212 GEOLOGICAL TIME 



Professor Tait seeks to deduce results from them, 

 we are fully justified in following Sir William 

 Thomson, who says that " the existing state of things 

 on the earth, life on the earth — all geological history 

 showing continuity of life, must be limited within 

 some such period of past time as 100,000,000 

 years. l 



More recently Professor Perry has entered the lists, 

 from the physical side, to challenge the validity of 

 the conclusions so confidently put forward in limitation 

 of the age of the earth. He has boldly impugned 

 each of the three physical arguments. That which 

 is based on tidal retardation, following Mr. Maxwell 

 Close and Professor Darwin, he dismisses as fallacious. 

 In regard to the argument from the secular cooling 

 of the earth, he contends that it is perfectly allowable 

 to assume a much higher conductivity for the interior 

 of the globe, and that this assumption would vastly 

 increase our estimate of the age of the planet. As 

 to the conclusions drawn from the history of the sun, 

 he maintains that, on the one hand, the sun may have 

 been repeatedly fed by infalling meteorites, and that 

 on the other hand, the earth, during former ages, may 

 have had its heat retained by a dense atmospheric 

 envelope. He thinks that ' almost anything is possible 

 as to the present internal state of the earth,' and he 

 concludes in these words : ' To sum up, we can find 

 no published record of any lower maximum age of 

 life on the earth, as calculated by physicists, than 

 400 millions of years. From the three physical 

 arguments, Lord Kelvin's higher limits are 1,000, 



1 Rep. Brit. Assoc, 1886, p. 517. 



