For the support of life and vegetation, light is also necessary, for without it no flowers would bloom, nor corn grow and ripen to maturity. To obtain this supply of light and heat it is necessary that a life-bearing planet should revolve at a suitable distance from, and in a nearly circular orbit round, a central sun. These conditions, it is hardly necessary to say, are fulfilled in the case of the earth. Were we much nearer to the sun than we are, we should suffer from excessive heat, and were we much further away, we should probably perish from the cold. For this reason the existence of life on the other planets of the Solar System seems very doubtful. Mercury is probably too hot, and the other planets are certainly too cold, so far as heat from the sun is concerned, unless, indeed, their internal heat is sufficient to raise the temperature of their surface to a point sufficient for the maintenance of life. Indeed, there is good reason to suppose that in the planets Jupiter, Saturn, Uranus, and Neptune, this internal heat is still so great that life would be quite impossible on their surface. Venus, inside the earth’s orbit, and Mars, outside, are the two planets which seem to approach nearest to the required conditions. We know that both these planets possess atmospheres somewhat similar to ours, and, in Mars at least, land and water most probably exist on its surface. Venus is, of course, much hotter than the earth, and Mars much colder, but possibly the polar regions of Venus and the equatorial regions of Mars may form suitable abodes for some forms, at least, of animal and vegetable life. Let us proceed, however, to consider some other conditions necessary for the existence of life on a planet. A suitable temperature is, of course, indispensable, but this is not all. There are other conditions which must be complied with. The planet must have a rotation on its axis, so that every portion shall in turn receive its due share of light and heat. Each point on its surface must have its day and night, the day for work and the night for rest. The axis of rotation must not lie in the plane of the planet’s orbit, but must have a suitable inclination, so that each hemisphere may enjoy its seasons, summer and winter, “seed-time and harvest,” in due course. Further the velocity of rotation on its axis must not be too rapid. If the earth rotated in a period of one and a quarter hours, bodies at the equator would have no weight, and life would be impossible in those regions. The planet must also possess a mass sufficient to retain bodies on its surface by the force of gravity. In the case of very small bodies, such as the moons of Mars and some of the minor planets between Mars and Jupiter, objects thrown into the air would pass away into space never to return.