Another vitally important effect of Mexican geography is the uncertainty and untoward distribution of rainfall. This has forced upon the Mexicans their diet of corn and with it their use of fiery condiments which are probable causes of the digestive disorders which ravage all classes. Moreover, the rainfall conditions have been vitally important in the determination of the entire agricultural tendency of the country. The seasons in Mexico are marked, not by winter cold and summer heat so much as by seasons of rain and drought. The winter is the dry season, roughly between October and May, and the summer is the salubrious rainy season, from June to September. The distribution of rain throughout the year and the failure of the rains in the important growing seasons in some of the otherwise fertile sections is due primarily to the geography of the country. Professor Ellsworth Huntington, of Yale, the great American climatologist, finds that Mexico’s summer rains are due to the vertical rays of the sun which cause the rapid rising of the heated air, with sudden expansion and condensation, first over the low-lying hot country and later on the rising uplands of the _tierra templada_, where the function of the mountains in bringing about condensation is amply proven by the well-watered eastern slopes and the dry western sides of the ranges. This mountain contour and the peculiar shape of the Mexican mainland (very wide at the north in proportion to the south) create another important climatological effect--the broad stretches of desert in the northern sections. The so-called continental type of climate which forms the American deserts further north combines with the mountain contour and the distance from the eastern seashore (driving the rain-clouds southward) to make immense sections of Mexico desert, capable of supporting, at best, only wandering herds. These deserts lie between the broad arms of the great “Y” of the Mexican mountain ranges, and combined with the mountainsides themselves render nearly three-fourths of the area of Mexico unfit for cultivation, even if irrigation were general. The result is that of the 500,000,000 acres of land in Mexico, not more than 25,000,000 acres are arable. Great sections are useless, so that in the state of Chihuahua, 90,000 square miles in extent, only about 125,000 acres, or less than two-tenths of one per cent can be cultivated--and most of that arable portion is irrigated.