Haeckel’s demonstration of the possibility of spontaneous generation is along these lines. He refers to the cytodes, the blood corpuscles, to alleged or actual non-nucleated cells, to bacteria, to the simplest forms of cell-structure, as proofs of the possibility of a descending series of connecting-links. He (and with him Nägeli) calls these links, below the level of the cell, Probia or Probions, and for a time he believed that he had discovered in _Bathybius Haeckeli_ presently existing homogeneous living masses, without cell division, nucleus or structure, the “primitive slime” which apparently existed in the abysmal depths of the ocean to this day. Unfortunately, this primitive slime soon proved itself an illusion.
Opinions differ as to whether spontaneous generation took place only in the beginning of evolution, or whether it occurred repeatedly and is still going on. Most naturalists incline to the former idea; Nägeli champions the latter. There are also differences of opinion as to whether the origin of life from the non-living was manifold, and took place at many different places on the earth, or whether all the forms of life now in existence have arisen from a common source (monophyletic and polyphyletic theories).
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 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.