Even so, if life is to obtain a footing, the planets must not be too hot or too cold. In the solar system, for instance, we cannot imagine life existing on Mercury or on Neptune; liquids boil on the former and freeze hard on the latter. These planets are unsuitable for life because they are too near to, or too far from, the sun. We can imagine other planets which are unsuitable because their substance itself generates energy at such a rate as to make them unsuitable for habitation. The inert atoms which form our earth seem to be the end products of a long series of atomic changes, a sort of final ash resulting from the combustion of the universe. We have seen how such atoms probably float to the top in every star, as being the lightest in weight, but it is by no means a foregone conclusion that all planets will consist of nothing but inert atoms, and so will cool down until life can obtain a footing on them. This has happened with our earth, but we do not know how many planets and planetary systems may be unsuited for life because it has not happened with them. All this suggests that only an infinitesimally small corner of the universe can be in the least suited to form an abode of life. Primaeval matter must go on transforming itself into radiation for millions of millions of years to produce a minute quantity of the inert ash on which life can exist. Then by an almost incredible accident this ash, and nothing else, must be torn out of the sun which has produced it, and condense into a planet. Even then, this residue of ash must not be too hot or too cold, or life will be impossible. Finally, after all these conditions are satisfied, will life come or will it not? We must probably discard the at one time widely accepted view that once life had come into the universe in any way whatsoever, it would rapidly spread from planet to planet and from one planetary system to another until the whole universe teemed with life; space now seems too cold, and planetary systems too far apart. Our terrestrial life must in all probability have originated on the earth itself. What we would like to know is whether it originated as the result of still another amazing accident or succession of coincidences, or whether it is the normal event for inanimate matter to produce life in due course, when the physical environment is suitable. We look to the biologist for the answer, which so far he has not been able to produce.