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.(66) Heredity. 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.