All organisms, from the lowest to the highest, whether plant or animal, consist very largely of water, and its constant presence either in the liquid or gaseous form is essential for organic life. On our earth oceans and seas occupy the greater part of the surface, while their average depth is so great that the quantity of water is sufficient to cover the whole of the globe free from inequalities two miles deep. It is this enormous amount of water that supplies the air with ample moisture, such as renders the life of the tropics so luxuriant. Yet even now the inequality of water-supply is such that large areas in all parts of the earth are what we term deserts, only supporting a very few forms of life that have become specially adapted to them, and certainly unfitted for the continuous development of life from lower to higher forms. [Sidenote: Water and the Atmosphere] Water is also of immense importance as an equaliser of temperature, the currents of the ocean conveying the warmth of the tropics to ameliorate the severity of temperate and Polar regions, while the amount of water-vapour in the atmosphere acts as a retainer of heat during the night, without which it is probable that the surface of the earth would freeze every night even in the tropics. When we consider that water consists of two gases--oxygen and hydrogen--in definite proportions, and that without their presence in these proportions and in the necessary quantity the development of organic life would have been impossible, we find that we have here a remarkable and very complex set of conditions which must be fulfilled in any planet to enable it to develop life. But this is not all. The atmosphere is so intimately associated with water in its life-relations, and is itself so absolutely essential to the existence from moment to moment of the higher animals, that the two require to be duly proportioned to each other and to the globe of which they form a part. [Sidenote: How Water Protects Earth by Night]