In the first place the appearance of the dark markings both on the Moon and Mars hints that though seas no longer, they were seas once upon a time. On the moon, not only does their shape suggest this previous condition, but the smooth and even look of their surfaces adds to the cogency of the inference. More important, however, than either of these characteristics, and confirmatory of both, is the fact that the great tracts in question seem to lie below the level of the corrugated surface, which is thickly strewn with volcanic cones. Their level and their levelness fay in explanation into one another. The first makes possible the former presence of water; the second speaks of its effect. For their flat character hints that these areas were held down at the time when the other parts of the surface were being violently thrown up. That they can themselves be cooled lava flows, their extent and position seem enough to negative; to say nothing of the fact that they should in that case lie above, not below, the general level. Something, therefore, covered them during the moon’s eruptive youth and disappeared later. Such superincumbence may well have been water, under which the now great plains lay then as ocean bottoms. Deep-sea soundings in our own oceans betray an ocean floor of the same extensive sort, diversified as on the moon. To call the lunar _maria_ seas may not be so complete a misnomer after all; but only a resurrecting in epitaph what was the truth in its day. Only doubtfully offered here for the Moon, for Mars the inference seems more sure. Here again the dark regions not only look as they should had they had an earlier history, but they, too, seem to lie below the level of the surface round about. When they pass over the terminator they invariably show as flattenings upon it, as if a slice of the surface had been pared off. Such profile in such pass is what ground at a lower level would present. Undoubtedly a part of the seeming depression is due to relative absence of irradiation consequent upon a more sombre tint, but loss of light hardly seems capable of the whole effect. In the case of Mars, then, as with the Moon, a mistaken inference builded better than it knew, if, indeed, we should rightly consider an inference to be mistaken which on half data lands us at the right door. From the aspects of the dark regions we are led, then, to regard Mars as having passed through that stage of existence in which the earth finds itself at the moment, the stage at which oceans and seas form a feature of its landscape and an impediment to subjugation of its surface in its entirety. What once were ocean beds have become ocean bottoms devoid of that which originally filled them.