Purely with reference to the development of physical strength, which alone is under consideration, any animal organism whatsoever must be considered simply in the light of a machine.

A compound machine having two parts, first an arrangement of levers and points of application of power, all of which is purely mechanical, together with an arrangement of parts, designed, first, to convert fuel or food into heat, and, secondly, to transform heat into force, which is purely a chemical change in the first instance, and a transformation of energy in the second.  So much for the animal—­man or beast—­as a machine physically considered.

What then is animal strength considered in the same light?  The animal is not creative.  It can make nothing—­it can only transform.  Does it create any strength or force?  No.  The strength it puts forth or exerts is merely the outcome of this transformation, which it is the office of the machine to perform.

What do we find transformed?  Simply the energy, or potential, contained in the fuel or food we put into the machine.  Its exact equivalent we find transformed to another form of energy, known as animal strength, which is simply heat within the system available for the working of its mechanical parts.  How, then, is this energy which exists in the shape of animal strength used and distributed?  This is the question the answer of which underlies this whole discussion as a principle.  It is distributed to the different parts of the machine in proportion to the relative amount of physical work that nature has made it the office of any particular part to perform.

Let us see how it is with the bird machine.  In course of flight he is called upon to remain in the air, which means that should he cease to make an effort to do this, i.e., should he cease to expend energy in doing it, he would fall during the first second of time after ceasing to make the effort some sixteen feet toward the center of the earth.  But he remains in the air for hours and days at a time.  What is he, then, doing every second of that time?  He is overcoming the force of gravitation, which is incessantly pulling him down.  That is, every second he is doing an amount of work equal to his weight—­say 10 lb. multiplied by 16—­say 160 lb. approximately; all this by beating the air with his wings.  Now let us institute a slight comparison—­and the work shall be performed by a man, who climbs a mountain 10,000 feet high in 10 hours.  The man weighs 150 lb.; he climbs 10,000 feet; 1,500,000 foot pounds is, then, the work done.  He does it in 10 hours, or 36,000 seconds, which gives an amount of work of only 42 foot pounds per second performed by his muscles of locomotion.

At the end of the ten hours the man is exhausted, while the bird delights in further flight.  To what is this difference of condition due? It is due simply to the difference in the machine; but this, you say, is not explaining the unknown in terms of the known.  Let us see, then, if we cannot do this.  In the two accounts of work done as above cited in the case of the man and the bird, an amount of energy, i.e., heat of the system, has been expended just proportional to the work done.