that the simplest substance manifesting the phenomena of life is found to be a machine, we can no longer find in chemical forces efficient causes for its formation.  Chemical forces and chemical affinity can explain chemical compounds of any degree of complexity, but they cannot explain the formation of machines.  Machines are the result of forces of an entirely different nature.  Man can manufacture machines by taking chemical compounds and putting them together into such relations that their interaction will give certain results.  Bits of iron and steel, for instance, are put together to form a locomotive, but the action of the locomotive depends, not upon the chemical forces which made the steel, but upon the relation of the bits of steel to each other in the machine.  So far as we have had any experience, machines have been built under the guidance of intelligence which adapts the parts to each other.  When therefore we find that the simplest life substance is a machine, we are forced to ask what forces exist in nature which can in a similar way build machines by the adjustment of parts to each other.  But this topic belongs to the second part of our subject, and must be for the present postponed.

==Reaction against the Cell Doctrine.==—­As the knowledge of cells which we have outlined was slowly acquired, the conception of the cell passed through various modifications.  At first the cell wall was looked upon as the fundamental part, but this idea soon gave place to the belief that it was the protoplasm that was alive.  Under the influence of this thought the cell doctrine developed into something like the following:  The cell is simply a bit of protoplasm and is the unit of living matter.  The bodies of all larger animals and plants are made up of great numbers of these units acting together, and the activities of the entire organism are simply the sum of the activities of its cells.  The organism is thus simply the sum of the cells which compose it, and its activities the sum of the activities of the individual cells.  As more facts were disclosed the idea changed slightly.  The importance of the nucleus became more and more forcibly impressed upon microscopists, and this body came after a little into such prominence as to hide from view the more familiar protoplasm.  The marvellous activities of the nucleus soon caused it to be regarded as the important part of the cell, while all the rest was secondary.  The cell was now thought of as a bit of nuclear matter surrounded by secondary parts.  The marvellous activities of the nucleus, and above all, the fact that the nucleus alone is handed down from one generation to the next in reproduction, all attested to its great importance and to the secondary importance of the rest of the cell.