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Show 202 ISLAND LIFE. (PART 1. proportions had been reversed, large areas ?f ~and would necessarily have been removed from the beneficial mfluence of aqueous currents or moisture-laden winds ; and slight geological changes might easily lead to half the land surface becoming covered with perpetual snow and ice, or being exposed to extremes of summer heat and winter cold, of which our water-permeated globe at present affords no example. We thus see that what are usually regarded as geographical anomaliesthe disproportion of land and water, the gathering of the land mainly into one hemisphere, and the singular arrangement of the land in three great southward-pointing masses-are really facts of the greatest significance and importance, since it is to these very anomalies that the universal spread of vegetation and the adaptability of so large a portion of the earth's surface for human ha,bitation is directly due. CHAPTER X. TilE EARTH'S AGE, AND THE RATE OF DEVELOPMENT OF ANIMALS AND PLANTS. Various estimates of Geological Time-Denu~lation and Deposition of Strata as a measure of 'fime-How to estimate the thickness of the Sedimentary Rocks-How to estimate the average rate of deposition of the Sedimentary Rocks-'l'he rate of Geological Change probably greater in very remote times-Value of the preceding estimate of Geological Time-Organic modification dependent on Change of ConditionsGeographical mutations as a Motive power in bringing about Organic Changes-Climatal revolutions as an agent in producing Organic Chang-es-Present condition of the Earth one of exceptional stability as regards Climate-Date of last Glacial Epoch and its bearing on the Measurement of Geological Time-Concluding Remarks. THE subjects discussed in the last three chapters introduce us to a diffi.culty which has hitherto been considered a very formidable one-that the maximum age of the habitable earth, as deduced from physical considerations, does not afford sufficient time either for the geological or the organic changes of which we have evidence. Geologists continually dwell on the slowness of the processes of upheaval and subsidence, of denudation of the earth's surface, and of the formation of new strata; while on the theory of development, as expounded by Mr. Darwin, the ·variation and modification of organic forms is also a very slow process, and has usually been considered to require an even longer series of ages than might satisfy the requirements of physical geology alone. As an indication of the periods usually contemplated by geolo-gists, we ma.y refer to Sir Charles Lyell's calculation in the tenth |