(a) At present the westward drift of warm waters, set in motion by the trade winds, is interrupted by land masses and turned poleward, producing the important Gulf Stream Drift and Japan Current in the northern hemisphere, and corresponding, though less important, currents in the southern hemisphere. During the past, quite different sets of ocean currents doubtless have existed in response to a different distribution of land. Repeatedly, in the mid-Cretaceous (Fig. 9) and several other periods, the present American barrier to the westward moving tropical current was broken in Central America. Even if the supposed continent of "Gondwana Land" extended from Africa to South America in equatorial latitudes, strong currents must still have flowed westward along its northern shore under the impulse of the peculiarly strong trade winds which the equatorial land would create. Nevertheless at such times relatively little warm tropical water presumably entered the North Atlantic, for it escaped into the Pacific. At several other times, such as the late Ordovician and mid-Devonian, when the isthmian barrier existed, it probably turned an important current northward into what is now the Mississippi Basin instead of into the Atlantic. There it traversed an epeiric, or mid-continental sea open to both north and south. Hence its effectiveness in warming Arctic regions must have been quite different from that of the present Gulf Stream. (b) We will next consider the influences of changes in the amount of equatorial and tropical land. As such lands are much hotter than the corresponding seas, the intensity and width of the equatorial belt of low pressure must be great when they are extensive. Hence the trade winds must have been stronger than now whenever tropical lands were more extensive than at present. This is because the trades are produced by the convection due to excessive heat along the heat equator. There the air expands upward and flows poleward at high altitudes. The trade wind consists of air moving toward the heat equator to take the place of the air which there rises. When the lands in low latitudes were wide the trade winds must also have dominated a wide belt. The greater width of the trade-wind belt today over Africa than over the Atlantic illustrates the matter. The belt must have been still wider when Gondwana Land was large, as it is believed to have been during the Paleozoic era and the early Mesozoic.