The leaf and the cloud are the signs of a habitable world. The leaf--that is to say, plant life, vegetation--is necessary because animal life is not capable of building itself up from inorganic material. This step must have been previously taken by the plant. The cloud, that is to say water-vapour, is necessary because the plant in its turn cannot directly assimilate to itself the nitrogen from the atmosphere. The food for the plant is brought to it by water, and it assimilates it by the help of water. It is, therefore, upon the question of the presence of water that the question of the habitability of a given world chiefly turns. In the physical sense, man is "born of water," and any world fitted for his habitation must "stand out of the water and in the water."
The Sun is, of all the heavenly bodies, the most impressive, and has necessarily, at all times, attracted the chief attention of men. There are only two of the heavenly bodies that appear to be more than points of light, only two that show a surface to the naked eye, and the Sun, being so much the brighter of the two, and the obvious source of all our light and heat, and the fosterer of vegetation, readily takes the premier place in interest. In the present day we know too much about the Sun for anyone to suppose that it can be the home of organic life; but it is not many years since its habitability was seriously suggested even by so high an authority as Sir William Herschel. He conceived that it was possible that its stores of light and heat might be confined to a relatively thin shell in its upper atmosphere, and that below this shell a screen of clouds might so check radiation downward that it would be possible for an inner nucleus to exist which should be cool and solid. This fancied inner globe would then necessarily enjoy perpetual daylight, and a climate which knew no variation from pole to pole. To its inhabitants the entire heavens would be generally luminous, the light not being concentrated into any one part of the vault; and it was supposed that, ignorant of time, a happy race might flourish, cultivating the far-spread solar fields, in perpetual daylight, and in the serenity of a perpetual spring that was distracted by no storm.