what extent do the conditions of life vary? As to heat and light, the range is startling. Neptune receives 6000 times less than Mercury. No doubt the atmosphere of the planets may modify much these extremes. The small difference of density in the atmosphere, experienced in the ascent of mountains, and the consequent cold, shews that the atmosphere of the planet, as well as its distance from the Sun, has much to do with the temperature. Still, the vast difference between 1 and 6000 is too much to be equalised by the modifying influence of the atmosphere.
Then as to gravity, there is a range of at least 20 to 1, if we take the asteroids into account. A man in Jupiter would be 20 times heavier than in one of the minor planets. If we descend in the scale, we shall, undoubtedly, find planets hardly large enough to accommodate a single family. Then as to density, there is the widest range. The comets are members of the solar system, and must be dealt with in any general argument, but most of them are probably not so dense as the vacuum of an air-pump. It would require no ordinary stretch of imagination to suppose them inhabited. And if we strike off the comets, we must also include the dark transparent ring of Saturn. Next, as to atmosphere, we have a very wide range if we take the Moon into account. If there is a lunar atmosphere, its weight must be 1800 times less than that of the Earth.
