The Mechanico-physiological Theory of Evolution, (Mechanisch-Physiologische Theorie der Abstammungslehre), by Carl von Nägeli, was published in Munich and Leipsic in 1884 in a large octavo volume of 822 pages, including two large appendices. The Abstammungslehre proper, including the summary, occupies 552 pages, and constitutes, in its way, one of the most important contributions to theoretical biology. It is difficult to understand how a work of so much consequence should have received such comparatively small notice in this country, especially as Nägeli's theories seemed calculated by nature to appeal much more strongly to American students than do, for instance, those of Weismann, who has been studied ten times as much as Nägeli. This is doubtless due, in part, to the fact that we have had no English translation of Nägeli's work, a circumstance much to be regretted.
The foregoing translation of the summary from Abstammungslehre goes but a small way toward making Nägeli's theories accessible to English-reading students, but it will, at least, be better than nothing. The work covers a great range of subjects, all, however, having a certain relationship to each other. In the main part of the book the discussion is presented in the following order: (1) Idioplasm as bearer of the inheritable determinants; (2) Spontaneous generation; (3) Causes of variation; (4) Determinants and visible characters, in which the origin and function of the determinants is presented; (5) Variety, race, "nutrition variety," heredity and variation; (6) Criticism of the Darwinian theory of natural selection, in which the author urges seven objections to that theory; (7) Laws of evolution of the plant kingdom; (8) Alternation of generations from the standpoint of phylogeny; (9) Morphology and classification as phylogenetic sciences; (10) A comprehensive summary of the whole work, a translation of which is given in the foregoing pages.
In the first part of the work Nägeli 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 At the present time its only competitor is Strasburger's lamellar theory, and even this has not superseded Nägeli's work to any great degree.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:
"Nägeli'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 micellæ, 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 micellæ, which, with their respective films of water, are held together by: (1) The attraction of the micellæ for each other, which varies inversely as the square of the distance. (2) The attraction of the micellæ 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, Nägeli inferred that in form the micellæ 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. Nägeli 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 Nägeli'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 Nägeli 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 Nägeli's conception of an automatic perfecting principle (Autonome Vervollkommnung). This conception may be briefly outlined as follows:
1. The essential part of the reproductive plasm, termed idioplasm, since it divides and passes over from generation to generation, in higher as well as in lower organisms, has a continuous or "immortal" existence.[I]