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."[199] 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."[200]
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 evolution, based on a great
variety of examples, has made this concept clearer and