It is true that life presupposes life in all the cases we have been able to investigate. These cases are exceedingly numerous because on the disbelief in _generatio spontanea_ rests a whole modern industry, the art of preserving, which in millions of cases daily verifies the hypothesis. But our experience, in spite of this, does not reach far. If we continue our observations, who can guarantee that we would not finally discover that Büchner, after all, was right, and one single case would suffice. The utmost we can attain by observation is a certain degree of probability, and if we undertook to prove Harvey’s hypothesis to be a law in this way, our experiments must be extended _in infinitum_. In order to reach certainty only under present conditions, we must study the generation of every now living organism, animals, plants, bacteria and the like. If it were found then that all these beings have had parents it would still be impossible to draw absolutely sure conclusions in regard to previous generations. We should be obliged to extend our researches through antiquity and primeval ages. If then no gap was to be found in the series and we perhaps finally traced life back to the “moss-clad fragment” from another world, we would again face the question, how the beings on that planet, once in time, had come into existence? Perhaps there the elements and forces of nature were such as to create life spontaneously. This question, of course, could not be decided except through continued observations, which would be obliged to extend to every point of an infinite universe and back to the dawn of time. First, then, we should know that Harvey’s hypothesis was a law, valid without limitations in the past—but also only in the past—and valid with one single exception, namely, the very first organism, of which we presently shall speak. In regard to the law’s validity in the future, we should no doubt possess a knowledge that approached certainty, but it could not be called absolutely sure. Because, even granted that no living being hitherto was without parents, it is not logically impossible that sometime in the future, lifeless matter might undertake to create organisms. To obtain certainty we must continue our observations until the end of time. CHAPTER V. The Materialistic Demonstration of _Generatio Spontanea_.