(e) As the extent of the ocean diminished, there would normally be a decrease in the amount of water vapor for three reasons: (1) Evaporation from the ocean is the great source of water vapor. Other conditions being equal, the smaller the ocean becomes, the less the evaporation. (2) The amount of water vapor in the air diminishes as convection increases, since upward convection is a chief method by which condensation and precipitation are produced, and water vapor removed from the atmosphere. (3) Nocturnal cooling sufficient to produce dew and frost is very much more common upon land than upon the ocean. The formation of dew and frost diminishes the amount of water vapor at least temporarily. (f) Any diminution in water vapor produced in these ways, or otherwise, is significant because water vapor is the most essential part of the atmosphere so far as regulation of temperature is concerned. It tends to keep the days from becoming hot or the nights cold. Therefore any decrease in water vapor would increase the diurnal and seasonal range of temperature, making the climate more extreme and severe. Thus a periodic increase in the area of the continents would clearly make for periodic increased climatic contrasts, with great extremes, a type of climatic change which has recurred again and again. Indeed, each great glaciation accompanied or followed extensive emergence of the lands.[84]
Whether or not there has been a _progressive_ increase from era to era in the area of the lands is uncertain. Good authorities disagree widely. There is no doubt, however, that at present the lands are more extensive than at most times in the past, though smaller, perhaps, than at certain periods. The wide expanse of lands helps explain the prominence of seasons at present as compared with the past.
II. The contraction of the earth, as we have seen, has produced great changes in the distribution as well as in the extent of land and water. Large parts of the present continents have been covered repeatedly by the sea, and extensive areas now covered with water have been land. In recent geological times, that is, during the Pliocene and Pleistocene, much of the present continental shelf, the zone less than 600 feet below sea level, was land. If the whole shelf had been exposed, the lands would have been greater than at present by an area larger than North America. When the lands were most elevated, or a little earlier, North America was probably connected with Asia and almost with Europe. Asia in turn was apparently connected with the larger East Indian islands. In much earlier times land occupied regions where now the ocean is fairly deep. Groups of islands, such as the East Indies and Malaysia and perhaps the West Indies, were united into widespreading land masses. Figs. 7 and 9, illustrating the paleography of the Permian and the Cretaceous periods, respectively, indicate a land distribution radically different from that of today.