"To elucidation of this principle of the Tendency to Stability," says Dr. Petzoldt analyzing Fechner's work, "we have only to call to remembrance a number of natural phenomena, such as the ebb and flow of the tide, the circulation of moisture, periodic changes of temperature, and so forth, which exhibit great periods of approximate stability and in which we notice in general no retrogression. "Not less does the constitution of organisms which are, 'so to speak, constituted dependent upon periodicity of their functions, and so upon stable relations of their life,' serve to confirm the theory. Only the concept of stability must be extended in their case, since not always the same, but only substitutive parts of the organic systems tend towards stability. "Experience never gives us an example of an isolated system; on the contrary, every system is a part of higher systems. The inner relations of its stability are not conditioned by its own parts only, but also, more or less, by those of other systems, so that the destruction of one part-system is always only in the direction towards the stability of a higher, ultimately of the highest, system; that is, of the system of the universe." "Thus the teleological principle coincides with the principle of the Tendency to Stability, and at the same time the latter constitutes the link between the former and the law of Causality. Though, in truth, this manner of looking at the matter signifies a generalization of the concept of 'end,' since it defines _all_ stable conditions as ends. The view is justified, however, by the fact that the greatest possible physical satisfaction--for us, the criterion of teleology--is always bound up with the longest possible preservation or slow change of a stable organic condition. The _physical_ Tendency to Stability 'bears with it a _psychical_ tendency to the attainment and conservation of just those conditions' towards which the physical tendency is directed." Of the fact that Lange "feels the lack of the proof of this 'Tendency to Stability,'" Dr. Petzoldt says: "But how is there a need of proof here? To prove is to refer back to known facts. But what is there in Fechner's remarks that stands in need of such a reference? They simply draw our attention to the result of evolution as a state which bears, in itself, the guarantee of some continuance. Can any one contest this? Is there anything further to prove? It is said that Gauss once remarked that Lagrange's equations of motion are not proved, but only historically stated. The case is exactly the same here. The fact is attested, merely, that evolution ends in a stable condition; and herein lies the pith and the great merit of the theory of the Tendency to Stability."