18 Astronomy and Geology compared. PT. i. 



ascertained as those relating to Astronomy. As I 

 think I already observed, they are derived from the 

 relics of former conditions of our Grlobe ; they cannot 

 be tested by comparison with what now exists ; 

 they are also very much scattered and unconnected. 

 We cannot refer to any former period of the world's 

 history as indicating a state of things complete in 

 itself; each of the various strata which geologists 

 trace beneath the earth's surface seems to refer to 

 a period of transition between the stratum imme- 

 diately below and that above it. Each of these 

 strata contains many of the older species of plants 

 and animals, and each contains many new animals. 

 At each step many of the older varieties disappear, 

 and many new ones appear ; but all this renders the 

 evidence of facts upon which they are founded 

 exceedingly complicated and difficult to trace. We 

 long for the certainties of mathematical demonstra- 

 tion, and for the power of examining the present. 

 Where all the theory is founded upon the evidence 

 of facts, and of facts so scattered and so remote, our 

 calculations are liable to be disturbed at every turn 

 by new facts inconsistent with the results we 

 imagined that we had realised. 



An example of this may be found in the different 



