PLANETARY EVOLUTION 



manifested by the four planets whose surfaces 

 have thus far been successfully studied are just 

 what the theory requires them to be. The in- 

 tense heat and furious volcanic activity of Ju- 

 piter and Saturn, the extreme loss of heat and 

 cessation of volcanic activity upon the moon, the 

 moderate temperature and habitable aspect of 

 Mars, are alike deducible from the nebular hy- 

 pothesis. 



I doubt if such persistent agreement between 

 deduction and observation has ever been wit- 

 nessed in the case of an erroneous or radically 

 inadequate hypothesis. If the sole ultimate test 

 of a theory is that it reconciles the order of con- 

 ceptions with the order of phenomena, may we 

 not say that the theory of Kant and Laplace, 

 having sustained the repeated application of this 

 test, may be accepted provisionally as a true 

 account of the past history of our system of 

 worlds ? It is true that the application of the 

 test has not yet been made exhaustive ; the veri- 

 fication is not yet complete. Some of the inter- 

 pretations above given are still, as I have ac- 

 knowledged, but partial ; and there are yet other 

 groups of phenomena with which I have not 

 ventured to meddle. To the various densities 

 of the planets I have alluded but incidentally ; 

 and the various angular velocities, as well as 

 the order of distances formulated in the law of 

 Titius, still await an explanation. Besides which, 

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