One of the primary questions to demand attention in this search is whether we are to find the explanation, at the bottom, a chemical or a mechanical one.  In the simplest form of life in which vital manifestations are found are we to attribute these properties simply to chemical forces of the living substance, or must we here too attribute them to the action of a complicated machinery?  This question is more than a formal one.  That it is one of most profound significance will appear from the following considerations: 

Chemical affinity is a well recognized force.  Under the action of this force chemical compounds are produced and different compounds formed under different conditions.  The properties of the different compounds differ with their composition, and the more complex are the compounds the more varied their properties.  Now it might be assumed as an hypothesis that there could be a chemical compound so complex as to possess, among other properties, that of causing the oxidation of food to occur in such a way as to produce assimilation and growth.  Such a compound would, of course, be alive, and it would be just as true that its power of assimilating food would be one of its physical properties as it is that freezing is a physical property of water.  If such an hypothesis should prove to be the true one, then the problem of explaining life would be a chemical one, for all vital properties would be reducible to the properties of a chemical compound.  It would then only be necessary to show how such a compound came into existence and we should have explained life.  Nor would this be a hopeless task.  We are well acquainted with forces adequate to the formation of chemical compounds.  If the force of chemical affinity is adequate under certain conditions to form some compounds, it is easy to conceive it as a possibility under other conditions to produce this chemical living substance.  Our search would need then to be for a set of conditions under which our living compound could have been produced by the known forces of chemical affinity.

But suppose, on the other hand, that we find this simplest bit of living matter is not a chemical compound, but is in itself a complicated machine.  Suppose that, after reducing this vital substance to its simplest type, we find that the substance with which we are dealing not only has complex chemical structure, but that it also possesses a large number of structural parts adapted to each other in such a way as to work together in the form of an intricate mechanism.  The whole problem would then be changed.  To explain such a machine we could no longer call upon chemical forces.  Chemical affinity is adequate to the explanation of chemical compounds however complicated, but it cannot offer any explanation for the adaptation of parts which make a machine.  The problem of the origin of the simplest form of life would then be no longer one of chemical but one of mechanical evolution.  It is plain then that the question of whether we can attribute the properties of the simplest type of life to chemical composition or to mechanical structure is more than a formal one.