In considering the habitability of various portions of the solar system, we have to draw a marked distinction between the planets which travel within the orbit of the earth, and those which lie beyond its range. So far, indeed, as our belief in these orbs being inhabited is concerned, we may apply the same processes of reasoning to one set of planets as to the other. Until it has been demonstrated that no form of life can exist upon a planet, the presumption must be that the planet is inhabited. But it is impossible to contemplate the various members of our solar system, without being led to consider their physical habitudes rather with relation to the wants of such creatures as exist upon our own earth than merely with reference to the existence of life of some sort upon their surface. Viewing Venus and Mercury in this way, we have a different set of relations to deal with than we find among the outer planets. We are struck, at once, with the marked effects which seem associable with their comparative proximity to the sun's orb. This feature and the shortness of their period of revolution—that is, of their year, are the characteristic peculiarities we have to deal with.