From being distributed thus in belts, it is evident that the deserts are general globe phenomena, and from their being found only in the zones of least rainfall, that the earth has itself entered, though not far as yet, upon the desert stage of its history. Once begun, the desert areas must perforce spread as water becomes scarce, invading and occupying territory in proportion as the rainfall there grows small. Now the axial tilt of Mars is almost exactly the same as that of our Earth, the latest determinations from the ensemble of measures giving 24° for it. Here, then, we have initial conditions reproducing those of the earth. But from the smaller size of the planet that body would age the earlier, since it would lose its internal heat the more rapidly, just as a small stone cools sooner than a larger one. On general principles, therefore, it should now be more advanced in its planetary career. In consequence, desertism should have overtaken more of its surface than has yet happened on earth, and instead of narrow belts of sterility we should expect to find there Saharas of relatively vast extent. Now, such a state of things is precisely what the telescope reveals. The ochre tracts occupy nine tenths of the northern hemisphere and a third of the southern. Three fifths, therefore, of the whole surface of the planet is a desert. [Illustration: Desert areas.] Of cosmic as well as of particular import is the correlation thus made evident between the physical principles that effect the aging of a planet and the aspect Mars presents. Experimental corroboration of those laws is thus afforded, while, reversely, confidence in their applicability is increased. With continued observation the planet appears more desiccate as improved conditions bring it nearer. Dry land as it was thought to be proves even drier, something which lacks water for the ordinary necessities of a living world. [Illustration: Desert areas.] The picture the planet offers to us is thus arid beyond present analogue on Earth. Pitiless as our deserts are, they are but faint forecasts of the state of things existent on Mars at the present time. Only those who as travelers have had experience of our own Saharas can adequately picture what Mars is like and what so waterless a condition means. Only such can understand what is implied in having the local and avoidable thus extended into the unescapable and the world-wide; and what a terrible significance for everything Martian lies in that single word: _desert_. CHAPTER XIV SUMMARY