The various organs are developed by the process of specialization during the course of the evolution of the organism. Organic life on earth divides into various species. And in the sphere of social life we have an example in the division of labor. — Whatever differentiation proceeds one-sidedly, dissolution quickly follows. A third characteristic of evolution must therefore be added, namely, that it consists of a determination which presupposes a definite harmony between integration and differentiation.
The concept of evolution just described applies to every particular phenomenon, and to every phenomenal sphere (but not, as some have misunderstood Spencer, to "the universe" as a whole). It has been discovered by induction, but it must also be verified by deduction. Here Spencer falls back on a principle which he regards the foundation of all real science: the principle of the persistence of energy. With Spencer this principle (as with Hamilton and even Descartes and Spinoza) is really identical with the principle of causality. Every experiment rests upon the assumption of this principle: for if energy could originate or be lost during the course of an experiment it would be impossible to draw any inference. It follows therefore that similar elements must be similarly affected by similar energies, which establishes the principle of integration. It follows further that similar elements must be differently affected by different energies; which establishes the principle of differentiation. Proof of the necessity of the third characteristic determination is lacking. It is not a mere accident that Spencer was unable to establish this principle. From the viewpoint of experience it is impossible to furnish any guarantee for the harmony of integration
