These are all great natural handicaps, and have affected the life of Mexico probably more than it will be possible to estimate. The mighty and costly work of the Diaz régime in building harbors is a monument to that “materialistic” era which will last through many years and has already played a tremendous part in furnishing the sinews of revolution to succeeding governments, for without that work Mexico would be far from capable of sustaining herself in the period of her agony to-day. But beyond all these factors of mountains and rivers and sea looms a yet greater problem, and still more far-reaching--the problem of climate. As noted, Mexico lies in about equal parts in the temperate and torrid zones. But the geological zones are far more important, for climate is affected not alone by latitude but by altitude as well. These geological zones are three, the hot country or _tierra caliente_, the temperate country or _tierra templada_ and the (relatively) cold country or _tierra fria_. The hot country is the lowland section along the coasts from sea level to 3,000 feet altitude, where the mean annual temperature varies from 76° to 88° Fahrenheit. The Mexican terminology includes not only the lowlands of the torrid zone, but the whole coastal plain up to the northern border. The _tierra templada_ lies along the mountain slopes and in the lower plateaus, between 3,000 and 6,500 feet altitude, where the temperature is between 65° and 76°. This zone takes in the higher northern sections which are within the temperate zone proper. The _tierra fria_ takes in the high plateaus and the mountains, between 6,500 and 12,500 feet, the yearly average temperatures varying from 30° to 60°, although the important sections record 50° or more. The three geological zones each include about equal portions of the country, but half of the inhabitants live in the cold zone, and only a quarter each in the temperate and hot sections. The mean temperatures of the cold zone are approximately those recognized as the most favorable for physical exertion, but in the hot country the body struggles against a handicap of almost 20° F. more than the 65° at which it normally functions best. More significant still are the temperatures of all the zones in their relation to mental activity. The human mind is at its best under the stimulus of a mean temperature of about 40° F., but even at Mexico City, 7,600 feet above sea level, the mean temperature of winter is as high as 53°. In the temperate and hot countries the handicaps under which the brain functions run to 20° and up to 45° above the 40° at which the human mind works at highest efficiency. No stimulating winters, no clarifying cool spells, even, in the midst of the endlessly beautiful summers of Mexico!