But we have to take account, too, of an entirely general reason, which lies in the fact that all life is transient, that it can be annihilated, and is not merely mortal! Everything that is distinctively organic may be destroyed to the extent of becoming inorganic. Not only may the phenomena of life disappear, and the living body as such cease to be, but the organic compounds which form the physical basis of all life are ceaselessly breaking up, and they fall back by stages to the level of the inorganic. It seems to me that we must necessarily conclude from this that the basis of Liebig's idea was incorrect, that is, the assumption that 'organic substances are everlasting and have existed from the first just in the same way as inorganic substances.' This is obviously not the case, for a thing that has an end cannot be everlasting; it must have had a beginning too, and consequently organic combinations are not everlasting, but are transitory; they come and go, they arise wherever the conditions suitable for them occur, and they break up into simpler combinations when these conditions cease to be present. It is only the elements which are eternal, not their combinations, for these are subject to more or less rapid continual change, whether they have arisen outside of organisms or within them.
It seems to me that these considerations destroy the foundations of the hypothesis of the cosmic origin of life on our earth; in any case they leave the hypothesis without great significance; for if we could even admit the possibility of a transference of living organisms from space, the question would only be pushed a little further back by the assumption, and not solved, for the organisms thus brought in must have had their origin on some other planet, since they are, ex hypothesi, not everlasting.
Thus we are directed to our earth itself as the place of the origin of the tellurian world of life, and I see no possibility of avoiding the assumption of spontaneous generation. It is for me a logical necessity.
Even about the middle of the nineteenth century there was acute discussion in regard to the occurrence of spontaneous generation. In the French Academy especially Pouchet brought forward arguments in favour of it, and Pasteur against it. Pouchet observed that living organisms made their appearance in infusions of hay and other vegetable material in which any possible living germs had presumably been destroyed by prolonged boiling. Living organisms, Algæ, and Infusorians appeared, notwithstanding the fact that the glass bottles in which they were kept were hermetically sealed. But Pasteur showed that the air contains numerous living germs of lowly organisms in its so-called motes, and that, if these were first removed, Pouchet's infusion would not exhibit any signs of life. He caused the air, which was continually passed through the tubes, to stream first along the heated barrel of a gun, and so destroyed these germs, and no organisms were obtained in the infusions. He showed that the air is teeming with germs by an experiment with boiled infusions which were allowed to lie undisturbed for a considerable time in bottles with open necks, one on the roof of the Institute at Paris, the other on the top of the Puy de Dôme in Auvergne, which was at that time still the highest mountain in France. In the Parisian experiment, organisms appeared in the bottles in a very few days, while in those exposed to the pure air at the mountain-top none were seen, even after months had elapsed.
Strangely enough, these and similar experiments were at the time regarded as conclusive proof against the existence of spontaneous generation, though it is obvious enough that the first living being on this earth cannot have sprung from hay, or from any other organic substance, since that would presuppose what we are attempting to explain. After the fiery earth had so far cooled that its outermost layer had hardened to a firm crust, and after water had condensed to a liquid form, there could at first only have been inorganic substances in existence. In order to prove spontaneous generation, therefore, it would be necessary to try to find out from what mingling of inorganic combinations organisms could arise; to prove that spontaneous generation could never have been possible is out of the question.
It would be impossible to prove by experiment that spontaneous generation could never have taken place; because each negative experiment would only prove that life does not arise under the conditions of the experiment. But this by no means excludes the possibility that it might arise under other conditions.
Up till now all attempts to discover these conditions have been futile, and I do not believe that they will ever be successful, not because the conditions must be so peculiar in nature that we cannot reproduce them, but above all, because we should not be able to perceive the results of a successful experiment. I shall be able to prove this convincingly without difficulty.
If we ask ourselves the question how the living beings which might have arisen through spontaneous generation must be constituted, and on the other hand, in regard to what kinds of living forms we can maintain with certainty that they could not have arisen thus, it is obvious that we must place on the latter list all organisms which presuppose the existence of others, from which they have been derived. But to this category belong all the organisms which possess a germ-plasm, an idioplasm that we conceive of as composed of primary constituents (Anlagen) which have gradually been evolved and accumulated through a long series of ancestors. Thus not only all multicellular animals and plants which reproduce by means of germ-cells, buds, and so forth, but also all unicellular organisms, must be placed in this class. For these last—as we have seen—possess in their nucleus a substance made up of primary constituents, without which the mutilated body is unable to make good its loss, in short, an idioplasm. That this plays the same rôle in unicellular as in multicellular organisms we can infer with the greatest certainty from the process of amphimixis, which runs its course in an analogous way in both cases.
Thus, even though we did not know what Ehrenberg demonstrated in the third decade of last century, that Infusorians in an encapsuled state can be blown about everywhere, and can even be carried across the ocean in the dust of the trade-winds, to re-awaken to life wherever they fall into fresh water, we should still not have remained at the standpoint of Leuwenhoek, who regarded Infusorians as having arisen through spontaneous generation. They cannot arise in this way, nor can they have done so at any time, because they contain a substance made up of primary constituents, which can only be of historic origin, and cannot therefore have arisen suddenly after the manner of a chemical combination.