Dr. Petzoldt criticises, among other things, especially Fechner's concept of approximate stability, in that no distinction is made by the author between three different cases. The first case comprises forms of motion in which periodicity is only approximate, but in which, nevertheless, no retrogression in stability takes place; this case is illustrated by our solar system. The second case comprises forms of motion in which the stability increases up to a certain point, but beyond this, despite relative constancy in outer conditions, decreases again until complete destruction of the system supervenes; an illustration of this form of motion is found in all organisms. The third case comprises forms of motion which we cannot concede to be stable. "For, if we ascribe periodic motion to pendula and musical strings which vibrate in a resisting medium, this is nevertheless a periodicity, which continually changes _in the same sense_, and we certainly cannot say that pendula and strings approach, in a resisting medium, a condition of absolute, through a condition of approximate, stability. We recognize in these vibrations, decreasing in amplitude, merely unstable changes which tend toward a final stable condition,--namely that of rest." The author finds a further ground of criticism in Fechner's assertion that organisms are entirely dependent upon the periodicity of their functions. Only a part of such functions are periodic. Periodicity is not conceivable without stability, but stability is conceivable without periodicity.