In regard to the second half of the question, whether natural forces are adequate to explain the running of the machine, we have again been able to reach a satisfactory positive answer.  Digestion, assimilation, circulation, respiration, excretion, the principal categories of physiological action, and at least certain phases of the action of the nervous system are readily understood as controlled by the action of chemical and physical forces.  In the accomplishment of these actions there is no need for the supposition of any force other than those which are at our command in the scientific laboratory.

==The Living Machine Constructive as well as Destructive.==—­In one respect the living machine differs from all others.  The action of all other machines results in the destruction of organized material, and thus in a degradation of matter.  For example, a steam engine receives coal, a substance of high chemical composition, and breaks it into more simple compounds, in this way liberating its stored energy.  Now if we examine all forms of artificial machines, we find in the same way that there is always a destruction of compounds of high chemical composition.  In such machines it is common to start with heat as a source of energy, and this heat is always produced by the breaking of chemical compounds to pieces.  In all chemical processes going on in the chemist’s laboratory there is similarly a destruction of organic compounds.  It is true that the chemist sometimes makes complex compounds out of simpler ones; but in order to do this he is obliged to use heat to bring about the combination, and this heat is obtained from the destruction of a much larger quantity of high compounds than he manufactures.  The total result is therefore destruction rather than manufacture of high compounds.  Thus it is a fact, that in all artificial machines and in all artificial chemical processes there is, as a total result, a degradation of matter toward the simpler from the more complex compounds.

As a result of the action of the living machine, however, we have the opposite process of construction going on.  All high chemical compounds are to be traced to living beings as their source.  When green plants grow in sunlight they take simple compounds and combine them together to form more complex ones in such a way that the total result is an increase of chemical compounds of high complexity.  In doing this they use the energy of sunlight, which they then store away in the compounds formed.  They thus produce starches, oils, proteids, woods, etc., and these stores of energy now may be used by artificial machines.  The living machine builds up, other machines pull down.  The living machine stores sunlight in complex compounds, other machines take it out and use it.  The living organism is therefore to be compared to a sun engine, which obtains its energy directly from the sun, rather than to the ordinary engine.  While this does not in the slightest militate against the idea of the living body as a machine, it does indicate that it is a machine of quite a different character from any other, and has powers possessed by no other machine. Living machines alone increase the amount of chemical compounds of high complexity.