l 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 ma