that this cell contains not a single substance, but a large number, including solids, jelly masses, and liquids, each of which has its own chemical composition.  The number of chemical compounds existing in the material formerly called protoplasm no one knows, but we do know that they are many, and that the different substances are combined to form a physical structure.  Which of these various bodies shall we continue to call protoplasm?  Shall it be the linin, or the liquids, or the microsomes, or the chromatin threads, or the centrosomes?  Which of these is the actual physical basis of life?  From the description of cell life which we have given, it will be evident that no one of them is a material upon which our chemical biologists can longer found a chemical theory of life.  That chemical theory of life, as we have seen, was founded upon the conception that the primitive life substance is a definite chemical compound.  No such compound has been discovered, and these disclosures of the microscope of the last few years have been such as to lead us to abandon hope of ever discovering such a compound.  It is apparently impossible to reduce life to any simpler basis than this combination of bodies which make up what was formerly called protoplasm.  The term protoplasm is still in use with different meanings as used by different writers.  Sometimes it is used to refer to the entire contents of the cell; sometimes to the cell substance only outside the nucleus.  Plainly, it is not the protoplasm of earlier years.

With this conclusion one of our fundamental questions has been answered.  We found in our first chapter that the general activities of animals and plants are easily reduced to the action of a machine, provided we had the fundamental vital powers residing in the parts of that machine.  We then asked whether these fundamental properties were themselves those of a chemical compound or whether they were to be reduced to the action of still smaller machines.  The first answer which biologists gave to this question was that assimilation, growth, and reproduction were the simple properties of a complex chemical compound.  This answer was certainly incorrect.  Life activities are exhibited by no chemical compound, but, so far as we know, only by the machine called the cell.  Thus it is that we are again reduced to the problem of understanding the action of a machine.  It may be well to pause here a moment to notice that this position very greatly increases the difficulties in the way of a solution of the life problem.  If the physical basis of life had proved to be a chemical compound, the problem of its origin would have been a chemical one.  Chemical forces exist in nature, and these forces are sufficient to explain the formation of any kind of chemical compound.  The problem of the origin of the life substance would then have been simply to account for certain conditions which resulted in such chemical combination as would give rise to this physical basis of life.  But now