2i 4 GEOLOGICAL TIME 



further alterations are in store for them. 1 He has 

 cut off slice after slice from the allowance of time 

 which at first he was prepared to grant for the 

 evolution of geological history, his latest pronounce- 

 ment being that c it was more than twenty and less 

 than forty million years, and probably much nearer 



1 October 1904. Since this Address was given the subject of radio- 

 activity has assumed high importance in reference to questions con- 

 nected with the evolution of the cosmos. Thus Prof. George Darwin, 

 in view of the newly discovered properties of radium, has stated 

 that he sees ' no reason for doubting the possibility of augmenting 

 the estimates of solar heat, as derived from the theory of gravi- 

 tation, by some such factor as ten or twenty' (Nature, 24th Sept., 

 1903). The same opinion is shared by Prof. Joly, who points 

 out that the establishment of the observed gradient of temperature 

 from the earth's surface inward ' may have been deferred indefi- 

 nitely during the exhaustion of stores of radium and similar bodies 

 at greater or shallower depths' (Nature, ist Oct., 1903). More 

 recently Prof. Rutherford, who has taken so leading a part in the 

 discussion of radio-activity, has made the following statement : 

 * I think we may conclude that the present rate of loss of heat 

 of the earth might have continued unchanged for long periods 

 of time in consequence of the supply of heat from radio-active 

 matter in the earth. It thus seems probable that the earth 

 may have remained for very long intervals of time at a tempera- 

 ture not very different from that observed to-day, and that in 

 consequence the time during which the earth has been at a 

 temperature capable of supporting the presence of animal and 

 vegetable life may be very much longer than the estimate made 

 by Lord Kelvin from other data' (Radio-activity, Cambridge, 

 1904, p. 346). Thus two of the three physical arguments are 

 impugned on new grounds, and the forecast of Prof. Darwin is 

 shown to have been reasonable when in 1886 he said: 'Although 

 speculations as to the future course of science are usually of 

 little avail, yet it seems as likely that meteorology and geology 

 will pass the word of command to cosmical physics as the 

 converse.' 



