It is certain that the comparatively recent science of Physiological Chemistry will have many of the decisive words to say on this score, in the future. "When we see the symmetrical and complex outgrowths caused by a single atom of the poison of a gall-insect, we may believe that slight changes in the chemical nature of the sap or blood would lead to extraordinary modifications of structure," says the great seer of evolution himself.[101] Among special theories of Evolution, a distinction may be made between: (1) such special theories as aim at biological simplification by reduction of all organic variation to one primary form of cellular process; (2) such theories as are content with less ultimate laws, by which the various ascertained forms of change are included in one general statement not involving special physiological or physical theory but applicable to all forms of life; (3) such theories as aim to give distinctive philosophic expression to a generalization like the last named, including in this statement both psychical and physiological phenomena; and (4) such theories as aim at an ultimate expression of the direction of evolution that shall include the phenomena of life, both physiological and psychical, under one head with all other natural phenomena. To the first class belong only "provisional" hypotheses, among the best known of which are those of Pangenesis, Perigenesis, and the Continuity of the Germ-Plasm. To the second, which are not merely tentative but have a broad foundation in known fact, belongs Haeckel's theory of Inheritance and Adaptation, a theory restated in substance, from independent research, by Eimer, whose ultimate general factors of analysis are the same with Haeckel's, though he deals, beyond these, with special facts and special theories of his own. Phases of the second class often entitle them to inclusion in the third. An example of the third class is found in Spencer's definition, "Life is the continual adjustment of inner relations to outer relations." The fourth and last class includes Fechner's "Tendency to Stability" and Spencer's theory of the rhythm of motion (see his "First Principles"), similar to which are certain ideas of Zoellner, Du Prel, and others; and similar elements to which are to be found in Haeckel's "Plastidule-Theory." In connection with this class, reference may be made to an article by Dr. J. Petzoldt in the "Vierteljahrschrift fuer wissenschaftliche Philosophie" under the title "Maxima, Minima, und Oekonomie," in which, among others, Fechner's views especially are discussed with reference to an ultimate principle of evolution. The first pages on the "Tendency to Stability" in Fechner's "Ideas concerning the Evolution of the Organic" ("Einige Ideen zur Schoepfungs- und Entwicklungsgeschichte der Organismen") are as follows:--