In regard to almost all the points to which we have referred, the most consistent and decided champion of Darwinism in its essential principles is the zoologist of Freiburg, August Weismann.(35) In long chapters on the protective coloration of animals, on the phenomena of mimicry—that resemblance to foreign objects (leaves, pieces of wood, bark, and well-protected animals) by which the mimics secure their own safety from enemies—on the protective devices in plants, the selective value of “the useful” is demonstrated. In regard to the marvellous phenomena of “carnivorous” plants, the still more marvellous instincts of animals, which cannot be interpreted on Lamarckian lines as “inherited habit,” but only as due to the cumulative influence of selection on inborn tendencies, as well as in regard to “symbiosis,” “the origin of flowers,” and so on, he attempts to show that the heterodox attempts at explanation are insufficient, and that selection alone really explains. At the same time the Darwinian principle is carried still further. It is not only among the individuals, the “persons,” that the selective struggle for existence goes on. Personal selection depends upon a “germinal selection” within the germ-plasm, influencing it, and being influenced by it—for instance, restrained.
In order to explain the mystery of heredity, Weismann long ago elaborated, in his germ-plasm theory, the doctrine that the developing individual is materially preformed, or rather predetermined in the “idants” and “ids” of the germ-cell. Thus every one of its physical characters (and, through these, its psychical characters), down to hairs, skin spots, and birth-marks, is represented in the “id” by “determinants” which control the “determinates” in development. In the course of their growth and development these determinants are subject to diverse influences due to the position they happen to occupy, to their quality, to changes in the nutritive conditions, and so on. Through these influences variations in the determinants may be brought about. And thus there comes about a “struggle” and a process of selection among the determinants, the result of which is expressed in changes in the determinates, in the direction of greater or less development. On this basis Weismann attempts to reach explanations of the phenomena of variation, of many apparently Lamarckian phenomena, and of recognised cases of “orthogenesis,” and seeks to complete and deepen Roux’s theory of the “struggle of parts,” which was just another attempt to carry Darwinism within the organism.