This extension of the evolutionary concept, in its widest sense, to the problem of the origin of life on our globe is known as the hypothesis of abiogenesis or spontaneous generation. It regards inorganic matter as the source of organic life not merely in the sense of a passive cause, out of which the primordial forms of life were produced, but in the sense of an active cause inasmuch as it ascribes the origin of life to the exclusive agency of dynamic principles inherent in inorganic matter, namely, the physicochemical energies that are native to mineral matter. Life, in other words, is assumed to have arisen spontaneously, that is, by means of a synthesis and convergence of forces resident in inorganic matter, and not through the intervention of any exterior agency.
The protagonists of spontaneous generation, therefore, assert not merely a passive, but an active, evolution of living, from lifeless matter. As to the fact of the origin of the primal organisms from inorganic matter, there is no controversy whatever. All agree that, at some time or other, the primordial plants and animals emanated from inorganic matter. The sole point of dispute is whether they arose from inorganic matter by active evolution or simply by passive evolution. The passive evolution of mineral matter into plants and animals is an everyday occurrence. The grass assimilates the nitrates of the soil, and is, in turn, assimilated by the sheep, whose flesh becomes the food of man, and mineral substance is thus finally transformed into human substance. In the course of metabolic processes, the inorganic molecule may doff its mineral type and don, in succession, the specificities of plant, animal, and human protoplasm; and this transition from lower to higher degrees of perfection may be termed an evolution. It is an ascent of matter from the lowermost grade of an inert substance, through the intermediate grades of vegetative and animal life, up to the culminating and ultimate term of material perfection, in the partial constitution of a human nature and personality, in the concurrence as a coagent in vegetative and sensile functions, and in the indirect participation, as instrument, in the higher psychic functions of rational thought and volition.
At the present time, the inorganic world is clearly the exclusive source of all the matter found in living beings. All living beings construct their bodies out of inorganic substances in the process of nutrition, and render back to the inorganic world, by dissimilation and death, whatever they have taken from it. We must conclude, therefore, the matter of the primordial organisms was likewise derived from the inorganic world. But we are not warranted in concluding that this process of derivation was an active evolution. On the contrary, all evidence is against the supposition that brute matter is able to evolve of itself into living matter. It can, indeed, be transformed into plants, animals, and men through the action of an appropriate external agent (i.e. solely through the agency of the living organism), but it cannot acquire the perfections of living matter by means of its own inherent powers. It cannot vitalize, or sensitize, itself through the unaided activity of its own physicochemical energies. Only when it comes under the superior influence of preëxistent life can it ascend to higher degrees of entitive perfection. It does not become of itself life, sensibility, and intelligence. It must first be drawn into communion with what is already alive, before it can acquire life and sensibility, or share indirectly in the honors of intelligence (as the substrate of the cerebral imagery whence the human mind abstracts its conceptual thought). Apart from this unique influence, inorganic matter is impotent to raise itself in the scale of existence, but, if captured, molded, and transmuted by a living being, it may progress to the point of forming with the human soul one single nature, one single substance, one single person. The evolution of matter exemplified in organic metabolism is obviously passive, and such an evolution of the primal organisms out of non-living matter even the opponents of the hypothesis of spontaneous generation concede. But spontaneous generation implies an active evolution of the living from the lifeless, and this is the point around which the controversy wages. It would, of course, be utterly irrational to deny to the Supreme Lord and Author of Life the power of vivifying matter previously inanimate and inert, and hence the origin of organic life from inorganic matter by a formative (not creative) act of the Creator is the conclusion to which the denial of abiogenesis logically leads.
The hypothesis of spontaneous generation is far older than the theory of transformism. It goes back to the Greek predecessors of Aristotle, at least, and may be of far greater antiquity. It was based, as is well known, upon an erroneous interpretation of natural facts, which was universally accepted up to the close of the 17th century. As we can do no more than recount a few outstanding incidents of its long and interesting history here, the reader is referred to the VII chapter of Wasmann’s “Modern Biology” and the VIII chapter of Windle’s “Vitalism and Scholasticism” for the details which we are obliged to omit.
§ 2. The Law of Genetic Continuity—
From time immemorial the sudden appearance of maggots in putrescent meat had been a matter of common knowledge, and the ancients were misled into regarding the phenomenon as an instance of a de novo origin of life from dead matter. The error in question persisted until the year 1698, when it was decisively disproved by a simple experiment of the Italian physician Francesco Redi. He protected the meat from flies by means of gauze. Under these conditions, no maggots appeared in the meat, while the flies, unable to reach the meat, deposited their eggs on the gauze. Thus it became apparent that the maggots were larval flies, which emerged from fertilized eggs previously deposited in decaying meat by female flies. Antonio Vallisnieri, another Italian, showed that the fruit-fly had a similar life-history. As a result of these discoveries, Redi rejected the theory of spontaneous generation and formulated the first article of the Law of Genetic Vital Continuity: Omne vivum ex vivo.
Meanwhile, the first researches conducted by means of the newly invented compound microscope disclosed what appeared to be fresh evidence in favor of the discarded hypothesis. The unicellular organisms known as infusoria were found to appear suddenly in hay infusions, and their abrupt appearance was ascribed to spontaneous generation. Towards the end of the 18th century, however, a Catholic priest named Lazzaro Spallanzani refuted this new argument by sterilizing the infusions with heat and by sealing the containers as protection against contamination by floating spores or cysts. After the infusions had been boiled for a sufficient time and then sealed, no organisms could be found in them, no matter how long they were kept. We now know that protozoa and protophytes do not originate de novo in infusions. Their sudden appearance in cultures is due to the deposition of spores or cysts from the air, etc.
The possibility that the non-germination of life in sterilized infusions kept in sealed containers might be due to the absence of oxygen, removed by boiling and excluded by sealing, left open a single loophole, of which the 19th century defenders of abiogenesis proceeded to avail themselves. Pasteur, however, by employing sterilized cultures, which he aerated with filtered air exclusively, succeeded in depriving his opponents of this final refuge, and thereby completely demolished the last piece of evidence in favor of spontaneous generation. Prof. Wm. Sydney Thayer, in an address delivered at the Sorbonne, May 22, 1923, gives the following account of Pasteur’s experiments in this field: “Then, naturally (1860-1876) came the famous studies on spontaneous generation undertaken against the advice of his doubting masters, Biot and Dumas. On the basis of careful and well-conceived experiments he demonstrated the universal presence of bacteria in air, water, dust; he showed the variation in different regions of the bacterial content of the air; he demonstrated the permanent sterility of media protected from contamination, and he insisted on the inevitable derivation of every living organism from one of its kind. ‘No,’ he said, ‘there is no circumstance known today which justifies us in affirming that microscopic organisms have come into the world, without parents like themselves. Those who made this assertion have been the playthings of illusions or ill-made experiments invalidated by errors which they have not been able to appreciate or to avoid.’ In the course of these experiments he demonstrated the necessity of reliable methods of sterilization for instruments or culture media, of exposure for half an hour to moist heat at 120° or to dry air at 180°. And behold! our modern procedures of sterilization and the basis of antiseptic surgery.” (Science, Dec. 14, 1923, p. 477.) Pasteur brought to a successful completion the work of Redi and Spallanzani. Henceforth spontaneous generation was deprived of all countenance in the realm of biological fact.
Meanwhile, the cytologists and embryologists of the last century were adding article after article to the law of genetic cellular continuity, thus forging link by link the fatal chain of severance that inexorably debars abiogenesis from the domain of natural science. With the formulation of the great Cell Theory by Schleiden and Schwann (1838-1839), it became clear that the cell is the fundamental unit of organization in the world of living matter. It has proved to be, at once, the simplest organism capable of independent existence and the basic unit of structure and function in all the more complex forms of life. The protists (unicellular protozoans and protophytes) consist each of a single cell, and no simpler type of organism is known to science. The cell is the building brick out of which the higher organisms or metists (i.e. the multicellular and tissued metazoans and metaphytes) are constructed, and all multicellular organisms are, at one time or other in their career, reduced to the simplicity of a single cell (v.g. in the zygote and spore stages). The somatic or tissue cells, which are associated in the metists to form one organic whole, are of the same essential type as germ cells and unicellular organisms, although the parallelism is more close between the unicellular organism and the germ cell. The germ cell, like the protist, is equipped with all the potentialities of life, whereas tissue cells are specialized for one function rather than another. The protist is a generalized and physiologically-balanced cell, one which performs all the vital functions, and in which the suppression of one function leads to the destruction of all the rest; while the tissue cell is a specialized and physiologically-unbalanced cell limited to a single function, with the other vital functions in abeyance (though capable of manifesting themselves under certain circumstances). Normally, therefore, the tissue cell is functionally incomplete, a part and not a whole, whereas the protist is an independent individual, being, at once, the highest type of cell and the lowest type of organism.
According to the classic definition of Franz Leydig and Max Schultze, the cell is a mass of protoplasm containing a nucleus, both protoplasm and nucleus arising through division of the corresponding elements of a preëxistent cell. In this form the definition is quite general and applies to all cells, whether tissue cells, germ cells, or unicellular organisms. Moreover, it embodies two principles which still further determine the law of genetic cellular continuity, namely: Omnis cellula ex cellula, enunciated by Virchow in 1855, and Flemming’s principle: Omnis nucleus ex nucleo, proclaimed in 1882. In this way, Cytology supplemented Redi’s formula that every living being is from a preëxistent living being, by adding two more articles, namely, that every living cell is from a preëxistent cell, and every new cellular nucleus is derived by division from a preëxistent cellular nucleus. Now neither the nucleus nor the cell-body (the cytoplasm or extranuclear area of the cell) is capable of an independent existence. The cytoplasm of the severed nerve fibre, when it fails to reëstablish its connection with the neuron nucleus, degenerates. The enucleated amœba, though capable of such vital functions as depend upon destructive metabolism, can do nothing which involves constructive metabolism, and is, therefore, doomed to perish. The sperm cell, which is a nucleus that has sloughed off most of its cytoplasm, disintegrates, unless it regains a haven in the cytoplasm of the egg. Life, accordingly, cannot subsist in a unit more simply organized than the cell. No organism lives which is simpler than the cell, and the origin of all higher forms of life is reducible, as we shall see, to the origin of the cell. Consequently, new life can originate in no other way than by a process of cell-division. All generation or reproduction of new life is dependent upon the division of the cell-body and nucleus of a preëxistent living cell.