Vegetation as we know it on our world requires certain physical and chemical conditions for its existence. For instance, a temperature which, at least during the growing season, is well above the freezing-point of water is requisite; yet the temperature must remain a long way below the boiling-point of water; neither could plants as we know them exist in the absence of an atmosphere containing oxygen, carbon dioxide, and water vapour, and incidentally, by its capacity for retaining heat, warding off violent extremes of temperature which otherwise would be a daily and nightly occurrence. What evidence is there as to the condition in these respects of those heavenly bodies which are sufficiently near to allow us to know something of them? To take first our own Moon. Astronomers are agreed that on the Moon there is neither air nor water; it is a dead mass of solid material, scorched by the Sun by day, held in the grip of appalling frost by night. The Moon was no doubt at some remote period of the Earth’s history cast off from that body, and it carried off with it a portion of the Earth’s atmosphere, or of the materials which later formed the Earth’s atmosphere. But the attraction of the Moon is so small that it was unable to retain these gases on its surface; they diffused into space, much of them returning probably to the Earth, leaving the Moon without any covering of nitrogen or oxygen or hydrogen or water vapour, and thus condemning it to permanent sterility. As regards Mercury, the planet nearest the Sun, conditions appear equally unfavourable. Mercury has ceased to revolve round the Sun, and continually presents one side towards that luminary. On the opposite side an extraordinarily low temperature prevails, low enough to solidify and bind permanently most of the gases of any possible atmosphere; while, on the other side, the very high temperature, due to perpetual and intense sunshine, has assisted the diffusion into space of the more volatile gases, such as hydrogen, which might have remained unfrozen.