122 LIFE ON THE EARTH. 



held consistently by any speculator who is unpre- 

 pared with proof, that the conditions under which 

 the laws of nature operate have always been the 

 same ; not indeed at each moment of time or at 

 each point of space, but when a cycle of time or 

 the average of such cycles is taken as the unit of 

 proportion, and the area of the globe is the field of 

 experiment. Now this proof requires that the pro- 

 portion of land and water and atmosphere should 

 be always cyclically the same, the land equally ele- 

 vated, the water equally deep that terrestrial climate 

 should on the whole have been unchanged atmo- 

 spheric precipitations always equal in total effect 

 the surface of the globe always equally destructible, 

 besides other conditions on which equality in the 

 rate of deposition of sediments depends. Still, in spite 

 of all these difficulties, the short measure of modern 

 physical effects in a given time is the only standard 

 to be applied to the immensity of past duration. 



They who reject the uniformity of natural effects, 

 as a principle of computation of past geological time, 

 do so on the ground that the earth has gone 

 through a series of different conditions since it be- 

 came a terraqueous globe; that the effect of such 

 different conditions may be perfectly seen in dif- 

 ferent parts of the globe at present, giving to one 

 quarter more rapid waste of the land and more rapid 



