57 To Aristotle the “Soul” (ψυχὴ ϕυτική Psyche, phytike) was in the first place a purely biological principle. But by means of his elastic formula of Potentiality and Actuality he was able to make the transition to the psychological with apparent ease. The biological is to him in “potentiality” what sensation, impulse, imagination are in “realisation.” But the biological and the psychological are not related to one another as stages. Growth, form, development, &c., cannot be carried over through any “actualisatio” into sensation, consciousness and the like. An essentially different question is, whether the biological may not be not indeed derivable from the psychological—that would be the same mistake—but dependent on, and conditioned by it, just as we regard the voluntary moving and directing of the body as dependent on it. An imaginative interpretation of the world will always take this course. 58 Of course all this still gives us no ground for drawing conclusions as to the correctness of the mechanistic theory, but only affords a reason for its power of persistence. Indeed, the very fact that, in investigating the problem of life, instinct directs us towards mechanical interpretations, should give added weight to the other fact, that among the ranks of naturalists themselves there constantly arise doubts and criticisms of the adequacy of this mode of interpretation, and that many of them go over more or less completely to the vitalistic point of view. 59 H. Helmholtz, “Ueber die Erhaltung der Kraft, eine physikalische Abhandlung,” Berlin, 1847. 60 Max Verworn, “Die Biogenhypothese,” Jena, 1903. _Cf._ criticisms by Czapek in the “Botanische Zeitung,” No. 2, 1903, and by Loeb in the “Biologisches Zentralblatt,” 1902. 61 Berlin, 1900. Edited by R. du Bois-Reymond. 62 Bütschli, “Untersuchungen über microscopische Schäume und das Protoplasma,” Leipzig, 1892. _Cf._ Berthold, “Studien zur Protoplasmamechanik.” 63 Rhumbler, “Zur Mechanik des Gastrulationvorganges ...” in “Archiv. f. Entwicklungsmechanik,” Bd. 14. 64 “Bewegung der lebendigen Substanz.” Jena, 1892. 65 A short, very attractive description of these mechanical methods, and one which appeals particularly to us laymen because of its excellent illustrations, is Dreyer’s “Ziele und Wege biologischer Forschung” (Jena, 1892), especially the first part, “Die Flüssigkeitsmechanik als eine Grundlage der organischen Form- und Gerüst-Bildung.” The astonishing and fascinating forms of Radiolarian frameworks and “skeletons” (the artistic appreciation of which was made possible to a wider public by Haeckel’s “Kunstformen der Natur”) are here made the subject of mechanical explanations, which are certainly in a high degree plausible.