==Summary.==--We may now notice in a brief summary the position which we have reached. In our attempt to explain the living organism on the principle of the machine, we are very successful so far as secondary problems are concerned. Digestion, circulation, respiration, and motion are readily solved upon chemical and mechanical principles. Even the phenomena of the nervous system are, in a measure, capable of comprehension within a mechanical formula, leaving out of account the purely mental phenomena which certainly have not been touched by the investigation. All of these phenomena are reducible to a few simple fundamental activities, and these fundamental activities we find manifested by simple bits of living matter unincumbered by the complicated machinery of organisms. With the few fundamental properties of these bits of organic matter we can construct the complicated life of the higher organism. When we come, however, to study these simple bits of matter, they prove to be anything but simple bits of matter. They, too, are pieces of complicated mechanism whose action we do not even hope to understand. That their action is dependent upon their machinery is evident enough from the simple description of cell activity which we have noticed. That these fundamental vital properties are to be explained as the result of chemical and mechanical forces acting through this machinery, can not be doubted. But how this occurs or what constitutes the guiding force which corresponds to the engineer of the machine, we do not know. Thus our mechanical explanation of the living machine lacks a foundation. We can understand tolerably well the building of the superstructure, but the foundation stones upon which that structure is built are unintelligible to us. The running of the living machine is thus only in part understood. The living organism is a machine or, it is better to say, it is a series of machines one within the other. As a whole it is a machine, and its parts are separate machines. Each part is further made up of still smaller machines until we reach the realm of the microscope. Here still we find the same story. Even the parts formerly called units, prove to be machines, and when we recognize the complexity of these cells and their marvellous activities, we are ready to believe that we may find still further machines within. And thus vital activity is reduced to a complex of machines, all acting in harmony with each other to produce together the one result--life. PART II. _THE BUILDING OF THE LIVING MACHINE_. * * * * * CHAPTER III. THE FACTORS CONCERNED IN THE BUILDING OF THE LIVING MACHINE.