Jbr the Advanceineni of Science. 103 



liave a truth of a superior kind to the facts ; to be certain inde- 

 pendently of its exemplification in particular cases: — if, when 

 exceptions to our propositions occur, instead of modifying the 

 theory, we explain away the facts ;— our theory then becomes 

 our tyrant ; and all who work under its bidding do the work of 

 slaves, they themselves deriving no benefit from the result of 

 their labours. For the sake of example we may point out the 

 Geological Society as a body which, labouring in the former 

 spirit, has enobled and enriched itself by its exertions : if any 

 body of men should employ themselves in the way last describ- 

 ed, they must soon expend the small stock of a 'priori plausibi- 

 lity with which they must of course begin the world. 



To exemplify the distinction for a moment longer, let it be 

 recollected that we have at the present time two rival theories of 

 the history of the Earth which prevail in the minds of geolo- 

 gists : — one which asserts that the changes of which we trace 

 the evidence in the Earth's materials have been produced by 

 causes such as are still acting at the surface : — another which 

 considers that the elevation of mountain chains and the transi- 

 tion from the organized world of one formation to that of the 

 next, have been produced by events which, compared with the 

 present course of things, may be called catastrophes and convul- 

 sions. Who does not see that all that those theories have hitherto 

 done, has been, to lead geologists to study more exactly the laws 

 of permanence and of change in the existing organic and inor- 

 ganic world, on the one hand ; and, on the other, the relations 

 of mountain chains to each other, and to the phenomena which 

 their strata present .? And who doubts, that, as the amount of 

 the full evidence may finally be (which may indeed perhaps re- 

 quire many generations to accumulate) geologists will give their 

 assent to the one or the other of these views, or to some inter- 

 mediate opinion to which lx)th may gradually converge ? 



On the other hand — to take an example from a science with 

 which I have had a professional concern — the theory that crys- 

 talline bodies are composed of ultimate molecules, which have a 

 definite and constant geometrical form, may properly and phi- 

 losophically be adopted, so far as we can, by means of it, reduce 

 to rules the actually occurring secondary faces of such substances. 

 But if we assume the doctrine of this composition, and then form 



