From northern New Mexico and Arizona to Mexico City summer rains, dry winters, and still drier springs, are the rule. Forests are few, and much of the country is desert. The more abundant the rains, the greater the number of people and the greater the opportunities for the accumulation of wealth, and thus for that leisure which is necessary to part of a community if civilization is to make progress. That is one reason why the civilization of the summer rain people becomes more highly developed as they go from north to south. The fact that the altitude of the country increases from the United States border southward also tends in the same direction, for it causes the climate to be cooler and more bracing at Mexico City than at places farther north. The importance of summer rains in stimulating growth and in facilitating the early stages of agriculture is noteworthy. Every one familiar with Arizona and New Mexico knows how the sudden summer showers fill the mountain valleys with floods which flow down upon the plain and rapidly spread out into broad, thin sheets, often known as playas. There the water stands a short time and then either sinks into the ground or evaporates. Such places are favored with the best kind of natural irrigation, and after the first shower it is an easy matter for the primitive farmer to go out and drop grains of corn into holes punched with a stick. Thereafter he can count on other showers to water his field while the corn sprouts and grows to maturity. All that he needs to do is to watch the field to protect it from the rare depredations of wild animals. As time goes on the primitive farmer realizes the advantage of leading the water to particularly favorable spots and thus begins to develop a system of artificial irrigation. In regions where such advantageous conditions prevail, the people who live permanently in one place succeed best, for the work that they do one year helps them the next. They are not greatly troubled by weeds, for, though grasses grow as well as corn in the places where the water spreads out, the grasses take the form of little clumps which can easily be pulled up. In the drier parts of the area of summer rain, it becomes necessary to conserve the water supply to the utmost. The Hopi consider sandy fields the best, for the loose sand on top acts as a natural blanket to prevent evaporation from the underlying layers. Sometimes in dry seasons the Hopi use extraordinary methods to help their seeds to sprout. For instance, they place a seed in a ball of saturated mud which they bury beneath several inches of sand. As the sand prevents evaporation, practically all the water is retained for the use of the seed, which thereupon sprouts and grows some inches by the time the first summer floods arrive.