Spontaneous Generation
4. This reduction of known biological phenomena to simpler terms, the
lessening of the gap between inorganic and organic chemistry, and the
formulation of the doctrine of the conservation of energy, have all
prepared the way for a fourth step, the establishment of the inevitable
theory of generatio spontanea sive equivoca, the spontaneous generation
of the living, that is to say, the gradual evolution of the living from
the not living. Since the earth, and with it the conditions under which
alone life is possible, have had a beginning in time, life upon the earth
must also have had a beginning. The assumption that the first living
organisms may have come to the earth on meteorites simply shifts the
problem a step farther back, for according to all current theories of the
universe, if there are in any of the heavenly bodies conditions admitting
of the presence of life, these conditions have arisen from others in which
life was impossible. Therefore, since this suggestion is on the face of it
a mere evasion of the difficulty, the theory of spontaneous generation
naturally arose. There is something almost comical in the change in the
attitude of the natural sciences to this theory. For centuries it was one
of the beliefs of popular superstition, with its naive way of regarding
nature, that earthworms "developed" from damp soil, and vermin from
shavings, and in general that the living arose from the non-living. On the
other hand it was one of the characteristics and axioms of scientific
thought to reject this naive generatio equivoca, and to hold fast to the
proposition, omne vivum ex ovo, or, at least, omne vivum ex vivo. And
it was regarded as one of the triumphs of modern science when, about the
middle of the last century, Pasteur gave definiteness to this doctrine,
and when through him, through Virchow, and indeed the whole younger
generation of naturalists, the proposition was modified, on the basis of
the newly discovered cell-theory, to omnis cellula ex cellula. But a
short time after Pasteur's discoveries, the ideas of Darwinism and the
theory of evolution gained widespread acceptance. And now it appeared
that, in rejecting the theory of generatio equivoca, naturalists had, so
to speak, sawn off the branch on which they desired to sit, and thus many,
like Haeckel, became enthusiastic converts to the theory which natural
science had previously rejected.
Constructing theories and speculations as to the possibilities of
spontaneous generation is regarded by some naturalists as somewhat
gratuitous (cf. Du Bois-Reymond). In general, it is regarded as
sufficient to point out that the reduction of the phenomena of life as we
know them to those of a simpler order, and the unification of organic and
inorganic chemistry, have made the problem of the first origin of life
essentially simpler, and that the law of the constancy and identity of
energy throughout the universe permits no other theory. But others go more
determinedly to work, and attempt to give concrete illustrations of the
problem. The most elementary form of life known to us is the cell. From
cells and their combinations, their products and secretions, all
organisms, plant and animal alike, are built up. If we succeed in deriving
the cell, the derivation of the whole world of life seems, with the help
of the doctrine of descent, a comparatively simple matter. The cell itself
seems to stand nearer to the inorganic, and to be less absolutely apart
from the inanimate world than a highly organised body, differentiated as
to its functions and organs, such as a mammal. It almost seems as if we
might regard the lowest forms of life known to us, which seem little more
than aggregated homogeneous masses of flowing rather than creeping
protoplasm, as an intermediate link between the higher forms of life and
the non-living. But the theory does not begin with the cell; it assumes a
series of connecting-links (which may of course be as long and as
complicated as the series from the cell upwards to man) between the cell
and matter which is still quite "inorganic" and which is capable only of
the everyday chemical and physical phenomena, and not of the higher
syntheses of these, which in their increasing complexity and diversity
ultimately come to represent "life" in its most primitive forms. As
proteid is the chief constituent of protoplasm, it is regarded as the
specific physical basis of life, and life is looked upon as the sum of its
functions. And 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).