Now, it will be observed that the principal theories assigned for the Drift go upon the hypothesis that it was produced by extraordinary masses of ice--ice as icebergs, ice as glaciers, or ice in continental sheets. The scientists admit that immediately preceding this Glacial age the climate was mild and equable, and these great formations of ice did not exist. But none of them pretend to say how the ice came or what caused it. Even Agassiz, the great apostle of the ice-origin of Drift, is forced to confess: "We have, as yet, no clew to the source of this great and _sudden_ change of climate. Various suggestions have been made--among others, that formerly the inclination of the earth's axis was greater, or that a submersion of the continents under water might have produced a decided increase of cold; but none of these explanations are satisfactory, and science has yet to find any cause which accounts for all the phenomena connected with it."[1] Some have imagined that a change in the position of the earth's axis of rotation, due to the elevation of extensive mountain-tracts between the poles and the equator, might have caused a degree of cold sufficient to produce the phenomena of the Drift; but Geikie says-- "It has been demonstrated that the protuberance of the earth at the equator so vastly exceeds that of any [1. "Geological Sketches," p. 210.] {p. 59} possible elevation of mountain-masses between the equator and the poles, that any slight changes which may have resulted from such geological causes could have had only an infinitesimal effect upon the. general climate of the globe."[1] Let us reason together:-- The ice, say the glacialists, caused the Drift. What caused the ice? Great rains and snows, they say, falling on the face of the land. Granted. What is rain in the first instance? Vapor, clouds. Whence are the clouds derived? From the waters of the earth, principally from the oceans. How is the water in the clouds transferred to the clouds from the seas? By evaporation. What is necessary to evaporation? _Heat_. Here, then, is the sequence: If there is no heat, there is no evaporation; no evaporation, no clouds; no clouds, no rain; no rain, no ice; no ice, no Drift. But, as the Glacial age meant ice on a stupendous scale, then it must have been preceded by heat on a stupendous scale. Professor Tyndall asserts that the ancient glaciers indicate the action of heat as much as cold. He says: "Cold will not produce glaciers. You may have the bitterest northeast winds here in London throughout the winter without a single flake of snow. Cold must have the fitting object to operate upon, and this object--the aqueous vapor of the air--is the direct product of heat. Let us put this glacier question in another form: the latent heat of aqueous vapor, at the temperature of its production in the tropics, is about 1,000° Fahr., for the latent heat augments as the temperature of evaporation descends.