68 PROGRESS OF SCIENCE 



address ; Huxley, at this his last public appearance, 

 worthily played his part of champion to the end. 

 Two years earlier (Edinburgh, 1892) Kelvin, in com- 

 menting upon Sir Archibald Geikie's presidential 

 address, had been able to refer with satisfaction to 

 developments ' according to which geologists find it 

 possible to hurry up the action without abandoning 

 any fundamental principle of the Huttonian theory/ 

 This is not the place to pursue the geological con- 

 troversy further, but it should be observed that 

 physicists now offer the supporters of evolution the 

 view that the age of the earth must be increased by 

 many millions of years above the figures for which 

 Kelvin argued. In a discussion on this subject at the 

 Edinburgh meeting in 1921, Lord Rayleigh showed 

 that Kelvin's calculations are upset by the discovery 

 of radio-active substances in the earth, since studies 

 of the duration of time needed for known changes 

 of these substances to take place have resulted in 

 assigning ' a moderate multiple of 1000 million years 

 as the possible and probable duration of the earth's 

 crust as suitable for the habitation of living beings/ 

 Lastly, the subject of petrology, concerned with 

 the minute examination of the composition of rocks, 

 may be said to have been established by Henry 

 Sorby's paper ' on a new method of determining 

 the temperature and pressure at which various 

 rocks and minerals were formed/ read at the Leeds 

 meeting of the Association in 1858. 



ZOOLOGY AND BOTANY 



There might be a superficial temptation to date 

 the progress of zoology and botany during our period 



