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 [pg 210] 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, [pg 211] 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