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 Changes--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 difficulty 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 {100} even longer series of ages than might satisfy the requirements of physical geology alone.