CHAP. IX GEOLOGICAL CLIMATES 209 



preponderating influence, and without very large areas of 

 high northern land to act as condensers, no perpetual snow 

 is possible, and hence the initial process of glaciation does 

 not occur. 



The theory as now set forth should commend itself to 

 geologists, since it shows the direct dependence of climate 

 on physical processes, which are guided and modified by 

 those changes in the earth's surface which geology alone 

 can trace out. It is in perfect accord with the most recent 

 teachings of the science as to the gradual and progressive 

 development of the earth's crust from the rudimentary 

 formations of the Azoic age, and it lends support to the 

 view that no important departure from the great lines of 

 elevation and depression originally marked out on the 

 earth's surface has ever taken place. 



It also shows us how important an agent in the pro- 

 duction of a habitable globe with comparatively small 

 extremes of climates over its whole area, is the great dis- 

 proportion between the extent of the land and the water 

 surfaces. For if these proportions had been reversed, large 

 areas of land would necessarily have been removed from 

 the beneficial influence of aqueous currents or moisture- 

 laden winds ; and slight geological changes might easily 

 have led 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 

 anomalies — the 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 signific- 

 ance and importance, since it is to these very anomalies 

 that the universal spread of vegetation and the adapt- 

 ability of so large a portion of the earth's surface for 

 human habitation is directly due. 



