It is only when we turn to tropical and sub-tropical regions that we find variations of temperature unable to account for increased glaciation. Not only were the changes of land and sea distribution on a very much smaller scale than further north, but the Appendix shows that the temperature value of a corresponding change of land area is also very much less. But the high intertropical mountains—the Andes and Kenya and Kilimanjaro in central Africa—which to-day bear glaciers, in Quaternary times carried much greater ones. We cannot call in a fall of temperature, for the reason above stated, and also because at lower levels there is no evidence of colder conditions. In the Glacial period the marine fauna was the same as to-day, and mountains which now fall short of the snow-line by a few hundred feet were still unglaciated even then. The only alternative is increased snowfall on the higher mountains. Fortunately this fits in well with meteorological theory. The rain and snowfall of tropical regions depends, first of all, on the evaporation over the oceans. But evaporation is profoundly influenced by the velocity of the wind; and the wind, which in the Tropics represents the strength of the atmospheric circulation, depends on the thermal gradient between the equator and the poles; since there is no evidence of any appreciable change of temperature over the Tropics as a whole, while there was a very great fall in cold temperate and polar regions, the thermal gradient, and therefore, ultimately, the tropical and sub-tropical, rain and snowfall must have been very greatly increased. Hence the increased glaciation of high mountains near the equator, and hence also the evidence of “Pluvial periods” in the sub-tropical arid regions on either side of the equator. Thus during Glacial periods we have: (1) Elevation in high latitudes caused a great increase of land areas there. (2) Both elevation and increase of land area resulted in a lowering of temperature, materially increased by the gradual development of great ice-sheets. (3) These ice-sheets caused the development of subsidiary ice-sheets on their southern and western borders. (4) The lowering of temperature in high latitudes increased the thermal gradient between equator and poles, resulting in: (_a_) Increased snowfall, and hence increased glaciation on high mountains near the equator. (_b_) Pluvial periods in the sub-tropical arid regions. BIBLIOGRAPHY Humphreys, W. J. “Physics of the air.” Philadelphia, 1920. [Pt. 4, pp. 556-629.] Chamberlin, T. C. “An attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis.” _Journal of Geology_ (American), Vol. 7, 1899, pp. 545-84, 667-85, 751-87. [Carbon dioxide theory.] Croll, J. “Climate and time in their geological relations.” London, 1875. “Discussions on climate and cosmology.” London, 1889. [Eccentricity of earth’s orbit.]