198 THE STORY OF THE EARTH AND MAN. 



The data for such calculations are very uncertain ; 

 and three elements of additional uncertainty closely 

 related to each other must also be noticed. The first 

 is the unknown length of the intervals in which no 

 deposition whatever may have been taking place 

 over the areas open to our investigation. The second 

 is the varying amounts in which material once de- 

 posited may have been swept away by water. The 

 third is the amount of difference that may have 

 resulted from the progressive change of the geo- 

 graphical features of our continents. These uncer- 

 tainties would all tend to diminish our estimate 

 of the relative length of the Mesozoic. Lastly, the 

 changes that have taken place in living beings, 

 though a good measure of the lapse of time, cannot be 

 taken as a criterion here, since there is much reason to 

 believe that more rapid changes of physical conditions 

 act as an inducing cause of rapid changes of life. 



On the whole, then, taking such facts as we have, 

 and making large deductions for the several causes 

 tending to exaggerate our conception of Palseozoic 

 time, we can scarcely doubt that the Palaeozoic may 

 have been three times as long as the Mesozoic. If 

 so, the continental pulsations, and the changes in 

 animal and vegetable life, must have gone on with 

 accelerated rapidity in the later period, a conclusion 

 to which we shall again have occasion to refer when 

 we arrive at the consideration of the Tertiary or 

 Neozoic time, and the age of man, and the probable 

 duration of the order of things under which we live. 



