heory 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 inportant[**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 production of a habitable
globe with comparatively small extremes of climates over its whole area, is
the great disproportion 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
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 habitation is directly due.

* * * * *

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CHAPTER X

THE EARTH'S AGE, AND THE RATE OF DEVELOPMENT OF ANIMALS AND PLANTS

Various Estimates of Geological Time--Denudation and Deposition of
Strata as a Measure of Time--How to Estimate the Thickness of the
Sedimentary Rocks--How to Estimate the Average Rate of Deposition of
the Sedimentary Rocks--The Rate of Geological Change Probably greater
in very Remote Times--Value of the Preceding Estimate of Geological
Time--Organic Modification Dependent on Change of
Conditions--Geographical Mutations as a Motive Power in bringing about
Organic Changes--Climatal Revolutions as an Agent in Producing Organic
Change