THE DATE OF THE GLACIAL PERIOD. 361 



has been, at any rate, only in slight degree deepened and 

 enlarged during post-Tertiary time. 



Summary. 



In briefly summarising our conclusions concerning the 

 question of man's antiquity as affected by his known re- 

 lations to the Glacial period, it is important, first, to re- 

 mark upon the changes of opinion which have taken place 

 with respect to geological time within the past generation. 

 Under the sway of Sir Charles LyelPs uniformitarian 

 ideas, geologists felt themselves at liberty to regard geo- 

 logical time as practically unlimited, and did not hesi- 

 tate to refer the origin of life upon the globe back to a 

 period of 500,000,000 years. In the first edition of his 

 Origin of Species Charles Darwin estimated that the time 

 required for the erosion of the Wealden deposits in Eng- 

 land was 306,662,400 years, which he spoke of as " a mere 

 trifle " of that at command for establishing his theory of 

 the origin of species through natural selection. In his 

 second edition, however, he confesses that his original 

 statement concerning the length of geological time was 

 rash ; while in later editions he quietly omitted it. 



Meanwhile astronomers and physicists have been grad- 

 ually setting limits to geological time until they have now 

 reached conclusions strikingly in contrast with those held 

 by the mass of English geologists forty years ago. Mr. 

 George H. Darwin, Professor of Mathematics at Cam- 

 bridge University, has from a series of intricate calcula- 

 tions shown that between fifty and one hundred million 

 years ago the earth was revolving from six to eight times 

 faster than now, and that the moon then almost touched 

 the earth, and revolved about it once every three or four 

 hours. From this proximity of the moon to the earth, it 

 would result that if the oceans had been then in existence 

 the tides would have been two hundred times as great as 

 now, creating a wave six hundred feet in height, which 



