The Mechanics of Development
Theory, Organism, Germ, Mechanical, Cells, Individual
5. The minds of the supporters of the mechanical theory had still to move along a fifth line in order to solve the riddle of the development of the living individual from the egg, or of the germ to its finished form, the riddle of morphogenesis. They cannot assume the existence of “the whole” before the part, or equip it with the idea of the thing as a spiritus rector, playing the part of a metaphysical controlling agency. Here as elsewhere they must demonstrate the existence of purely mechanical principles. It is simply from the potential energies inherent in its constituent parts that the supply of energy must flow, by means of which the germ is able to make use of inorganic material from without, to assimilate it and increase its own substance, and, by using it up, to maintain and increase its power of work, to break up the carbonic acid of the atmosphere and to gain the carbon which is so important for its vital functions, to institute and organise the innumerable chemico-physical processes by means of which its form is built up. Purely as a consequence of the chemico-physical nature of the germ, of the properties of the substances included in it on the one hand, and of the implicit structure and configuration of its parts, down to the intrinsic specific undulatory rhythm of its molecules, it must follow that its mass grows exactly as it does, and not otherwise, that it behaves as it does and not otherwise, duplicating itself by division after division, and by intricate changes arranging and rearranging the results of division until the embryo or larva, and finally the complete organism, is formed.
An extraordinary amount of ingenuity has been expended in this connection, in order to avoid here, where perhaps it is most difficult of all, the use of “teleological” principles, and to remain faithful to the orthodox, exclusively mechanical mode of interpretation. To this category belong Darwin's gemmules, Haeckel's plastidules, Nägeli's micellæ, Weismann's labyrinth of ids, determinants, and biophors within the germ-plasm, and Roux's ingenious hypothesis of the struggle of parts, which is an attempt to apply the Darwinian principle within the organism in order here also to rebut the teleological interpretation by giving a scientific one.
6. With this fifth line of thought a sixth is associated and intertwined. The problem of development is closely bound up with that of “heredity.” A developing organism follows the parental type. The acorn in its growth follows the type of the parent oak, repeating all its morphological and physiological characters down to the most intimate detail. And the animal organism adds to this also the whole psychical equipment, the instincts, the capacities of will and consciousness which distinguish its parents. The problems of the fifth and sixth order are closely inter-related, the sixth problem being in reality the same as the fifth, only in greater complexity.
A step towards the mechanical solution of this problem was indicated in the “preformation theory” advanced by Leibnitz, and elaborated by Bonnet. According to this theory the developing organism is enclosed in the minutest possible form within the egg, and is thus included in the parental organism, in miniature indeed, but quite complete. Thus the problem of the “development of form” or of “heredity” was, so to speak, ruled out of court; all that was assumed was continuous growth and self-unfolding.
Opposed to this theory was one of later growth, the theory of epigenesis, which maintained that the organism developed without preformation from the still undifferentiated and homogeneous substance of the egg. The supporters of the first theory considered themselves much more scientific and exact than those of the second. And not without reason. For the theory of epigenesis obviously required mysterious formative principles, and equally mysterious powers of recollection and recapitulation, which impelled the undifferentiated ovum substance into the final form, precisely like that of its ancestors. Nor need the preformationists have greatly feared the reproach, that the parental organism must have been included within the grand-parental, and so on backwards to the first parents in Paradise. For this “Chinese box” encapsulement theory only requires that we should grant the idea of the infinitely little, and that idea is already an integral part of our thinking.
Modern biologists ridicule the preformation hypothesis as altogether too artificial. And undoubtedly it founders on the facts of embryology, which disclose nothing to suggest the unfolding of a pre-existent miniature model, but show us how the egg-cell divides into two, into four, and so on, with continued multiplication followed by varied arrangements and rearrangements of cells—in short, all the complex changes which constitute development. But a preformation in some sense or other there must be;—some peculiar material predisposition of the germ, which, as such, supplies the directing principle for the development, and the sufficient reason for the repetition of the parental form. This is of such obvious importance from the mechanical point of view that the speculations of to-day tend to move along the old preformationist lines. To these modern preformationists are opposed the modern upholders of epigenesis or gradual differentiation, who attempt to elaborate a mechanical theory of development. And with the contrast between these two schools there is necessarily associated the discussion as to the inheritance or non-inheritance of acquired characters.
Darwin's contribution to the problem of the sixth order was his rather vague theory of “Pangenesis.” The living organism, according to him, forms in its various organs, parts, and cells exceedingly minute particles of living matter (gemmules), which, “in some way or other,” bear within them the special characteristics of the part in which they are produced. These may wander through the organism and meet in the germ-plasm, and then, when a child-organism is produced, they “swarm,” so to speak, in it again “in some way or other,” and in some fashion control the development. This gemmule-theory was too obviously a quid pro quo to hold its ground for long. Various theories were elaborated, and the world of the invisibly minute was flooded with speculations.
The most subtle of these, on the side of consistent Darwinism, is that of Weismann, a pronounced preformation theory which has been increasingly refined and elaborated in the course of years of reflection. According to Weismann, the individual parts and characteristics of the organism are represented in the germ-plasm, not in finished form, but as “determinants” in a definite system which is itself the directing principle in the building up of the bodily system, and with definite characteristics, which determine the peculiarities of the individual organs and parts, down to scales, hairs, skin-spots, and birth-marks. As the germ-cells have the power of growth, and can increase endlessly by dividing and re-dividing, and as each process of division takes place in such a way that each half (each product of division) maintains the previous system, there arise innumerable germ-cells corresponding to one another, from which, therefore, corresponding bodies must arise (inheritance). It is not in reality the newly developed bodies which give rise to new germ-cells and transfer to them something of their own characters; the germ-cells of the child-organism develop from that of the parent (“immortality” of the germ-cells). Therefore there can be no inheritance of acquired characters, and no modifications of type through external causes; and all variations which appear in a series of generations are due solely to internal variations in the germ-cells, whether brought about by the complication of their system through the fusion of the male and female germ-cells, or through differences in the growth of the individual determinants themselves. The numerous subsidiary theses interwoven in Weismann's theory are entirely coherent, and have been thought out to their conclusions with praiseworthy determination. To the theory as a whole, because of its fundamental conception of preformation, and to its subsidiary hypotheses, piece by piece, there has been energetic opposition on the part of the upholders of the modern mechanical theory of epigenesis. This opposition is most concretely and comprehensively expressed in Haacke's “Gestaltung und Vererbung.” The infinitely complex intricacy of Weismann's minute microcosm within the germ-cell, indeed within every id in it, is justly described as a mere duplication, a repetition in the infinitely little of the essential difficulties to be explained. The complicated processes of developing in the growing and inheriting organism cannot be explained, they say, in terms of processes of the equally complex and likewise developing germ-plasm. The complex, if it is to be explained at all, must be explained by the simple—in this case by the functions of a homogeneous uniform plasm.
At an earlier date Haeckel had made an attempt in this direction in his theory of the “perigenesis of the plastidules.” Peculiar states of oscillation and rhythm in the molecules of the germ-substance, handed on to it from the parent organism and transferable to all the assimilated matter of the offspring, represent, according to this theory, the principle which impels development to follow a particular course corresponding to the type of the parents. This was a physical way of interpreting the matter. Other investigators have given a chemical expression to their theoretical schemes for explaining heredity.
Haacke declares both these to be unsatisfactory, and replaces them by morphological formative principles. It is the structure of the otherwise homogeneous living matter that explains morphogenesis and inheritance. Minute “gemmæ,” homogeneous fundamental particles of living substance, not to be compared to or confused with Darwin's “gemmules,” are aggregated in “Gemmaria,” whose configuration, stability, symmetrical or asymmetrical structure, and so on, are determined by the relative positions of the gemmæ to each other, and these in their turn control the organism and give it a corresponding symmetrical or asymmetrical, a firmly or loosely aggregated structure. The completed organism then forms a system in organic equilibrium, which is constantly exposed to variations and influences due to external causes (St. Hilaire), and to use and disuse of organs (Lamarck). These influences affect the structure of the gemmaria, and as the germ-cells consist of gemmaria, like those of the rest of the organism, the possibility of the transmission of acquired new characters is self-evident. The importance of correlated growth and orthogenesis is explained on a similar basis, and the Darwinian conceptions of the independent variation of individual parts, of the exclusive dominance of utility, of the influence of the struggle for existence in regard to individual selection, and of the omnipotence of natural selection, are energetically denied.
Oscar Hertwig, de Vries, Driesch and others attempt to reconcile the preformationist and the epigenetic standpoints, and “to extract what is good and usable out of both.” Hertwig and Driesch, however, can only be mentioned with reservations in this connection.
We cannot better sum up the whole tendency of the construction of mechanical theories on these last lines than in the words of Schwann: “There is within the organism no fundamental force working according to a definite idea; it arises in obedience to the blind laws of necessity.”
So much for the different lines followed by the mechanical theories of to-day. An idea of their general tenor can be gained from a series of much quoted general treatises, of which we must mention at least the “classics.” In Wagner's “Handwörterbuch der Physiologie,” 1842, Vol. I., Lotze wrote a long introductory article to the whole work, on “Life and Vital Force.” It was the challenge of the newer views to the previously vitalistic standpoint, and at the same time it was based on Lotze's general principles and interspersed with philosophical criticism of the concepts of force, cause, effect, law, &c. A similar train of ideas to Lotze's is followed to-day by O. Hertwig, especially in his “Mechanismus und Biologie.” Lighter and more elegant was the polemic against vital force, and the outline of a mechanical theory which Du Bois-Reymond prefaced to his great work, “Untersuchungen über die tierische Electricität” (1849). It did not go nearly so deep as Lotze's essay, but perhaps for that very reason its phrases and epigrams soon became common property. We may recall how he speaks of vital force as a “general servant for everybody,” of the iron atom which remains the same whether it be in the meteorite in cosmic space, in the wheel of the railway carriage, or in the blood of the thinker, and of analytic mechanics which may be applied even to the problem of personal freedom.
The most comprehensive and detailed elaboration of the mechanical theory of life is to be found in Herbert Spencer's “Principles of Biology.” Friedrich Albert Lange's “History of Materialism” is a brilliant plea for mechanical theories, which he afterwards surpassed and neutralised by his Kantian Criticism. Verworn, too, in his “Physiology” gives a clear example of the way in which the mechanical theory in its most consistent form is sublimed, apparently in the idealism of Kant and Fichte, but in reality in its opposite—the Berkeleyan psychology. A similar outcome is in various ways indicated in the modern trend of things.
|Written By Dr. Rudolf Otto|
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