the seasons, stamp it for vegetation in fact. Thus these regions must be more humid than the rest of Mars. They must, therefore, be lower. That they are thus lower and possess a modicum of water to-day marks them out for the spots where seas would be, were there any seas to be. As we know of a vera causa which has for ages been tending to deplete them, extrapolation from what is now going on returns them the water they have lost and rehabilitates their ancient aquatic character. To the far-sight of inference, seas they again become in the morning of the ages long ago when Mars itself was young.
Nor is this the end of the evidence. When we compare quantitatively the areas occupied by the quondam seas on Mars and on the Moon, we find reason to increase our confidence in our deduction. For the smaller body, the Moon, should have had less water relatively, at the time when the seas there were laid down, than the larger, Mars. Because from the moment its mass began to collect, it was in process of parting with its gases, water-vapor among the rest, and, as we shall see more in detail in the next chapter, it had from the start less hold on them than Mars. Its oceans, therefore, should have been less extensive than the Martian ones. This is what the present lunar Mare seem to attest. They are less extended than the dark areas of Mars. A fact which becomes the more evident
