EUROPE BEFORE ARRIVAL OF MAN 



ture as we go beneath the surface, Sir William 

 Thomson concludes that the crust of the earth 

 cannot possibly have existed in the solid state 

 for more than four hundred million years, and 

 in all probability has not been solidified and in 

 fit condition for the support of vegetable and 

 animal life for more than one hundred million 

 or two hundred million years. This conclusion 

 is largely speculative, including several data of 

 which our knowledge is far from complete, and 

 it is of course extremely indefinite. It makes a 

 good deal of difference whether life has existed 

 on the earth for one hundred million years or 

 for two hundred millions, since one period is 

 just twice as long as the other. Still, in spite 

 of this indefiniteness, there is a growing dispo- 

 sition among geologists to accept Sir William 

 Thomson's estimate, as showing at least the 

 order of magnitudes with which the geological 

 chronologer must deal. That is to say, while 

 it may not be clear whether life has existed for 

 one or for two hundred millions of years, it is 

 not at all probable that it has existed for a 

 thousand millions or for any greater period. 

 Even this amount of limitation is of some value 

 as giving definite shape to our ideas, and as re- 

 minding us that geologists who have habitually 

 reasoned as if there were an infinite fund of past 

 time at their disposal have probably been in 

 error. Provided we do not forget that Sir Wil- 

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