Another hypothesis is based upon variations in quantity of carbonic acid gas and water vapor in the air. Increase or decrease of these constituents causes increase or decrease of temperature because they have high capacities for absorbing heat. "The great elevation of the land at the close of the Tertiary seems to afford conditions favorable both for the consumption of carbon dioxide in large quantities (by weathering of rocks) and for the reduction of the water content of the air. Depletion of these heat-absorbing elements was equivalent to the thinning of the thermal blanket which they constitute. If it was thinned, the temperature was reduced.... By variations in the consumption of carbon dioxide, especially in its absorption and escape from the ocean, the hypothesis attempts to explain the periodicity of glaciation (i.e., glacial and interglacial stages)." (Chamberlin and Salisbury.) Still another suggested explanation is based upon variability of amount of heat radiated by the sun. Slight variations are now known to take place, and possibly in the past during certain periods of time these variations may have been sufficiently great to cause a glacial climate with interglacial stages. Here, as in the case of so many other great natural phenomena, a single, simple explanation does not seem sufficient to account for all the features of the several well-known glacial epochs of geologic time. Two or more hypotheses, or parts of hypotheses, must more than likely be combined to explain a particular Ice Age. CHAPTER XVII EVOLUTION OF PLANTS