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.(67) 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.