of definite products, the efficiency of animals must depend upon the amount of work performed for a given supply of energy in their food.
An ordinary steam-engine formerly converted less than one tenth of the potential energy of the fuel consumed into useful work, and the attention of engineers has been directed to improvements in construction to secure greater economy and efficiency in the work performed, by a more complete utilization of the potential energy supplied in the form of coal or other fuel. The remarkable industrial development of the past few years, resulting in a material reduction of the cost of production and transportation, is largely owing to improvements in the steam-engine which have been brought about by a more intelligent application of the principle of the conservation of energy.
There are good reasons for the belief that the animal machine works with greater economy than the steam-engine, even in its improved form, but, according to the most favorable estimates, only a small proportion of the potential energy of foods is utilized in useful work, and there is a broad margin for improvement, even in what we call our improved breeds, to secure a more efficient expenditure of energy.
The problem of paramount interest in animal husbandry is essentially the same the mechanical engineer has been dealing with in his efforts to improve the steam-engine. It is simply to obtain the largest net returns in useful work from the potential energy of the food consumed. It is evident that improvements in the animal machine itself must be the leading object to receive attention, and the breeders of pure-bred stock must recognize this principle in their efforts for improvement. The form and proportions in which the chemical constituents of food are provided are of far less importance than the inherited capacity and capabilities of the animal machine to utilize and economize energy in the work involved in the manufacture of animal products.
When speaking of foods we should bear in mind the fact that there is but a limited demand in the animal economy for the so-called nutritive constituents, aside from their agency in the transformations of energy involved in the metabolism of the system. But a small proportion of the chemical constituents of foods are stored up in the body, even during the period of growth, when the demands for new materials in constructive metabolism are most active, while an abundant supply of energy in an avail-
