Carl Ngeli

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 At the present time its only competitor is Strasburger's lamellar theory, and even this has not superseded NÄgeli's work to any great degree.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]

2. During this continuous life the idioplasm goes through a development of its own, just as an individual organism goes through a certain cycle of development during its individual life. This development consists in a constantly increasing complexity of structure and differentiation of function.

3. This development is automatic, resulting from internal forces or movements, (Vervollkommnungs-bewegungen).

4. As a result of the increasing complexity of structure in the idioplasm the entire organism, which in each generation rearises therefrom, becomes, from generation to generation, more and more complex with greater and greater differentiation of function. Thus the progression of the idioplasm controls the phylogeny of the race. It marks out the course of evolution.

5. Since, according to NÄgeli, new life with new idioplasms, may arise wherever and whenever the necessary conditions combine, the present organic world is not made up from branchings of a single original idioplasm, but each race or group may have its own specific idioplasm; and, since this has its own characteristic structure and its own specific internal perfecting forces, it passes through its own peculiar evolution, carrying with it its own depending race of organisms.

The fact that animals and plants at the present time show such various degrees of organization is also accounted for on the last supposition, for those of lowlier organization are merely of more recent origin and have not progressed so far in idioplasmic development.

This automatic perfecting principle has been the mark of much criticism. Some have confounded it with the mystical nisus formativus, or formative principle of preceding theorists. But, as Weismann remarks, NÄgeli's phyletic force is conceived as a thoroughly scientific mechanical principle. NÄgeli has simply made application in the organic world of the principle of entropy, as stated in the mechanical theory of heat. NÄgeli himself also compares his internal perfecting principle to mechanical inertia. He says, "the force of evolution once started in a given direction, tends to continue in the same direction. This constitutes the law of inertia in the organic world."

Two other matters remain to be noticed. The first of these is NÄgeli's use of the German word Anlage. We have been unable to give a perfectly satisfactory translation of this word in its technical meaning. We have received some comfort, though but little help, from the experience of the translators of similar works. Selmar Schoenland, in translating from Weismann, renders it variously as "germ," "germ of structure," "germ (of NÄgeli)," "germ of NÄgeli," "NÄgeli's preformed germ of structure," "preformed germs," "tendency." Another translator renders the word as "constitutional element." The translation, "determinant," which we have selected is an appropriation of an analogous but not absolutely identical technical term from Weismann's Germinal Selection. The use of the word in this connection is open to the objection that it has previously been used technically for a somewhat different idea by another author. M. C. Potter, in his translation of Warming's Systematic Botany, following Dr. E. L. Mark, renders the word Anlage as "fundament." Dr. H. C. Porter, in his translation of the Bonn Text-Book of Botany, renders the same word as "rudiment."

In general the word Anlage means beginning, plan, disposition to anything, and hence involves the ideas of origin, organization and tendency. Sanders defines the word in one of its meanings as: "The act of planning or beginning anything; the act of laying the foundation of any work intended to be carried on toward completion, in order that from the beginning made, a definite thing may be developed or may develop itself"; (i.e., to determine, in the sense of limiting to a particular purpose or direction, hence determinant). "Also, the thing begun or planned, considered as the basis and germ of the further development of that which has already originated."

In its restricted use as applied to organisms it would mean "germ," in the sense of embryonic starting point. More specifically, it is a portion of plastic, organized substance, functioning as an individual and containing potentially an elemental organ plus a formative power. In NÄgeli's own words, "There exists an essential difference between the substance of a mature organism which does not possess the capability of further development, and the substance of an egg, which does possess this capability. By virtue of this difference the egg-substance is characterized as the Anlage, or germ of the mature organism. All characteristics of the adult condition are potentially contained in the ovum."

NÄgeli was not the first to assume the existence of a unit of organization intermediate between the molecule and the cell. E. B. Wilson, in his The Cell in Inheritance and Development, states the case as follows:

NÄgeli's Laws of Evolution are also worth special notice. As stated in the body of Abstammungslehre they are as follows:

1. Asexual reproductive cells which arise by division, remain united and become tissue cells.

2. Asexual reproductive cells which arise by budding, instead of separating, become cell branches or branched cell threads.

3. Reproductive cells which arise by free cell formation become bodies which form a part of the cell contents.

4. Parts of a plant which arise by differentiation lie side by side and form a body of web-like or tissue-like structure.

5. A definite and previously limited growth continues, or a definite formation of parts of an ontogeny which has previously been present but once, is repeated. (Ampliation.)

6. The parts of an ontogeny become dissimilar, since the functions which were previously united become differentiated and since new dissimilar functions are produced in the various parts. This differentiation is either one of space between the parts of the ontogeny that appear near each other, or one of time between those that are derived from each other.

7. Parts which have become dissimilar by differentiation undergo a reduction, in which the intermediate forms are suppressed and at last only the qualitatively dissimilar forms with qualitatively dissimilar functions remain.

8. The environment in which plants live operates in different ways, directly as a stimulus or indirectly as a felt necessity and by this means lends to their forms and activities a definite expression of time and place, and thus brings about different adaptations. These become permanent through heredity, but are again gradually lost if other adaptations supersede them.

Laws 1 to 4 may be expressed as one—the law of combination: Similar parts that are wholly or partly separated have the tendency to unite more and more completely and intimately into one continuous tissue.

The laws of ampliation (5), differentiation (6), and reduction (7), may be summarized in one as follows: While increasing in size the similar parts of an ontogeny become internally dissimilar and the dissimilarity increases as the transition forms of the dissimilar parts vanish. Hence only the extreme forms remain.

It may also interest the reader to know that NÄgeli was the first to propose the general theory of cell formation as accepted at the present day.