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all, however, depending upon considerations of heat or of" energy; and 

 the result to which he has come is that he cannot grant more than about 

 10,000,000 years for the production of all those results to which Geology 

 bears witness. You may think this a tolerably liberal allowance ; but 

 some of the Geologists and Mr. Darwin appear to' think that even 

 300,000,000 of years would be insufficient for the work to be done, 

 which has actually been accomplished. Professor Tait says that if 

 Geology really cannot do without so long a period, so much the worse 

 for Geology, inasmuch as the theory of heat and energy cannot, in its 

 most liberal mood, grant it anything Hke such a long innings. I do not 

 regret this present apparent conflict between different sciences ; it is by 

 such conflicts that truth is not unfrequently secured ; if upon more 

 careful inquiry it should appear that Geology and other natural sciences 

 cannot have so much time to expatiate in as has been till lately imagined, 

 I doubt not that further investigation will prove that the enormous 

 length of time is not really required ; ultimate harmony will assuredly 

 prevail amongst the sciences, however different may be the starting 

 points of their respective investigations. 



Let me say a word here upon the term Theory, as I have been lately 

 using it. I have spoken of the Theory of the Conservation and Trans- 

 formation of Energy ; and you' may speak, if you please, of Mr. Darwin's 

 Theory of the Transformation of Species, and you may be disposed to 

 say that one Theory is as good as another. But it should be borne in 

 mind that when a Mathematician speaks of a Theory, he generally means 

 something different from speculation, different from ingenious guess, 

 diff'erent from plausible hypothesis : he speaks of the Lunar Theory, the 

 Planetary Theory, the Undulatory Theory of Light, and so forth, as 

 doctrines respectively concerning the Moon, the Planets, and Light, 

 which can be proved to be true by such evidence as no reasonable man, 

 who follows the steps of the investigation, can find it possible to doubt. 

 These Theories are absolutely different in kind from mere hypotheses 

 which cannot be verified ; and it is in this sense that we speak of the 

 Theory of the Conservation and Transformation of Energy. I cannot, 

 of course, prove to you in an address like this that the Theory is of the 



