Rationally viewed, then, the general problem of life in other worlds reduces itself to a question of conditions. Since certain physical results follow inevitably upon certain physical premises, if we can assure ourselves of the proper premises we may look to nature for their conclusion. _A priori_, then, the possibility of life becomes one of habitat. If the environment be suitable life will ensue. What makes for such a mediary _milieu_ is, like most cosmic processes, in its fundamentals of interesting simplicity; for the production of a proper nidus depends primarily upon the mere size of the body parentally concerned. If a planet be big enough it will inevitably bring forth life, because of conditions suitable to its generating; if too small it will remain sterile to the end of time. That size should be the determining factor whether a planet shall be fecund or barren may seem at first thought strange. Yet that it is so admits of no rational doubt. All that we see of bodies about us shows its truth, and what we have learnt of cosmic process enables us in some sort to discern why. In order for evolution, such as we mark it upon the earth, to be possible, the parent body must have been at one time at a high temperature, since only under great heat can the primal processes occur. But for this generation of caloric the aggregate mass of the particles, the falling together of which makes the planet, and their stoppage its internal heat, must be large. The sun’s rays alone are insufficient to cause the necessary temperature; the heat must come from within, though it be helped from without. Even here the action is abetted by a large body. For a planet to entrap the sun’s rays or even to preserve its own internal warmth, an atmosphere is needed, and it takes a large body to retain an atmospheric covering sufficiently long. Yet without it not only would there be no suitable state, but no medium in which organic or even inorganic reactions could go on. Lastly, water, the essential nidus for the organism’s early stages, has its presence similarly conditioned. For this, like the atmosphere, would from a small body speedily vanish away. Thus the planet itself is the life-producing body, although the sun furthers the process when once begun. That the needed substances are planetarily present, what we know of the distribution of matter astronomically sufficiently attests. What we find in meteorites shows that the catastrophe which preceded our present solar system’s birth scattered its elemental constituents throughout its domain, and thus when they came to be gathered up again into planets that these must have been materially the same. The manner, not the matter, then, is alone that about which we are concerned.