The question of spontaneous generation has undergone manifold vicissitudes. Aristotle made use of spontaneous generation with perfect ingenuousness, not to say unstinted lavishness. The more, however, continued experiment taught that where one had hitherto imagined one beheld the arising of new life, serious mistakes had been made--that germs of life had found their way into the medium, all the more did men turn away from the idea of a _generatio spontanea_. The experiments of Pasteur seemed to give the decisive blow. Wherever life is present, life is presupposed. To-day men give their opinion on the subject of the possibility of spontaneous generation with that cautious reserve which has been learnt from the calculation of probabilities. A modern physiologist expresses himself as follows:-- “The question as to whether out of dead substance a living cell can be produced, whether so-called spontaneous generation is a possibility, does not in the present condition of our knowledge permit of being answered in a decided negative. We are bound to admit the possibility, even though all experiments yield a negative result.”[33] The necessity which, despite all negative results, compels one to cling to the possibility of spontaneous generation, is the truly heroic violence with which biology identifies “life” and “cell.” The entire sum of biological wisdom comes to a point in the saying, _Omnis cellula e cellula_--against which as little objection is to be urged as against the statement of the fact that every living being arises from another living being. At this point, however, geology steps in and plays the spoil-sport by producing indubitable proofs of the one-time molten condition of our globe, thereby setting an insurmountable limit to “life” in the biological acceptation of the word. This fact served as spur to all sorts of attempts at imparting a more scientific character to the belief in spontaneous generation. In these endeavours the main support received came from organic chemistry. The first achievement on the road to the chemical “synthesis” of life was Wöhler’s demonstration of artificial urea. But this event has been so far outstripped that to-day one only looks back at it in order to bring visibly before the eye the progress that has been made in a comparatively short space of time. To-day one is already beginning to talk of the possibility of producing living albumen. The following passage from Huxley’s _On our knowledge of the causes of the phenomena of organic nature_ may serve as a sample of the “scientific circumspection” with which one sets to work upon this most difficult of tasks also. After laying it down that there are two possible proofs of the origin of life: first, the historical one as found in geology; and second, that derived from experiment--of which the former is unsatisfactory and the latter not carried out, the writer proceeds:--