Nearly all the theories which have been put forward to account for geological changes of climate, and especially the occurrence of the last or Quaternary Ice Age, are based on the abnormal variation of one or other of the above factors, and we may consider them briefly in turn. Very few have ever been taken seriously. In the first place, we can at once dismiss fluctuations in the radiation emitted by the sun as a cause of _great_ changes of climate. It is true that many small fluctuations are traceable directly to this cause, such as the eleven-year periodicity of temperature and rainfall; but these fluctuations are, and must be, greater at the equator than at the poles, while the fall of temperature during the Glacial period reached its maximum near the poles and was least at the equator. Moreover, there is not the slightest direct evidence in support of such a theory, and it can only be admitted when all other hypotheses have failed. The “astronomical” theory of the cause of climatic fluctuations is associated chiefly with the name of James Croll. Croll’s theory connects abnormal variations of climate with variations, firstly of the eccentricity of the earth’s orbit, and secondly of the ecliptic. In periods of high eccentricity the hemisphere with winter in aphelion is cold because the long severe winter is far from being balanced by the short hot summer; at the same time the opposite hemisphere enjoys a mild equable climate. This theory commanded instant respect, and still finds a place in the text-books, but difficulties soon began to appear. The evidence strongly suggests that glacial periods did not alternate in the two hemispheres, but were simultaneous over the whole earth; even on the equator the snow-line was brought low down. Moreover, on Mars the largest snow-cap appears on the hemisphere with its winter in perihelion. Although Croll’s reasoning was beautifully ingenious he gave very few figures; while the date which he gives for the conclusion of the Ice Age, 80,000 years ago, has been shown by recent research to be far too remote, 15,000 years being nearer the mark. Croll’s theory has recently been revived in an altered form by R. Spitaler, a Czecho-Slovakian meteorologist, who calculated the probable alteration in the mean temperature of each latitude under maximum eccentricity (0.7775) and maximum obliquity (27° 48′), the distribution of land and water remaining unchanged. The results are shown in the attached table, where - means that the temperature was so much below the present mean, and + that it was so much above.