This making of deserts is not a sporadic, accidental, or local matter, although local causes have abetted or hindered it. On the contrary, it is an inevitable result of planetary evolution, a phase of that evolution which follows from what has been said in Chapter XII on the abandonment of a planet by its water. Deserts are simply another sign of the same process. The very aging which began by depriving a body of its seas takes from it later its forest and its grass. A growing scarcity of water is bound to depauperate the one, as it depletes the other. We have positive proof of the action in our own deserts. For these bear testimony, in places at least, to not having always been so, but to have gradually become so within relatively recent times. But we have more general proof of the action from the position occupied on the earth’s surface by its deserts. The significant fact about the desert-making so stealthily going on is that only certain zones of the earth’s surface are affected. Those belting the two tropics of Cancer and Capricorn, for several degrees on either side of them, most exhibit the phenomenon. Such positioning of the deserts is not due to chance. Directly, of course, desertism is due to dearth of rain. This in turn depends on the character and condition of the winds. If a wind laden with moisture travel into a colder region of the globe, its moisture is precipitated in rain and we have a fertile country; if it voyage into a warmer clime it takes up what little moisture may be there already and a desert is the result. Now our system of winds is such as to produce a fall of rain for the different latitudes, as tabulated by Supan, thus:— Zone I 40°N-27°N Little rain in summer but much in winter. II 27 N-19 N Little rain at all seasons. III 19 N- 7 N Little rain in winter but much in summer. IV 7 N- 1 N Abundant rain at all seasons. V 1 N-17 S Little rain in winter but much in summer. VI 17 S-30 S Little rain at all seasons. VII 30 S-35 S Little rain in summer but much in winter. Zones II and VI, the zones of minimum rainfall, are also those in which the deserts occur. The northern one traverses southern California, Arizona, New Mexico, the Sahara, Arabia, and the Desert of Gobi; the southern, Peru, the South African veldt, and central Australia. The belts are wavy bands which by their form betray both a general underlying trend to drought at these parallels and also the effect of local topography in the matter.