2i6 GEOLOGICAL TIME 



to which the physicists would restrict it. 1 Let me 

 merely remark that the reasoning is essentially based 

 on observations of the present rate of geological and 

 biological changes upon the earth's surface. It is not, 

 of course, maintained that this rate has never varied 

 in the past. But it is the only rate with which we 

 are familiar, which we can watch and in some degree 

 measure, and which, therefore, we can take as a guide 

 towards the comprehension and interpretation of the 

 past history of our planet. 



It may be, and has often been, said that the present 

 scale of geological and biological processes cannot be 

 accepted as a reliable measure for the past. Starting 

 from the postulate, which no one will dispute, that 

 the total sum of terrestrial energy was once greater 

 than it is now and has been steadily declining, the 

 physicists have boldly asserted that all kinds of geo- 

 logical action must have been more vigorous and 

 rapid during bygone ages than they are to-day; that 

 volcanoes were more gigantic, earthquakes more 

 frequent and destructive, mountain-upthrows more 

 stupendous, tides and waves more powerful, and com- 

 motions of the atmosphere more violent, with more 

 ruinous tempests and heavier rainfall. Assertions of 

 this kind are temptingly plausible and are easily made. 

 But it is not enough that they should be made ; they 

 ought to be supported by some kind of evidence to 



!The geological arguments are briefly given in my Presidential 

 Address to the British Association at the Edinburgh Meeting of 

 1892 {ante p. 182). The biological arguments were well stated, 

 and in some detail, by Professor Poulton in his Address to the Zoo- 

 logical Section of the Association at the Liverpool Meeting of 1896. 



