It must be confessed that here we are at present forced to stop.  We can proceed no further with any certainty, or even probability.  We may say that variation and heredity are only phases of reproduction, and reproduction is a property of the living cell.  We may say that this power of reproduction is dependent upon the power of assimilation and growth, for cell division is a result of cell growth.  We may further say that growth and assimilation are chemical processes resulting from the oxidation of food, and that thus all of these processes are to be reduced to chemical forces.  In this way we may seem to have a chemical foundation for life phenomena.  But clearly this is far from satisfactory.  In the first place, it utterly fails to explain why the living cell has these properties, while no other body possesses them, nor why they are possessed by living protoplasms alone, ceasing instantly with death.  Indeed it does not tell us what death can be.  Secondly, it utterly fails to explain the marvels of cell division with resulting hereditary transmission.  For all this we must fall back upon the structure of protoplasm, and say that the cell machinery is so adjusted that the machine, when acting as a whole, is capable of transforming the energy of chemical composition in certain directions.  These fundamental properties are then the properties of the cell machine just as surely as printing is the property of the printing press.  We can no more account for the life phenomena by chemical powers than we can for printing by chemical forces manifested in the burning of the coal in the engine room.  To be sure, it is the chemical forces in the engine room that furnishes the energy, but it is the machinery of the press that explains the printing.  So, while chemical forces supply life energy, it is the cell machinery that must explain the fundamental living factors.  So long as this machine is intact it can continue to run and perform its duties.  But it is a very delicate machine and is easily broken.  When it is broken its activities cease.  A broken machine can not run.  It is dead.  In short, we come back once more to the idea of the machinery of protoplasm, and must base our understanding of its properties upon its structure.

It is proper to state that there are still some biologists who insist that the ultimate explanation of protoplasm is purely chemical and that life phenomena may be manifested in mixtures of compounds that are purely physical mixtures and not machines.  It is claimed that much of this cell structure described above is due to imperfection in microscopic methods and does not really exist in living protoplasm, while the marvellous activities described are found only in the highly organized cell, but do not belong to simple protoplasm.  It is claimed that simple protoplasm consists of a physical mixture of two different compounds which form a foam when thus mixed, and that much of the described structure of protoplasm is only the appearance