We must notice, however, that this power of construction in distinction from destruction, is possessed only by one special class of living machines. Green plants alone can thus increase the store of organic compounds in the world.  All colourless plants and all animals, on the other hand, live by destroying these compounds and using the energy thus liberated; in this respect being more like ordinary artificial machines.  The animal does indeed perform certain constructive operations, manufacturing complex material out of simpler bodies; as, for example, making fats out of starches.  But in this operation it destroys a large amount of organic material to furnish the energy for the construction, so that the total result is a degradation of chemical compounds rather than a construction.  Constructive processes, which increase the amount of high compounds in nature, are confined to the living machine, and indeed to one special form of it, viz., the green plant.  This constructive power radically separates the living from other machines; for while constructive processes are possible to the chemist, and while engines making use of sunlight are possible, the living machine is the only machine that increases the amount of high chemical compounds in the world.

==The Vital Factor.==—­With all this explanation of life processes it can not fail to be apparent that we have not really reached the centre of the problem.  We have explained many secondary processes, but the primary ones are still unsolved.  In studying digestion we reach an understanding of everything until we come to the active vital property of the gland-cells in secreting.  In studying absorption we understand the process until we come to what we have called the vital powers of the absorptive cells of the alimentary canal.  The circulation is intelligible until we come to the beating of the heart and the contraction of the muscles of the blood-vessels.  Excretion is also partly explained, but here again we finally must refer certain processes to the vital powers of active cells.  And thus wherever we probe the problem we find ourselves able to explain many secondary problems, while the fundamental ones we still attribute to the vital properties of the active tissues.  Why a muscle contracts or a gland secretes we have certainly not yet answered.  The relation of the actions to the general problems of correlation of force is simple enough.  That a muscle is a machine in the sense of our definition is beyond question.  But the problem of why a muscle acts is not answered by showing that it derives its energy from broken food material.  There are plainly still left for us a number of fundamental problems, although the secondary ones are soluble.