There is still another bold hypothesis which we have to mention--namely, _that the organic germs were once thrown from other spheres upon the earth by aerolites_. Years ago this idea was declared by Helmholtz to be scientifically conceivable; then it was formally asserted and brought into general notice by Sir William Thompson, in his opening address before the annual assembly of the British Association at Edinburgh, in 1871, but rejected as formally and materially unscientific by Zoellner, in the preface to his work, "Nature of Comets," and again defended by Helmholtz in his preface to the second volume of a translation of Thompson and Tait's Theoretical Physics. However, this hypothesis also only defers the solution of the question, and, supposing its scientific possibility, leads either to the remoter question, how life did originate in those other spheres, or to the metaphysical assertion of the eternity of life and of the eternal continuity of the living in the world, and shows therewith very clearly the impossibility of its explanation. This inexplicability would still exist, if what is quite improbable should happen, namely, that the experimental attempts at _artificially producing organic life_ should be successful, and if thus the question as to the _generatio aequivoca_, which during the past decades so much alarmed the minds of scientists and theologians, should be experimentally solved and answered in the affirmative. For in view of the hopes of a possible explanation of life, which is expected to be the reward for the success of {139} these attempts, Zoellner is fully right in saying: "That the scientists to-day set such an extremely high value on the inductive proof of the _generatio aequivoca_, is the most significant symptom of how little they have made themselves acquainted with the first principles of the theory of knowledge. For, suppose they should really succeed in observing the origin of organic germs under conditions entirely free from objection to any imaginable communication with the atmosphere, what could they answer to the assertion that the organic germs, in reference to their extension, are of the order of ether-atoms, and, with these, press through the intervals of the material molecules which form the sides of our apparatus?"