DARWIN AND THE ORIGIN OF SPECIES 245 



intensity, and have since the beginning diminished in intensity 

 and will diminish, till further sensible change ceases, and a dead 

 monotony is the final physical result of the mechanical laws 

 which matter obeys. 



Once this is granted, the calculations as to the length of 

 geological periods, from the present rates of denudation and 

 deposit, are blown to the winds. They are rough, very rough, 

 at best. The present assumed rates are little better than 

 guesses ; but even were these really known, they could by no 

 means be simply made use of in a rule-of-three sum, as has 

 generally been done. The rates of denudation and deposition 

 have been gradually, on the whole, slower and slower, as the 

 time of fusion has become more and more remote. There has 

 been no age of cataclysm, in one sense, no time, when the phy- 

 sical laws were other than they now are, but the results were as 

 different as the rates of a steam engine driven with a boiler first 

 heated to 1,500 degrees Fahrenheit, and gradually cooling to 

 200. 



A counter argument is used, to the effect that our argument 

 cannot be correct, since plants grew quietly, and fine deposits 

 were formed in the earliest geological times. But, in truth, 

 this fact in no way invalidates our argument. Plants grow just 

 as quietly on the slope of Vesuvius, with a few feet between 

 them and molten lava, as they do in a Kentish lane ; but they 

 occasionally experience the difference of the situation. The law 

 according to which a melted mass cools would allow vegetation 

 to exist, and animals to walk unharmed over an incredibly thin 

 crust. There would be occasional disturbances ; but we see that 

 a few feet of soil are a sufficient barrier between molten lava 

 and the roots of the vine ; each tendril grows not the less slowly 

 and delicately because it is liable in a year or two to be 

 swallowed up by the stream of lava. Yet no one will advance 

 the proposition that changes on the surface of a volcano are 

 going on at the same rate as elsewhere. Even so in the 

 primeval world, barely crusted over, with great extremes of 

 climate, violent storms, earthquakes, and a general rapid 

 tendency to change, tender plants may have grown, and deep 

 oceans may have covered depths of perfect stillness, interrupted 

 occasionally by huge disturbances. Violent currents or storms 



