There are, however, some matters connected with the water-supply of the earth, and its relation to the development of life, that call for a few remarks here. What has determined the total quantity of water on the earth or on other planets does not appear to be known; but presumably it would depend, partially or wholly, on the mass of the planet being sufficient to enable it to retain by its gravitative force the oxygen and hydrogen of which water is composed. As the two gases are so easily combined to form water, but can only be separated under special conditions, its quantity would be dependent on the supply of hydrogen, which is but rarely found on the earth in a free state. The important fact, however, is, that we do possess so great a quantity of water, that if the whole surface of the globe was as regularly contoured as are the continents, and merely wrinkled with mountain chains, then the existing water would cover the whole globe nearly two miles deep, leaving only the tops of high mountains above its surface as rows of small islands, with a few larger islands formed by what are now the high plateaus of Tibet and the Southern Andes. Now there seems no reason why this distribution of the water should not have occurred--in fact it seems probable that it would have occurred, had it not been for the fortunate coincidence of the formation of enormously deep ocean basins. So far as I am aware, no sufficient explanation of the formation of these basins has been given but that of Mr. Osmond Fisher, as here described, and that depends upon three unique circumstances: (1) the formation of a satellite at a very late period of the planet's development when there was already a rather thick crust; (2) the satellite being far larger in proportion to its primary than any other in the solar system; and (3) its having been produced by fission from its primary on account of extremely rapid rotation, combined with solar tides in its molten interior, and a rate of oscillation of that molten interior coinciding with the tidal period.[17]