The origin of life is as completely unknown to us as its essence. For thousands of years the assumption was lightheartedly made that under certain, somewhat vague circumstances, life originated of its own accord. Pasteur showed that a _generatio spontanea_ cannot be proved to exist, that every living thing comes from another living thing, a parent organism, and that the old philosophers were right in propounding "_omne vivum ex ovo_" as a law, although they only guessed it and had not proved it experimentally. A very few critics, who are hard to convince, still dare to assert in a small voice that Pasteur's work and all the facts established by microbiology do not prove conclusively that life does not nevertheless originate from inorganic matter under conditions which we cannot nowadays reproduce in our laboratories. No answer can be made to this objection. An experiment is only conclusive for the conditions in which it is made, and not for others. All that we can positively assert is that on earth the genesis of life without a demonstrable parent organism has never been observed. To go farther, and to assert that a _generatio spontanea_ is absolutely impossible under any conditions, on earth or elsewhere, is arbitrary, just as it is to assert the contrary. Those who are supporters of the theory that life can be developed from non-living matter for a long time thought they had conclusively proved their case; they argued as follows: At the present time life exists on our planet; according to the Kant-Laplace hypothesis our planet was formed from a cosmic nebula and passed through a state of fluid incandescence; in this state life is impossible; therefore life must have originated spontaneously one day after the Earth had cooled down; consequently either the Kant-Laplace hypothesis is wrong or the assertion that life can only be generated by life is erroneous; the two assumptions are incompatible. This conclusion no longer presents any insuperable difficulties. It has been observed that spores which have been kept for months at the temperature of frozen hydrogen, that is, very nearly at absolute zero, have retained their germinative power and have developed when they were brought back to a favourable temperature. Therefore they would not be killed by the cold of interstellar space on their way from one heavenly body to another, and could become the seeds of life on another hitherto inanimate star. That large numbers of tiny particles of matter exist in interstellar space and are precipitated on the heavenly bodies is proved by the cosmic dust that arctic explorers have collected from the surface of snow and ice. Therefore the Earth may well have been in an incandescent state, and may yet have received from interstellar space the germs of life which developed and multiplied when the Earth's crust had cooled sufficiently to provide the conditions favourable to their existence; and these germs may have been the