The chief reason for this failure to penetrate appreciably nearer to the central mystery of life appears to be the fact that an organism is something more than the sum of its various parts and functions.  In tracing the behaviour of any one part or function, whether it be the conduction of a nervous impulse, the supply of oxygen to the tissues by the blood, or the transmission of inherited characters by the germ-cells, we may be able to give a more or less complete physico-chemical or mechanical account of the process.  But we seem to get little or no nearer to an explanation of the fact that although every one of these processes may be explicable by laws familiar in the non-living, in the living organism they are co-ordinated in such a way that none of them is complete in itself; they are parts of a whole, but the whole is not simply a sum of its parts, but is in itself a unity, in which all the parts are subject to the controlling influence of the whole.  An organism, alone among the material bodies which we know, is constantly and necessarily in a state of unstable equilibrium, and yet has a condition of normality which is maintained by the harmonious interaction of all its parts.  Every function of the body, if not thus co-ordinated with the rest, would very quickly destroy this condition of normality, but in consequence of the co-ordination each is subject to the needs of the whole, and normality is maintained.  When the normality is artificially disturbed, all the functions of the body adapt themselves to the change, and, if the disturbance be not too great, co-operate in the restoration of the normal condition.  It is in these phenomena of adaptation and organic unity and co-ordination that up to the present time the efforts to reduce the phenomena of living things to the operation of physico-chemical laws have most conspicuously failed.

From what has been said it will be evident that, fundamentally, all biological research, whether its authors are conscious of it or not, is directed towards the solution of one central problem—­the problem of the real and ultimate nature of life.  And the main outcome of the work of sixty years has been that this problem has begun clearly to emerge as the central aim of the science.  The theory of evolution made the problem a reality, for without evolution the mystery of life must for ever be insoluble, but whatever direction biological investigation has taken, it has led, often by devious paths, to the borderland between the living and the inorganic, and in that borderland the central problem inevitably faces us.

Many suggestions for its solution have been made.  On the one hand there is still, as there always has been, a considerable body of opinion that the solution will be a mechanical one—­using the word mechanical in the widest sense—­and that the living differs from the non-living not in kind, but only in degree of complexity.  The upholders of the mechanistic or materialist theory, however,