it is not doubted that, if the conditions of the universe
brought about a natural combination of carbon, hydrogen, nitrogen and
oxygen in certain proportions, so that proteid resulted, the transition to
proteid which forms itself and renews itself from the surrounding
elements, to assimilating, growing, dividing proteid, and ultimately to
the most primitive plasmic structure, to non-nucleated, nucleated, and
finally fully formed cells, could also come about.

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 Naegeli) 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; Naegeli 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).

The Mechanics of Development.

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,