Altogether the problem of how the sun can have remained so uniform and how the earth's atmosphere and other conditions can also have remained so uniform throughout hundreds of millions of years is one of the most puzzling in the whole realm of nature. If appeal is taken to radioactivity and the breaking up of uranium into radium and helium, conditions can be postulated which will give the required amount of energy. Such is also the case if it be supposed that there is some unknown process which may induce an atomic change like radioactivity in bodies which are now supposed to be stable elements. In either case, however, there is as yet no satisfactory explanation of the _uniformity_ of the earth's climate. A hundred million or a thousand million years ago the temperature of the earth's surface was very much the same as now. The earth had then presumably ceased to emit any great amount of heat, if we may judge from the fact that its surface was cool enough so that great ice sheets could accumulate on low lands within 40 deg. of the equator. The atmosphere was apparently almost like that of today, and was almost certainly not different enough to make up for any great divergence of the sun from its present condition. We cannot escape the stupendous fact that in those remote times the sun must have been essentially the same as now, or else that some utterly unknown factor is at work. FOOTNOTES: [Footnote 1: W. A. Setchell: The Temperature Interval in the Geographical Distribution of Marine Algae; Science, Vol. 52, 1920, p. 187.] [Footnote 2: J. Barrell: Rhythms and the Measurements of Geologic Time; Bull. Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.] [Footnote 3: Pirsson and Schuchert: Textbook of Geology, 1915, pp. 538-550.] [Footnote 4: From Charles Schuchert in The Evolution of the Earth and Its Inhabitants: Edited by R. S. Lull, New Haven, 1918, but with revisions by Professor Schuchert.] [Footnote 5: J. H. Poynting: Radiation in the Solar System; Phil. Trans. A, 1903, 202, p. 525.] [Footnote 6: L. J. Henderson: The Fitness of the Environment, 1913.] [Footnote 7: Henderson: _loc. cit._, p. 138.] [Footnote 8: F. R. Moulton: Introduction to Astronomy, 1916.] [Footnote 9: Moulton: _loc. cit._] CHAPTER II THE VARIABILITY OF CLIMATE The variability of the earth's climate is almost as extraordinary as its uniformity. This variability is made up partly of a long, slow tendency in one direction and partly of innumerable cycles of every conceivable duration from days, or even hours, up to millions of years. Perhaps the easiest way to grasp the full complexity of the matter is to put the chief types of climatic sequence in the form of a table.