MODIFIED UNIFORMITARIANISM 177 



in the extreme doctrine of uniformitarianism. And 

 though, owing to uncertainty in regard to some of the 

 data, wide limits of time were postulated by him, he 

 insisted that within these limits the whole evolution 

 of the earth and its inhabitants must have been com- 

 prised. While, therefore, the geological doctrine that 

 the present order of Nature must be our guide to the 

 interpretation of the past remained as true and fruitful 

 as ever, it had now to be widened by the reception of 

 evidence furnished by a study of the earth as a plane- 

 tary body. The secular loss of heat, which demon- 

 strably takes place both from the earth and the sun, 

 made it quite certain that the present could not 

 have been the original condition of the system. This 

 diminution of temperature with all its consequences 

 is not a mere matter of speculation, but a physical fact 

 of the present time as much as any of the familiar 

 physical agencies that affect the surface of the globe. 

 It points with unmistakable directness to that beginning 

 of things of which Hutton and his followers could find 

 no sign. 



Another modification or enlargement of the uni- 

 formitarian doctrine was brought about by continued 

 investigation of the terrestrial crust and consequent 

 increase of knowledge respecting the history of the 

 earth. Though Hutton and Playfair believed in 

 periodical catastrophes, and indeed required these to 

 recur in order to renew and preserve the habitable 

 condition of our planet, their successors gradually came 

 to view with repugnance any appeal to abnormal, and 

 especially to violent manifestations of terrestrial vigour, 

 and even persuaded themselves that such slow and 



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