enerations, so that the nature and functions of the individual are only
to be understood through its connection with the life of the species.
In 1857, in his essay on "Progress", he propounded the law of
differentiation as a general law of evolution, verified by examples from
all regions of experience, the evolution of species being only one of
these examples. On the effect which the appearance of "The Origin of
Species" had on his mind he writes in his "Autobiography": "Up to
that time... I held that the sole cause of organic evolution is the
inheritance of functionally-produced modifications. The "Origin of
Species" made it clear to me that I was wrong, and that the larger part
of the facts cannot be due to any such cause... To have the theory of
organic evolution justified was of course to get further support for
that theory of evolution at large with which... all my conceptions were
bound up." (Spencer, "Autobiography", Vol. II. page 50, London, 1904.)
Instead of the metaphorical expression "natural selection," Spencer
introduced the term "survival of the fittest," which found favour with
Darwin as well as with Wallace.

In working out his ideas of evolution, Spencer found that
differentiation was not the only form of evolution. In its simplest form
evolution is mainly a concentration, previously scattered elements being
integrated and losing independent movement. Differentiation is only
forthcoming when minor wholes arise within a greater whole. And the
highest form of evolution is reached when there is a harmony between
concentration and differentiation, a harmony which Spencer calls
equilibration and which he defines as a moving equilibrium. At the same
time this definition enables him to illustrate the expression "survival
of the fittest." "Every living organism exhibits such a moving
equilibrium--a balanced set of functions constituting its life; and the
overthrow of this balanced set of functions or moving equilibrium is
what we call death. Some individuals in a species are so constituted
that their moving equilibria are less easily overthrown than those of
other individuals; and these are the fittest which survive, or, in Mr
Darwin's language, they are the select which nature preserves." (Ibid.
page 100.) Not only in the domain of organic life, but in all domains,
the summit of evolution is, according to Spencer, characterised by such
a harmony--by a moving equilibrium.

Spencer's analysis of the concept of