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Show 120 ISLAND LIFE. (l'.A.R'l' I. the Alps, and then by the Mediterranean; whe.rcas in eastern America and Asia the mountain chains run In a north and south direction, and there is nothing to prevent the flora from having been preserved by a southward migration into a milder region. Our next two chapters will be devoted to a discussion of the causes which brought about the glacial epoch, and that still more extraordinary climatic phenomenon-the mild climate and luxuriant vegetation of the Arctic zone. If my readers will follow me with the care and attention so difficult and interesting a problem requires and deserves, they will find that I have grappled with all the facts which have to be accounted for, and offered what I believe is the first complete and sufficient explanation of them. The important influence of climatal changes on the dispersal of animals and plants is a sufficient justification for introducing such a discussion into the present volume. CHAPTER VIII. THE C.AUSES OF GL.ACI.AL EPOCIIS. Various suggested causes- Astronomical causes of changes of ClimateDifference of Temperature caused by varying distance of the SunProperties of air and water, snow and ice, in relation to ClimateEffects of snow on Climate- High land and great moisture essential to the initiation of a Glacial Epoch- Perpetual snow nowhere exists on lowlands- Conditions determining the presence or absence of perpetual Snow- Efficiency of Astronomical causes in producing GlaciationAction of meteorological causes in intensifying Glaciation- Summary of causes of Glaciation- Effect of clouds and fog in cutting off the Sun's heat- South Temperate America as illustrating the influence of Astronomical causes on Climate- Geographical changes how far a cause of Glaciation- Land acting as a barrier to ocean-currents- The theory of interglacial periods and their probable character- Probable effect of winter in aphelion on the climate of Britain- The essential principle of climatal change restated-Probable date of the last Glacial Epoch-Changes of the sea-level dependent on Glaciation- The planet Mars as bearing on the theory of excentricity as a cause of Glacial Epochs. No less than seven different causes have been at various times advanced to account .for the glacial epoch and other changes of climate which the geological record proves to have taken place. These, as enumerated by Mr. Searles V. Wood, Jun., are as follows:- 1. A decrease in the original heat of our planet. 2. Changes in the obliquity of the ecliptic. 3. The combined effect of the precession of the equinoxes and of the excentricity of the earth's orbit. |