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22 markings. Here, then, we have at once a suspiciously suggestive hint that they once held water, and that that water flowed. Corroborating this deduction is the fact that the northern sides of all the dark areas are very perceptibly darker than the southern ones; for the northern side is the one which a descending current would plough out, since it is the northern coasts that would be constantly opposing the current's northerly inertia. Consequently, although at present the descending stream be quite inadequate to such task, it still finds its way, from preference, to these lowest levels, and makes them greener than the rest. Though seas no longer, we perceive, then, that there is some reason to believe the so- called seas of Mars to have been seas in their day, and to be at the moment midway in evolution from the seas of the earth to the seas of the moon. Now, if a planet were at any stage of its career able to support life, it is probable that a diminishing water supply would be the beginning of the end of that life, for the air would outlast the available water. Those of its inhabitants who had succeeded in surviving would find themselves at last face to face with the relentlessness of fate,--a scarcity of water constantly growing greater, till at last they would all die of thirst, either directly or indirectly; for either they themselves would not have water enough to drink, or the plants or animals which constituted their diet would perish for lack of it,--an alternative of small choice to them, unless they were conventionally particular as to their mode of death. Before this lamentable conclusion was reached, however, there would come a time in the course of the planet's history when water was not yet wanting, but simply scarce and requiring to be husbanded; when, for the inhabitants, the one supreme problem of existence would be the water problem,--how to get water enough to sustain life, and how best to utilize every drop of water they could get. Mars is, apparently, in this distressing plight at the present moment, the signs being that its water supply is now exceedingly low. If, therefore, the planet possess inhabitants, there is but one course open to them in order to support life. Irrigation, and upon as vast a scale as possible, must be the all-engrossing Martian pursuit. So much is directly deducible from what we have learned recently about the physical condition of the planet, quite apart from any question as to possible inhabitants. What the physical phenomena assert is this: if there be inhabitants, then irrigation must be the chief material concern of wwotoat their lives. At this point in our inquiry, when direct deduction from the general physical phenomena observable on the planet's surface shows that were there inhabitants there a system of irrigation would be an all-essential of their existence, the telescope presents us with perhaps the most startling discovery of modern times,--the so-called canals of Mars. These strange phenomena, together with the inferences to be drawn from them, will form the subject of the next paper. MARS. Ill. CANALS.