THE EARLIEST MEN 169 



different estimates arrived at in selected cases by 

 different observers. Thus, for example, Sir Charles 

 Lyell, a great authority in geological matters, 

 estimated that the amount of time required for 

 the erosion of a certain stretch of the Niagara 

 Gorge was forty thousand years, an estimate, of 

 course, based upon his calculation as to the annual 

 amount of erosion. Yet, in 1907, C. K. Gilbert 

 stated, in the publications of the United States 

 Geological Survey, that in his opinion the amount 

 of time required for the piece of work in question 

 would be no more than seven thousand six hun- 

 dred years.* In fact, with all respect be it said, 

 these estimates are and must, so it would appear, 

 always be, guesses guesses made by men more 

 likely to guess right than wrong perhaps, but, in 

 the end, only guesses, and thus very far removed 

 from being scientific facts as the more enthusiastic 

 papers and magazines are prone to represent them. 

 But there is yet another source of fallacy, and it is 

 this : The conditions cannot be shown to be con- 

 stant, nay more, everything points to the fact that 

 they have been extremely inconstant during past 

 ages. But this state of affairs would wholly upset 

 the accuracy of our clock, and render any calcu- 

 lations based upon its record wholly fallacious. 



In a very interesting little book on ancient 

 human remains,! Professor Keith claims that the 

 Thames is a reliable clock. " The Thames itself," 

 he says (p. 22), "is to be our clock one which 

 has never ceased to mark time and record history 



* See p. 220 for further account of this matter, 

 t Ancient Types of Man. 



