310 CONCLUSION 



paper caused, for a time, some consternation among both geologists 

 and biologists. He returned to the subject in other communica- 

 tions in 1865, in 1868, and again in 1899 arguing from certain 

 physical and astronomical data. His views as to the amount of 

 time that could be conceded as possible to have elapsed since the 

 consolidation of the earth have varied in these different communica- 

 tions, though his tendency has been to limit it more and more, till 

 in his latest utterance on this subject ' he put it as " more than 

 twenty and less than forty millions of years, and probably much 

 nearer twenty than forty." 



His views and reasonings have been criticised by other physicists, 

 and especially by Professors Perry and George Darwin, who have 

 attempted to show, with much success, the uncertain nature of 

 the data and assumptions upon which Lord Kelvin's conclusions 

 have been founded. 



Moreover, the time limits assigned by him have been considered 

 altogether inadequate by geologists and biologists alike. The most 

 authoritative demurrer on the former side may be found in Sir 

 Archibald Geikie's address = to the Geological Section of the 

 British Association in 1899, in which the whole subject is dis- 

 cussed in a full and interesting manner. He considered that 

 nothing short of 100 million years " would suffice for that portion 

 of the history which is registered in the stratified rocks of the 

 crust" {loc. cit., p. 500) — and, of course, very much more for the 

 total duration of life upon the planet. On the other hand Pro- 

 fessor Poulton, three years previously, in his address 3 to the 

 Zoological Section of the British Association, had dealt in an 

 elaborate manner with the problem from the biological standpoint. 

 He seemed to consider that a period " several times " as long as 

 that which had previously been indicated by Geikie in 1893 would 

 be needed to account for the evolution of all the forms of life upon 

 the globe from the Cambrian epoch onwards. The pre-Cambrian 

 period, in his estimation, may have been three or four times as 

 prolonged, and would certainly be double as long, as the subsequent 

 ages during which the whole stratified crust of the globe had been 

 laid down and all the forms of life known to us had been evolved. 

 This view as to the probable vast duration of the pre-Cambrian 

 ages is shared by geologists generally, and is also in accord with 



' "Phil. Mag.," January, 1899, p. 75, 

 " "Nature," September 21, 1899. 

 3 Ibid., September 24, 1896. 



