While the wide-reaching seasonal changes within the tropical province depend on the migrations of the planetary winds northward and southward with the sun, and are due primarily to astronomical conditions, there are equally well-marked variations in rainfall dependent on or regulated by geographical conditions. These are of two principal classes: First, elevation of the land; and second, the relation of broad land areas to the direction of the prevailing winds, each of which is abundantly illustrated in the tropical province. The mountainous islands in the West Indies receive a heavy rainfall, especially on their windward or easterly slopes, while the low islands, and less markedly the southwestern border of the higher islands, are much less humid. The eastern side of Porto Rico, for example, has an abundant rainfall, and was originally clothed with a luxuriant native flora, including large forest-trees, while the lowlands on its southern and southwestern border are subject to drought, and irrigation is desirable to insure the growth of crops. Again, while the mountainous islands of the Lesser Antilles, with their luxuriantly forested slopes, present ideal pictures of tropical loveliness, low-lying Sombrero, Barbuda, St. Martin, and other similar islands are so arid that guano has accumulated on them to such an extent as to be of commercial importance. The reason for these striking contrasts within narrow geographical limits is readily seen in the influence of the highlands on the trade-winds. These air-currents blowing from the Atlantic are warm and contain a high percentage of moisture. As they advance, however, they invade regions that are progressively warmer and warmer, and the capacity of the air for moisture is correspondingly increased. For this reason the trade-winds in crossing low land become drying winds. When the warm humid air-currents are forced upward, however, they are cooled in part by contact with the land, but to a greater extent on account of expansion due to decreased pressure; the dew-point is thus lowered, and when saturation is reached precipitation follows. This explanation applies also to the low peninsula of Yucatan, which is within the trade-wind belt, and is exceptional among the lands of Central America, on account of its dearth of forests, but in this instance, and also in reference to the similar barren condition of the Bahama Islands, in addition to the lowness of the land, the influence of the underlying porous, limestone rocks on the vegetable growths should be considered. In the instances just mentioned the rain that falls is quickly absorbed by the cavernous rocks, and surface streams are rare.