452 Evolution and Modern Philosophy 
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 function- 
ally-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’.” 
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”.” 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 more definite 
than before. It contains the three elements ; integration, differentia- 
tion and equilibration. It is true that a concept which is to be valid 
for all domains of experience must have an abstract character, and 
between the several domains there is, strictly speaking, only a relation 
of analogy. So there is only analogy between psychical and physical 
evolution. But this is no serious objection, because general concepts 
do not express more than analogies between the phenomena which 
1 Spencer, Autobiography, Vol. a. p. 50, London, 1904. 2 Ibid. p. 100. 
