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 [pg 206] 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 naÏve 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 naÏve 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 [pg 207] 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. [pg 208] 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 NÄgeli) calls these links, below the level of the cell, Probia or Probions, and for a time he believed that he [pg 209] 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).