In the last chapter of that most fascinating book _Worlds in the Making_,[26] Arrhenius discusses the possibility of life being eternal and of living germs of very small dimensions--_e. g._, the spores of micro-organisms--being carried through space from one planet to another or even from one solar system to another. If it be true that there is no spontaneous generation; if it be true that all cells are potentially immortal, we may indeed seriously raise the question: May not life after all be eternal? Such ideas were advocated by Richter in a rather phantastic way and more definitely by Helmholtz as well as Kelvin. The latter authors assumed that in the collision of planets or worlds on which there is life, fragments containing living organisms will be torn off and these fragments will move as seed-bearing stones through space. “If at the present instant no life existed upon this earth, one such stone falling upon it might ... lead to its becoming covered with vegetation.” Arrhenius points out the difficulties which oppose such a view, as, _e. g._, the fact “that the meteorite in its fall towards the earth becomes incandescent all over its surface and any seeds on it would therefore be deprived of their germinating power.” [26] Arrhenius, S., _Worlds in the Making_, London and New York, 1908, p. 212. Arrhenius suggests another and much more ingenious idea based on the fact that for particles below a certain size the mechanical pressure produced by light waves--the radiation pressure--can overcome the attractive force of gravitation. Bodies which according to Schwarzschild would undergo the strongest influence of solar radiation must have a diameter of 0.00016 mm. supposing them to be spherical. The first question is therefore: Are there any living seeds of such extraordinary minuteness? The reply of the botanist is that spores of many bacteria have a size of 0.0003 or 0.0002 mm., and there are no doubt much smaller germs which our microscopes fail to disclose. This assumption is undoubtedly correct. We will, in the first instance, make a rough calculation of what would happen if such an organism were detached from the earth and pushed out into space by the radiation pressure of our sun. The organism would first of all have to cross the orbit of Mars; then the orbits of the smaller and of the outer planets.... The organisms would cross the orbit of Mars after twenty days, the Jupiter orbit after eighty days, and the orbit of Neptune after fourteen months. Our nearest solar system would be reached in nine thousand years.