242 HEREDITY AND EVOLUTION IN PLANTS 



Sixty years ago Lord Kelvin gave a mean estimate of 

 100,000,000 years. With this estimate two geologists, 

 Walcott and Geikie, have nearly' concurred; but since the 

 discovery of radium it has been estimated that certain 

 carboniferous iron ores have an age of 140,000,000 years. 

 Figures of such magnitude convey but little meaning to 

 our minds; they are too large for us to grasp their real 

 value. "Therefore," as Darwin has said, "a man should 

 examine for himself the great piles of superimposed strata, 

 and watch the rivulets bringing down mud, and the waves 

 wearing away the sea-cliffs, in order to comprehend some- 

 thing about the duration of past time, the monuments of 

 which we see all around us." 



