But we have plainly not reached the bottom of the matter in our attempt to explain the machinery of living things. We have based the whole process upon three factors. Reproduction, variation, and heredity are the properties of all living matter; but they are not, like gravity and chemism, universal forces of nature. They occur in living organisms only. Why should they occur in living organisms, and here alone? These three properties are perhaps the most marvellous properties of nature; and surely we have not finished our task if we have based the whole process of machine building upon these mysterious phenomena, leaving them unintelligible. We must therefore now ask whether we can proceed any farther and find any explanation of these fundamental powers of the living machine. 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