great uplift of the lands which produces high mountains and large continents in subtropical and middle latitudes. Hence, uplift alone cannot account for extensive glaciation in subtropical latitudes during the Permian and Proterozoic. [Illustration: _Fig. 7. Permian geography and glaciation._ (_After Schuchert._)] The assumption of a great general lowering of temperature is also not adequate to explain glaciation in subtropical latitudes. In the first place this would require a lowering of many degrees,--far more than in the Pleistocene glacial period. The marine fossils of the Permian, however, do not indicate any such condition. In the second place, if the lands were widespread as they appear to have been in the Permian, a general lowering of temperature would diminish rather than increase the present slight efficiency of the monsoons in producing glaciation. Monsoons depend upon the difference between the temperatures of land and water. If the general temperature were lowered, the reduction would be much less pronounced on the oceans than on the lands, for water tends to preserve a uniform temperature, not only because of its mobility, but because of the large amount of heat given out when freezing takes place, or consumed in evaporation. Hence the general lowering of temperature would make the contrast between continents and oceans less than at present in summer, for the land temperature would be brought toward that of the ocean. This would diminish the strength of the inblowing summer monsoons and thus cut off part of the supply of moisture. Evidence that this actually happened in the cold fourteenth century has already been given in Chapter VI. On the other hand, in winter the lands would be much colder than now and the oceans only a little colder, so that the dry outblowing monsoons of the cold season would increase in strength and would also last longer than at present. In addition to all this, the mere fact of low temperature would mean a general reduction in the amount of water vapor in the air. Thus, from almost every point of view a mere lowering of temperature seems to be ruled out as a cause of Permian glaciation. Moreover, if the Permian or Proterozoic glacial periods were so cold that the lands above latitude 30 deg. were snow-covered most of the time, the normal surface winds in subtropical latitudes would be largely equatorward, just as the winter monsoons now are. Hence little or no moisture would be available to feed the snowfields which give rise to the glaciers.