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.