In the first part of the work Naegeli sets forth his micellar theory of the structure of organized bodies. This is one of his most important contributions to science. Until recent years it has been the only theory given in botanical text-books. At the present time its only competitor is Strasburger's lamellar theory, and even this has not superseded Naegeli's work to any great degree. The reader who may not be familiar with the micellar theory will find the general idea from the following brief sketch adapted from Vines's _Plant Physiology_: "Naegeli's micellar theory was developed from his study of organized bodies, especially of cell walls and starch grains. From the behavior of organized substance toward water absorbed by it, he concluded that water does not penetrate into the micellae, but only among them, thus merely separating them more from each other. He reasoned that if water should penetrate into the micella, its structure would be disintegrated. Hence he argued that organized bodies consist of solid micellae, which, with their respective films of water, are held together by: (1) The attraction of the micellae for each other, which varies inversely as the square of the distance. (2) The attraction of the micellae for water, which varies inversely as some higher power of the distance. (3) The force which holds together the ultimate chemical molecules of which each micella consists. "Since the swelling up of organized bodies does not take place equally in all three dimensions of space, and on account of their double refraction, Naegeli inferred that in form the micellae are crystals, probably parallelopipedal, with rectangular or rhomboidal bases." The law that "bodies attract each other with a force which varies inversely as the square of the distance," has been proven only in its application to the heavenly bodies. Naegeli has applied this law to molecules, unsupported, however, by any evidence other than that of analogy. On the other hand, there is evidence that molecules do not invariably act according to this law. Spontaneous generation (p. 4) was an important item in Naegeli's doctrine, and might almost be said to be fundamental to it, although it is not really necessary to the internal perfecting principle, which may be regarded as the chief feature of the Mechanico-Physiological Theory. Up to 1865 Naegeli believed in the spontaneous origin of many fungi, and thought that it could be demonstrated. He was obliged to abandon the experimental evidence, but to the close of his life held the views of abiogenesis presented in the accompanying translation. The characteristic and most interesting feature of the Mechanico-Physiological Theory is certainly Naegeli's conception of an automatic perfecting principle (_Autonome Vervollkommnung_). This conception may be briefly outlined as follows: