The answer alleged is that, of course, the experiments of Pasteur and Tyndall, to which some reference must afterwards be made here, merely demonstrated the impossibility of the spontaneous generation of life in our own day or under any conditions similar to those of our own day; but doubtless the first few simple forms of living matter arose by natural processes at some distant epoch “when the conditions were very different from those that obtain to-day.” Now it happens to be true that every difference between past and present conditions which physics and geology and chemistry can assert tends to the probability that if spontaneous generation is impossible now, it must have been a hundredfold more impossible a hundred million years ago. Yet for some three decades the great majority of biologists have been content to believe that spontaneous generation is impossible now, even though land and sea and sky are packed with organic matter under the very conditions which obviously favour life--as the all but omnipresence of life abundant to-day demonstrates--but that spontaneous generation was possible in the past when, by the hypothesis, there was no organic matter present at all, and when life had to arise in the union and architecture of such simple substances as inorganic carbonates! Such biologists are like those who know that the human organism can be developed from the microscopic germ in a few years, but find it incredible that man can have been developed from lowly organisms in æons of æons. Nor has any living biologist even attempted to make an adequate answer to the question, why what is impossible now should have been possible a hundred million years ago. On the contrary, so soon as the matter is looked at philosophically, we see that all the probabilities, all the analogies, all the great generalisations of science, are in favour of the belief that life must be arising from the lifeless now, as in the past, whenever certain conditions, such as the assemblage of carbon, oxygen, nitrogen and hydrogen in the presence of liquid water, are satisfied. For the moment, however, I propose to postpone this question of the truth of “Omne vivum ex vivo” at the present day, for I desire to throw into the forefront of my argument two quite recent developments of science, unreckoned with because non-existent in the controversy of the ’seventies, and in my judgment not yet duly appraised to-day. In the present and future discussion of the manner and causation of that supreme event in the earth’s history, the beginning of life upon it, we must reckon with two new orders of inquiry relating to facts unthinkably contrasted in physical magnitude yet equally relevant to our subject. The first series of facts with which I will deal are _astronomic_, and the second _atomic_. [Sidenote: The Evidence from Other Worlds] [Sidenote: Vegetable Life on Mars]